Requirements for meteorological observations at aerodromes

Requirements for meteorological observations at aerodromes
Safety and Airspace Regulation Group
Requirements for meteorological observations
at aerodromes
CAP 746
CAP 746
Published by the Civil Aviation Authority, 2017
Civil Aviation Authority,
Aviation House,
Gatwick Airport South,
West Sussex,
RH6 0YR.
You can copy and use this text but please ensure you always use the most up to date version and use it in context so as not to
be misleading, and credit the CAA.
First published 2003
Issue 2 - October 2012
Issue 3 - May 2014
Issue 4 - March 2017
Enquiries regarding the content of this publication should be addressed to: metauthority@caa.co.uk
Safety and Airspace Regulation Group, Civil Aviation Authority, Aviation House, Gatwick Airport South, West Sussex, RH6
0YR.
The latest version of this document is available in electronic format at www.caa.co.uk, where you may also register for e-mail
notification of amendments.
March 2017
CAP 746
Contents
Contents
Contents ..................................................................................................................... 1
Foreword .................................................................................................................... 7
Revision history .......................................................................................................... 9
Chapter 1................................................................................................................. 12
Introduction .............................................................................................................. 12
Chapter 2................................................................................................................. 16
General requirements for aerodrome observations .................................................. 16
Chapter 3................................................................................................................. 20
Accreditation and competence of observers ............................................................ 20
Introduction ....................................................................................................... 20
The meteorological observer’s certificate .......................................................... 21
The restricted meteorological observer’s certificate ................................... 21
Continued accreditation and refresher training .................................................. 23
Aerodrome meteorological observing service provider contingency ................. 24
Non-accredited meteorological reports.............................................................. 24
Chapter 4................................................................................................................. 25
METAR structure and UK coding rules..................................................................... 25
Introduction ....................................................................................................... 25
Aviation weather report for METARs – symbolic code ...................................... 26
UK METAR coding variations ............................................................................ 27
Surface wind .............................................................................................. 28
CAVOK....................................................................................................... 31
Visibility – meteorological minimum and maximum .................................... 31
Runway visual range .................................................................................. 34
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Contents
Present weather ......................................................................................... 36
Cloud .......................................................................................................... 50
Air temperature and dew point ................................................................... 52
Atmospheric pressure ................................................................................ 53
Recent weather .......................................................................................... 54
Runway states ............................................................................................ 57
TREND forecast ................................................................................................ 62
AUTO METAR ................................................................................................... 64
Chapter 5................................................................................................................. 68
Weather reports to air traffic services ....................................................................... 68
Introduction ....................................................................................................... 68
Surface wind ..................................................................................................... 69
Visibility ............................................................................................................. 70
RVR................................................................................................................... 70
Present weather ................................................................................................ 71
Cloud ................................................................................................................. 71
Temperature...................................................................................................... 71
Pressure ............................................................................................................ 71
Recent weather ................................................................................................. 72
Supplementary information ............................................................................... 72
Special reports for air traffic services purposes ................................................. 73
Chapter 6................................................................................................................. 76
General requirements for observing equipment ....................................................... 76
Introduction ....................................................................................................... 76
General requirements........................................................................................ 76
Operation and maintenance requirements of meteorological equipment .......... 77
Installation of meteorological observing equipment ........................................... 78
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Contents
Chapter 7................................................................................................................. 80
Design requirements for meteorological equipment ................................................. 80
Introduction ....................................................................................................... 80
Meteorological displays ..................................................................................... 81
Surface wind speed and direction equipment .................................................... 82
Pressure measurement equipment ................................................................... 84
Temperature and dew point measurement ........................................................ 87
Cloud base recorder systems ............................................................................ 88
Visibility measuring systems ............................................................................. 90
Present weather detectors ................................................................................ 92
Integrated Met measurement systems ....................................................... 93
Chapter 8................................................................................................................. 95
Dissemination of weather reports ............................................................................. 95
Introduction ....................................................................................................... 95
Timing requirements.......................................................................................... 95
METAR.............................................................................................................. 96
Chapter 9................................................................................................................. 97
Reliability and availability of reporting ...................................................................... 97
Completeness of reports ................................................................................... 97
Contingency arrangements for the failure of meteorological observing sensors
and systems ...................................................................................................... 97
Missing meteorological aerodrome reports ....................................................... 98
Timeliness ......................................................................................................... 98
Chapter 10............................................................................................................. 100
Records and archives............................................................................................. 100
Introduction ..................................................................................................... 100
Meteorological information records ................................................................. 101
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Contents
Chapter 11............................................................................................................. 103
Definitions, abbreviations and bibliography ............................................................ 103
Glossary .......................................................................................................... 103
Abbreviations .................................................................................................. 107
Bibliography .................................................................................................... 108
Appendix A ........................................................................................................... 111
Purpose of the aerodrome meteorological regulatory oversight audit .................... 111
Introduction ..................................................................................................... 111
Conduct of meteorological regulatory oversight audit ..................................... 112
Appendix B ........................................................................................................... 114
Frequently asked questions on the compilation of the METAR .............................. 114
Introduction ..................................................................................................... 114
FAQs ............................................................................................................... 114
Wind ......................................................................................................... 114
Visibility .................................................................................................... 114
Present weather ....................................................................................... 115
Cloud ........................................................................................................ 115
Temperature............................................................................................. 117
Pressure ................................................................................................... 117
Recent significant weather ....................................................................... 117
Runway state group ................................................................................. 118
Observing from outside the building ......................................................... 118
Final checks ............................................................................................. 118
Restricted meteorological observer’s (RMO) certificate ........................... 118
Annex A to Appendix B ................................................................................... 119
Assessing prevailing visibility ................................................................... 119
Appendix C ........................................................................................................... 123
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Contents
Human observed RVR conversion table ................................................................ 123
Foreword ......................................................................................................... 123
Distance based method................................................................................... 123
Reference copies ............................................................................................ 124
Annex A to Appendix C ................................................................................... 125
Distance based method example ............................................................. 125
Appendix D ........................................................................................................... 126
Daily atmospheric pressure equipment QNH check ............................................... 126
Appendix E ........................................................................................................... 127
Theoretical observer training requirements ............................................................ 127
Introduction ..................................................................................................... 127
Training programme ........................................................................................ 127
Syllabus.................................................................................................... 127
Examinations ................................................................................................... 128
Appendix F ............................................................................................................ 129
Practical observer training requirements for a meteorological observer’s certificate
(manual observed weather reports) ....................................................................... 129
Introduction ..................................................................................................... 129
Training organisations ..................................................................................... 129
Training programme ........................................................................................ 130
Assessment details during the practical training. ............................................ 131
Meteorological observers competence-based document ................................ 132
Operational competence .......................................................................... 132
Continuous assessment .................................................................................. 133
Annex A to Appendix F.................................................................................... 133
Appendix G ........................................................................................................... 135
Training requirements for a restricted meteorological observer’s certificate ........... 135
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Contents
Introduction ..................................................................................................... 135
Training programme ........................................................................................ 135
Syllabus.................................................................................................... 136
Practical training for a restricted meteorological observer’s certificate ............ 137
Operational competence ................................................................................. 137
Continuous assessment .................................................................................. 137
Relocation training........................................................................................... 138
Appendix H ........................................................................................................... 139
Competency of observers ...................................................................................... 139
Appendix I ............................................................................................................. 142
Calibration requirements for wind and pressure measuring equipment ................. 142
Wind measuring equipment ............................................................................. 142
Analogue cup or vane systems ................................................................ 142
Digital cup or vane systems ..................................................................... 142
All cup or vane systems ........................................................................... 143
Ultrasonic wind sensors ........................................................................... 143
Pressure measuring equipment ............................................................... 143
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Foreword
Foreword
1.
The Secretary of State for the Department for Transport (DfT) has
delegated the responsibilities for the function of the UK Meteorological
Authority to the Civil Aviation Authority (CAA) through the (Air Navigation)
Directions 2001, in exercise of the powers conferred by Section 66(1) of
the Transport Act 2000. Under the Directions, the CAA is required to
develop, promulgate, monitor and enforce a policy for the sustainable use
of UK airspace and for the provision of necessary supporting
infrastructure for air navigation.
2.
In particular, the Directions state that the CAA shall discharge the
responsibilities of the UK Meteorological Authority:

in accordance with International Civil Aviation Organisation (ICAO),
Meteorological Service for International Air Navigation, Annex 3 to
the Chicago Convention and other international obligations; and

subject to international obligations, in such a manner as the CAA
may determine from time to time.
3.
The CAA has also been appointed as the UK National Supervisory
Authority (NSA) under the Single European Sky Regulations and,
pursuant to Article 246 of the ANO 2009, the CAA is also the National
Aviation Authority (NAA) and the competent authority of the UK for the
purposes of the EASA Regulations.
4.
Meteorological observations at aerodromes are provided by the
aerodrome licensee for use by aviation users, operators and other
providers of services to aviation.
5.
Whereas ICAO is responsible for establishing aeronautical meteorological
requirements, the World Meteorological Organisation (WMO) through its
Commission for Aeronautical Meteorology, is responsible for specifying
the technical methods and practices recommended for use in providing
aeronautical meteorological services.
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CAP 746
6.
Foreword
It is the policy of the UK Meteorological Authority that, unless a difference
has been declared, meteorological services for both national and
international flights are compliant with ICAO Annex 3 Standards and
Recommended Practices (SARPs). Where a difference from ICAO SARPs
has been declared, requirements for meteorological services will be as
specified in the UK Aeronautical Information Publication (UK AIP).
7.
The contact address for the UK Meteorological Authority is:
The UK Meteorological Authority
Safety and Airspace Regulation Group
Civil Aviation Authority
CAA House, K6G1
45-59 Kingsway
London WC2B 6TE
United Kingdom
E-mail: metauthority@caa.co.uk
8.
The CAA publishes Civil Aviation Publications (CAPs) that provide details
of means of compliance with international and European Regulations. In
particular, this publication (CAP 746) contains procedures and information
which describe the provision of meteorological observations to civil
aviation in the UK, and the related regulatory requirements as specified in
ICAO Annex 3 and Commission Regulation (EU) No. 1035/2011.
9.
This is a living document and will be revised at intervals to take account of
changes in regulations, feedback from industry, and recognised best
practice. Contact addresses, should you have any comments concerning
the content of this document or wish to obtain subsequent amendments,
are given on the inside cover of this publication.
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Revision history
Revision history
Issue 1

4 December 2003
Minor editorial amendments.
Amendment 1/2004
27 April 2004

Introduction of prevailing visibility.

Enabling provision for dissemination of AUTO METAR under certain
circumstances.

Clarification of requirements for cloud ceilometer.
Amendment 1/2005

Revised Met Observers competency requirements.

Introduction of term ‘no significant cloud’ (NSC) in METAR.

Introduction of new code for AUTO METAR.
Amendment 1/2006

6 October 2005
31 October 2006
Clarification regarding when certain present weather phenomena are required
to be reported.

Clarification on how visibility should be reported in reports to air traffic.

Guidance on how to report prevailing visibility.

Introduction of requirements for documentation of contingency equipment.

Introduction of requirements for recording the originator of an observation.

Clarification on completion times for the METAR.

Explanatory Note and Foreword unchanged but pages now numbered with
Arabic instead of Roman numerals.

Introduction of a Revision History.
Amendment 1/2007

Addition of e-mail address for UK Meteorological Authority.

Changes in the definition of prevailing visibility.

Definition of Aerodrome Reference Point.
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7 November 2007
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CAP 746

Revision history
Requirement for observer to self-brief on the expected weather conditions prior
to taking over watch.

Changes to requirement for AUTO METARs.

Revised Appendix C on requirement for Human Observed RVR Conversion
Tables.
Amendment 1/2008
5 November 2008

Changes in the reporting of CAVOK.

Requirement for a quality system to be in place.

Requirement for AUTO METARs to be issued when duty breaks are taken.

Clarification of IRVR reporting requirements.

Clarification of wind reporting requirements to ATS.

Clarification of siting requirements for visiometers and temperature sensors.

Clarification of requirements for visibility measuring systems.

Update to purpose of Aerodrome Meteorological Liaison Visits.
Issue 2
15 October 12

New title for CAP 746.

Revision to applicability of CAP 746.

Requirement for CAT II and III aerodromes to have Integrated Met
Measurement System.

Change in the number of observations required by an observer to remain
current.

Staff carrying out Met competency checking role required to undergo refresher
training every five years.

Changes to Runway State Message format.

Revision of sections related to AUTO METAR.

Introduction of guidance related to calibration of wind and pressure sensors.
Issue 3
21 May 2014

Minor editorial amendments.

Clarification of Runway State Codes - R88/ and R99/.

Runway Visual Range Special Reports, change thresholds - updated.
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CAP 746

Revision history
Cloud Base Recorders, Visibility Measuring Systems and Present Weather
Detectors are mandatory at aerodromes providing automated observations.

Visibility Measuring Systems are mandatory at CAT ll and CAT lll aerodromes.

NATS HORVR Calibration Team Contact Details - updated.

Frequently Asked Questions - Clarification of reporting Sky Obscured, VV///.

Clarification of Automated Cloud Coding - NCD (no cloud discernible).
Issue 4
20 March 2017

Minor editorial amendments

Removal of the Gold Visibility Meter as a method for producing Human
Observed RVR Conversion Tables; Copies of HORVR conversion tables are
not required to be forwarded to the UK Met Authority.

Clarification of reporting Mist.

RETSRA, RETSSN, RETSGR, RETSGS added to the list of permitted recent
weather codes

Clarification of the requirement for records of annual observer competency
assessments to be dated, and for the assessments to include a check that
observers can use local backup procedures.

Clarification of reporting SH in observations generated by automatic observing
systems.

Updates to Frequently Asked Questions.
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CAP 746
Chapter 1: Introduction
Chapter 1
Introduction
1.1
Requirements and standards for aerodrome meteorological observations,
both within the United Kingdom as well as the surrounding offshore areas
are determined in accordance with the standards and recommended
practices (SARPs) of the International Civil Aviation Organisation (ICAO)
and the guidance issued by the World Meteorological Organisation
(WMO).
1.2
At licensed aerodromes, the aerodrome licensee is responsible for
arranging the provision of aerodrome weather observations and other
meteorological information to users.
1.3
It is recognised that this function may be performed by staff directly
employed or contracted by the aerodrome licensee. In order to maintain
clarity of responsibilities in respect of meteorological data, the
arrangements for the compilation of aerodrome weather reports are
described within this document as the responsibility of the Aerodrome
Meteorological Observing Service Provider. Within the UK, the Aerodrome
Meteorological Observing Service Provider is commonly the air traffic
service (ATS) provider organisation. This document recognises, however,
that aerodrome weather reports may be produced by any suitably
competent and qualified person.
1.4
At aerodromes with an Air Traffic Control unit, weather reports are
provided and utilised in accordance with ICAO PANS ATM Doc 4444, and
CAP 493, Manual of Air Traffic Services Part 1. At aerodromes that do not
have an ATC unit, the procedures for ensuring that weather reports are
made available to pilots and other users should be described locally.
1.5
All the requirements contained within this document apply to aerodromes
that are certificated by the CAA under the EASA Common Requirements
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Chapter 1: Introduction
(EC REG 1035/2011) as Air Navigation Service Providers which routinely
disseminate MET information beyond the aerodrome.
1)
Aerodromes that are certificated under the EASA Common
Requirements (EC REG 1035/2011) as an Air Navigation Service
Provider, have published or airport owned instrument approach
procedures but do not routinely disseminate MET information
beyond the aerodrome are only required to ensure that all the Met
equipment used on the aerodrome meets the specifications stated in
Chapter 7 and calibrated in accordance with Chapter 6, Paragraphs
6.10 to 6.17, and Appendix l.
2)
Aerodromes that are certificated under EASA Common
Requirements (EC REG 1035/2011) as an Air Navigation Service
Provider but do not routinely disseminate MET information beyond
the aerodrome and are without published or airport owned
instrument approach procedures may use less stringent
requirements for pressure and wind sensors as detailed in Chapter
7, Paragraphs 7.26, 7.49, 7.51, Chapter 6, Paragraphs 6.10 to 6.17,
and Appendix l.
3)
For all other aerodromes this document should be used as guidance
on best practice.
1.6
Aerodrome meteorological observations are used for flight planning
purposes and to facilitate safe operation of aircraft in the take off and
landing phases of flight. The information includes direction and speed of
the surface wind; horizontal visibility; prevailing weather; atmospheric
pressure information; surface temperature and dew point; cloud amounts
and height of the cloud base. Equipment used to provide real-time
information to ATC is subject to requirements specified in CAP 670 ATS
Safety Requirements. If the same equipment is used to originate METAR
reports and to provide real-time information to ATS, the sensing and data
processing equipment will normally be subject to the requirements of the
UK Meteorological Authority and the display equipment used within the
ATS unit will normally be subject to the relevant requirements of CAP 670
ATS Safety Requirements.
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CAP 746
1.7
Chapter 1: Introduction
Accurate, timely and complete aerodrome meteorological observations
are necessary to support safe and efficient air navigation.
1.8
All aerodromes that are certificated under the EASA Common
Requirements (EC REG 1035/2011) as an Air Navigation Service Provider
are required to ensure that the unit’s quality management system includes
an appropriate level of detail of the meteorological processes which are
applicable to the provision of meteorological services.
1.9
The quality management system should include a system that assures
users that the quality of meteorological information supplied complies with
the requirements in this document. When the quality system indicates that
the meteorological information does not comply with the requirements in
this document it should not be issued.
1.10
The purpose of this document is to describe how ICAO standards and
recommended practices are applied in the UK and to specify the
requirements for observers and equipment to achieve this.
1.11
Two types of aerodrome meteorological observations may be provided.

Official Meteorological Reports which, if in the form of a
meteorological aerodrome report (METAR), may be disseminated
beyond an aerodrome to pilots and other meteorological service
providers using processes that adhere to ICAO Standards. An
observer shall be accredited and competent to produce these
observations, as described in Chapter 3, and, the instrumentation
used shall comply with the requirements in Chapters 6 and 7.

All other meteorological observations. Whilst the observer does not
need to be accredited to give an opinion on the latest meteorological
conditions, a basic competency in assessing conditions and reading
instrumentation is required. Such opinions shall be regarded as
unofficial for air navigation purposes.
1.12
METAR information is used by Meteorological Forecast Offices in the
production of Aerodrome Forecasts (TAF) and other forecasts.
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CAP 746
1.13
Chapter 1: Introduction
The Met Observers at an aerodrome should be located, as far as
possible, in a position that enables them to supply observations which are
representative of the aerodrome and its vicinity.
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CAP 746
Chapter 2: General requirements for aerodrome observations
Chapter 2
General requirements for aerodrome observations
2.1
Timely and accurate meteorological information shall be available to
aircraft operators and ATS providers. Also, Aerodrome Forecasts (TAF)
are provided only on receipt of valid METARs and due account should be
taken by aerodrome licensees of operator requirements for these
forecasts.

A METAR shall contain the following items of information:

Identification of the type of report (e.g. METAR)

Location indicator

Time of Observation, in UTC

Surface wind direction and speed (including variations in
direction)

Visibility* - see 2.2 below

Runway visual range (where applicable and
equipment/procedures have been approved)* - see 2.2 below

Present weather* - see 2.2 below

Cloud amount (and type, if applicable) and height of base* see 2.2 below
2.2

Air temperature and dew point temperature

QNH and, where applicable QFE

Recent Weather, when applicable

Runway States, when applicable
Asterisked elements are included as necessary. The term CAVOK may
replace visibility, present weather and cloud information under certain
conditions (see glossary for definition).
2.3
Within the UK, METAR reports shall be provided every 30 minutes during
the operational hours of the aerodrome unless otherwise agreed with the
UK Meteorological Authority.
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CAP 746
2.4
Chapter 2: General requirements for aerodrome observations
For ATS purposes, the measurements of meteorological elements should
be representative of the landing and take-off areas on the runways. For
the METAR, the measurement should be representative over the whole
aerodrome operating area. Instrumentation used in the measurement of
meteorological elements for METAR reports may also be used for reports
to ATS providers, providing that the exposure of the instruments is
suitable to provide representative readings for both purposes.
NOTE:
Although the content of meteorological reports for ATS purposes and METAR
reports are similar, the averaging periods for certain elements in the reports
differ. Refer to Chapters 4 and 5 for the requirements for each type of report.
2.5
A method to ensure that observing staff are aware of, and competent in,
local observing and reporting procedures shall be established.
2.6
Local observing and reporting procedures include the way in which
observations are recorded and disseminated both within and beyond the
aerodrome, including any necessary backup arrangements.
NOTE:
Prior to taking over watch, observers shall obtain full information regarding the
weather to be expected during the period of their watch. This may be
accomplished by a study of forecasts and charts routinely supplied by the Met
Office.
2.7
The observer shall continuously monitor the weather. Observations must
be updated, as necessary, in a timely manner. During any period that
routine METAR reports are being produced, special reports shall be
produced as dictated by the weather conditions. Unless otherwise agreed
by the UK Meteorological Authority, the criteria for the production of a
special report shall be those given in Chapter 5, Weather Reports to Air
Traffic Services. All special reports shall be issued to the ATS provider
and, where possible, should be passed to the Meteorological Forecast
Office.
2.8
The observer shall provide a full non-routine observation at the time of an
aircraft accident on or in the vicinity of the aerodrome. This is to ensure
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CAP 746
Chapter 2: General requirements for aerodrome observations
that complete details of the weather at the time of the incident will be
available to an official inquiry.
2.9
The UK Meteorological Authority is responsible for arranging regulatory
oversight audits of Aerodrome Meteorological Observing Units at regular
intervals to ensure that a high standard of observations is maintained, that
instruments and their indicators are functioning correctly, and to validate
the exposure of the instrumentation. Appendix A, Purpose of aerodrome
meteorological regulatory oversight audits, provides further information on
such audits.
2.10
Automated sensors used to measure certain meteorological elements
(typically visibility, present weather and cloud) shall be considered to be
an aid to an observer rather than a direct source of information for the
official weather report. Such sensors can measure each of these
elements, but are limited by the spatial coverage of the sensor and the
capability to resolve certain weather phenomena. Procedures shall be in
place to ensure that any observation generated by a semi automatic
observing system is not disseminated unless it has been checked and
qualified by an accredited observer.
2.11
Where an aerodrome is closed for more than two hours (for example
overnight), two consecutive METARs shall be produced before the
aerodrome opens, unless otherwise agreed by the UK Meteorological
Authority. The METARs shall be produced by an accredited observer and
separated by an interval of not less than 20 minutes and not more than 1
hour.
2.12
Aerodromes that operate 24 hours a day are required to provide METAR,
or where agreed, AUTO METAR observations, at all times, and to ensure
that the ATIS is updated as required.
2.13
At aerodromes where a Met observing system is installed that can be
operated automatically, it may be possible for the system to generate fully
automated weather reports in the form of AUTO METARs. Should an
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Chapter 2: General requirements for aerodrome observations
aerodrome wish to provide AUTO METARs then the permission of the
CAA is required.
2.14
The Meteorological Forecast Office will monitor the quality of the METAR
reports and provide feedback, as appropriate.
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CAP 746
Chapter 3: Accreditation and competence of observers
Chapter 3
Accreditation and competence of observers
Introduction
3.1
The Aerodrome Licensee is responsible for arranging provision of
aerodrome weather reports and other meteorological information to users.
For the purposes of this document, and to distinguish these
responsibilities from other functions that are the responsibility of the
Aerodrome Licensee, the responsibilities relating to meteorological
information are deemed to be delegated to the Aerodrome Meteorological
Observing Service Provider.
3.2
The Aerodrome Meteorological Observing Service Provider is responsible
for ensuring the competence of each aerodrome meteorological observer
employed at the aerodrome. This includes following initial training, during
routine observing duties, following changes to observing equipment or
METAR coding rules, and following the relocation of an observer from
another aerodrome.
3.3
The Aerodrome Meteorological Observing Service Provider should ensure
that observing staff are sufficiently familiar with all meteorological
phenomena that can reasonably be expected to occur at the aerodrome
as to permit their competent observation and reporting.
3.4
It is recommended that a formal agreement, such as a Service Level
Agreement, be reached between the Aerodrome Meteorological
Observing Service Provider and the ATS Provider and other agencies to
which accurate and timely meteorological information is essential for safe
operations.
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CAP 746
Chapter 3: Accreditation and competence of observers
The meteorological observer’s certificate
3.5
In order to gain a Meteorological Observer’s Certificate, each aerodrome
meteorological observer must successfully complete a recognised course
of training on the preparation of aerodrome weather reports and must
demonstrate basic competence in compiling such reports.
3.6
Information on recognised training courses for aerodrome observers is
published and updated each year in the UK AIP, Section 3.5. A provider
wishing to utilise an alternative training organisation is advised to consult
the UK Meteorology Authority to ascertain the suitability of the training
scheme.
3.7
The certification process comprises two parts; theory and practical. The
theory part will provide the necessary background information on all
elements of aerodrome meteorological observing; this is examined to
ensure that the concepts have been fully understood. The practical part
puts the aerodrome meteorological observer in the company of an
experienced observer in order to enable observing techniques to be
practised and allows the new observer’s basic competence to be
assessed.
3.8
The theoretical training syllabus is given in Appendix E, Theoretical
Practical Observer Training Requirements; requirements for practical
training requirements for observers are given in Appendix F, Practical
Observer Training Requirements for a Meteorological Observer’s
Certificate (Manual Observed Weather Reports).
3.9
Following certification, the trainee observer shall continue to carry out all
operational observing duties under supervision until such time that the
observer can meet the competency requirements listed in Appendix H,
Competency of Observers.
The restricted meteorological observer’s certificate
3.10
Semi-automated observing systems are utilised on many aerodromes for
the provision of weather reports. Such systems process data from
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Chapter 3: Accreditation and competence of observers
external sensors located at the aerodrome and compile the basic METAR
report incorporating the measurements made by the sensors. Elements
such as surface wind, temperature and pressure are acceptable as
measured by the sensors, without verification by an observer.
3.11
Whilst automated sensors can measure visibility, weather type, and cloud
height and amount, they are limited by the spatial coverage of the sensor
and the capability to resolve present weather types.
3.12
To comply with internationally agreed practices, the Met Authority requires
the reporting of prevailing visibility, lowest visibility (if certain criteria are
met), present weather phenomena such as thunderstorms, snow, freezing
precipitation (including hail and freezing rain), towering cumulus and
cumulonimbus cloud. However, the current automatic observing systems
in use in the UK are either incapable of reporting these elements or do not
have an appropriate high degree of accuracy and consistency. It is for this
reason that measurements of horizontal visibility, present weather and
cloud must be validated by an accredited observer before being issued as
a METAR.
3.13
Where a semi-automated observing system is used on an aerodrome,
aerodrome meteorological observers need only be competent to provide
the visual elements. Reduced training may be provided, concentrating on
observing visibility, weather types and cloud details.
3.14
Under these circumstances, following successful completion of the
appropriate training course, a restricted met observer’s certificate may be
awarded. The programme for this training is given in Appendix G, Training
Requirements for a Restricted Meteorological Observer’s Certificate.
3.15
Contingency procedures and the provision of contingency observing
equipment shall take account of the limits of the observers’ accreditation.
Where a semi-automated observing system is used on an aerodrome,
appropriate contingency observing equipment and training shall be
provided to enable a suitable level of accuracy and regularity of
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Chapter 3: Accreditation and competence of observers
observations to be maintained. Contingency observing arrangements shall
be tested on a regular basis.
3.16
Aerodrome Met Observers should note that a restricted met observer’s
certificate may only be transferred to another aerodrome where a
semiautomated observing system is in operation. In these cases, the
Aerodrome Meteorological Observing Service Provider should ensure that
the observer is provided with sufficient training to ensure that the observer
can competently use the primary and backup observing system at the new
aerodrome.
Continued accreditation and refresher training
3.17
The Aerodrome Meteorological Observing Service Provider shall ensure
that all accredited aerodrome met observers maintain their observing
competence. Competency requirements are listed in Appendix H,
Competency of Observers. The process of continuing accreditation shall
be documented.
3.18
The Aerodrome Meteorological Observing Service Provider shall ensure
that all accredited aerodrome met observers maintain their observing
competance.
3.19
Certificated observers should carry out a minimum of fifteen observations
over a consecutive period of ninety days to maintain observing and
METAR coding skills; ideally, one of these observations should be during
conditions when the visibility is less than 5 km, one should be during a
precipitation event and one should be during ‘CAVOK’ conditions. Where
observers do not meet the minimum requirements, the Manager, or other
nominated person, of the Aerodrome Met Observing Service Provider
should ensure that the observer can demonstrate observing and METAR
coding competence before resuming operational observing duties.
3.20
Following changes to observing practices or aeronautical codes, the
Aerodrome Meteorological Observing Service Provider shall ensure that
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Chapter 3: Accreditation and competence of observers
all staff are aware of the changes, additional training arranged as
necessary and that the changes are implemented accordingly.
3.21
Met Observers nominated to carry out the met observing competency
checking role at an aerodrome shall ensure that they have refreshed their
theoretical and practical met observing skills every 5 years by attending
an approved course, thereby ensuring they are up to date with the latest
coding requirements and observing techniques.
3.22
Other observing staff for instance those who have not observed for more
than a year and wish to acquaint themselves with changes to observing
practices and METAR codes should consider attending a met observing
refresher course. Additionally staff who are unable to have their met
observing competency checked locally may do so by attending such a
course.
Aerodrome meteorological observing service provider
contingency
3.23
The Aerodrome Meteorological Observing Service Provider shall identify
contingency and other mitigation measures as agreed between the
Licensee and the Provider in case of such events as observer
incapacitation or equipment failure.
Non-accredited meteorological reports
3.24
A weather report that has not been validated by an accredited observer
may, by agreement with the UK Meteorological Authority, be distributed
locally and to the meteorological forecast office. Such a report will need to
be clearly identified as an unofficial report and prefixed as such when
being passed to aircraft or other agency.
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Chapter 4: METAR structure and UK coding rules
Chapter 4
METAR structure and UK coding rules
Introduction
4.1
In the United Kingdom the standard codes used in composing a METAR
report are based on WMO Document No. 306, Manual on Codes. The full
METAR message may contain up to 18 groups. The Meteorological
Authority for each State determines the applicability of codes and practice
to that State. The specific coding rules and practice detailed in this
chapter are those applicable in the United Kingdom.
4.2
Entries for surface wind (including variations of speed and direction),
surface visibility (including directional variation), present weather, cloud
details, air temperature, dew point, QNH, QFE and supplementary
information are normally completed. The quality of such reports shall
conform to ICAO Standards and Recommended Practices (SARPs) as
specified in ICAO Annex 3.
4.3
Appendix B, Frequently Asked Questions on METAR coding provides
additional guidance on the compilation of the METAR.
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Chapter 4: METAR structure and UK coding rules
Aviation weather report for METARs – symbolic code
Code name
Location
Date/time of report
Automated
Wind
velocity/gust
Extremes in
direction
METAR (or METAR COR)
CCCC
YYGGggZ
( AUTO )
dddffGfmfmKT
( dndndnVdxdxdx )
Prevailing visibility
MNM visibility/direction
Runway visual range
Present
weather
VVVV
( VNVNVNVNDV )
( RDRDR/VRVRVRVR )
( w’w’ )
( or CAVOK )
Cloud
Air temperature and dew
point
QNH
Recent weather Wind shear
NsNsNs hshshs(CC)
( or NSC )
( or NCD )
( or VVhshshs )
T’T’ / Td’Td’
QPHPHPHPH
( REw’w’ )
Sea surface temperature
and sea state
Runway state
Trend
Remarks
( RDRDR/ERCReRERBRBR )
( BECMG … )
or
( TEMPO … )
or
( NOSIG)
( WTSTS/SS )
or
( WtSTS/HHSHSHS )
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( WS RDRDR )
( or WS ALL
RWY )
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4.4
Chapter 4: METAR structure and UK coding rules
The table above shows the full METAR coding as given in WMO
Document No. 306, Manual on Codes, Volume 1, Part A; FM15-X Ext.
The UK does not use all of the codes; variations are listed below.
Parentheses indicate groups to be included, as appropriate.
UK METAR coding variations
‘AUTO’ indicates that the report has been prepared by an automated observing
system, without any human input or supervision. Automatic observations shall
indicate the limitations of the observing equipment through the use of additional
codes, where applicable, as given in Paragraphs 4.158 to 4.171.
RVR tendencies and significant variations are not reported in the United Kingdom.
Wind shear groups (WS.........) are not reported in the United Kingdom.
Sea surface temperature and sea state are only reported from certain UK offshore
installations.
Not all UK METARs include a TREND forecast. A TREND forecast is a short period
forecast, predicting significant weather changes that are likely to occur at the
aerodrome in the two hours following the time of the meteorological observation. The
TREND forecast may be appended to the METAR either by the forecaster or by the
observer at aerodromes where procedures exist for obtaining the TREND message
from the meteorological forecasting office.
RMK’ indicates that an optional remarks section follows. The remarks section is not
used in the United Kingdom.
A correction to a METAR shall be indicated by the use of METAR COR before the
ICAO location indicator in the body of the METAR message. The time of the
observation shall not be changed (unless the time of the observation requires
correction).
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Chapter 4: METAR structure and UK coding rules
Surface wind
Surface wind coding requirements
4.5
General format: dddffGfmfmKTdndndnVdxdxdx
Where

ddd is the mean surface wind direction over the previous 10 minutes
(but see paragraphs 4.9 and 4.12)

ff is the mean surface wind speed over the previous 10 minutes (but
see paragraph 4.9)

fm is the maximum surface wind gust speed over the previous 10
minutes (but see paragraph 4.10)

dn and dx describe the variation in surface wind speed (in clockwise
order) over the previous 10 minutes is the surface wind speed (but
see paragraph 4.10)
4.6
Surface wind information for the METAR report should be taken from an
anemometer located on the aerodrome. The anemometer should be sited
in a suitable location to provide a representative measurement of
conditions over the whole of the runway where there is only one runway or
the whole runway complex where there are two or more. The
requirements of ATS providers for surface wind information described in
Chapter 5, Weather Reports to Air Traffic Services should be noted.
4.7
Where buildings or other obstructions disturb the airflow to an
anemometer in certain wind directions, exceptionally a second
anemometer may be used to provide surface wind information for the
METAR report. Procedures for the implementation of a change in the
anemometer used for the METAR should be documented.
4.8
The direction from which the surface wind is blowing shall be given in
degrees from true North and the speed shall be given in knots.
4.9
The surface wind direction and speed reported is the average taken over
a ten minute period immediately preceding the time of the observation.
The only exception is when a marked discontinuity occurs. This is defined
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Chapter 4: METAR structure and UK coding rules
as a change in the mean direction of 30 degrees or more with a mean
speed of 10 knots or more before or after the change, or an increase or
decrease in the wind speed of 10 knots or more, sustained for at least 2
minutes. In this case, the mean over this latter period shall be reported.
4.10
The maximum wind (gust) within the last 10 minutes (or since the marked
discontinuity) shall be reported only if it exceeds the mean speed by 10
knots or more.
4.11
Variations in wind direction shall be reported only when the total variation
in direction over the previous ten-minute period (or since the marked
discontinuity) is 60 degrees or more or but less than 180 degrees and the
average wind speed is greater than 3 knots. Variations are reported in
clockwise order (e.g. 290V090 or 170V250).
4.12
The mean wind direction shall not be included for variable winds when the
total variation in direction over the previous ten-minute period (or since the
marked discontinuity) is 60 degrees or more or but less than 180 degrees
and the wind speed is 3 knots or less; the wind in this case shall be
reported as variable.
4.13
The mean wind direction shall not be included for variable winds when the
total variation in direction over the previous ten-minute period (or since the
marked discontinuity) is 180 degrees or more or where it is not possible to
report a mean direction e.g. when a thunderstorm passes over the
aerodrome. The wind should be reported as variable and no reference
should be made to the two extreme directions between which the wind
has varied.
4.14
When the wind speed is less than 1 knot, this should be reported as calm.
4.15
It is recommended that averages of wind speed and variations in the wind
direction and speed should be generated by automatic equipment.
Range and increments
4.16
The surface wind direction mean and variations in direction shall be
rounded to the nearest 10 degrees in the METAR.
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Chapter 4: METAR structure and UK coding rules
4.17
Wind directions of 005, 015 degrees etc. should be rounded down.
4.18
Surface wind direction is reported between 010 and 360 degrees.
4.19
The surface wind mean speed and maximum speed shall be rounded to
the nearest knot in the METAR. Surface wind speed is reported between
01 and 99 knots. If the speed is 100 knots or more, the wind speed should
be encoded as “P99” (see example 7 below).
4.20
Calm is encoded as ‘00000KT’.
4.21
Variable is encoded ‘VRB’.
Examples of METAR surface wind coding
1.
02008KT
wind zero two zero degrees, 8 knots
2.
00000KT
wind calm
3.
VRB02KT
wind variable, 2 knots (the variation in direction over the previous tenminute period has been 60 degrees or more or but less than 180 degrees
and the wind speed is 3 knots or less)
4.
33022G34KT
wind three three zero degrees, 22 knots, max 34 knots
5.
16016KT 120V190
wind one six zero degrees, sixteen knots, varying between 120 degrees
and 190 degrees
6.
21015G28KT 180V270
wind two one zero degrees, 15 knots, max 28 knots varying between 180
degrees and 270 degrees
7.
27070GP99KT
wind two seven zero degrees, 70 knots, max 100 knots or more
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CAVOK
Coding requirements
4.22
General format: CAVOK
4.23
The visibility, RVR, weather and cloud groups are replaced by CAVOK
(Cloud And Visibility OK) when the following conditions exist
simultaneously:
1)
Prevailing visibility is 10 km or more.
2)
No minimum visibility is reported.
3)
No cloud below 5000 ft or below the highest Minimum Sector
Altitude, whichever is the greater.
4)
No towering cumulus or cumulonimbus clouds.
5)
No significant weather phenomena at or in the vicinity of the
aerodrome.
4.24
Example - CAVOK should be used when the meteorological visibility is 20
km, there is no weather and there is broken cloud (5-7 oktas) at 5000 ft.
Visibility – meteorological minimum and maximum
Visibility coding requirements
4.25
General format: VVVV VNVNVNVNDv
Where

VVVV = prevailing meteorological visibility

VNVNVNVN = minimum meteorological visibility

DV = one or two letters indicating one of the eight points of the
compass that best describes the direction of the meteorological
visibility, relative to the aerodrome meteorological observer’s station.
4.26
Prevailing visibility is defined as “the greatest visibility value that is
reached within at least half the horizon circle or within at least half of the
surface of the aerodrome. These areas could comprise contiguous or noncontiguous sectors.”
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4.27
Chapter 4: METAR structure and UK coding rules
In the METAR, the visibility reported is the prevailing visibility and, under
certain circumstances, the minimum visibility. In order to determine the
prevailing visibility and any requirement to report the minimum visibility,
the variation of visibility in all directions around the aerodrome should be
considered.
4.28
The visibility reported in the METAR should be assessed at a height of
about 1.5 m above the ground at the observing site. Observers should be
aware of possible errors generated by reporting ‘slant’ visibility when
meteorological visibility is assessed at heights greater than 1.5 m above
the ground.
4.29
If the visibility in one direction which is not the prevailing visibility, is less
than 1500 m or less than 50% of the prevailing visibility, the lowest
visibility observed should be reported after the prevailing visibility and its
general direction in relation to the aerodrome indicated by reference to
one of the eight points of the compass. If the lowest visibility is observed
in more than one direction, then the most operationally significant
direction should be reported. When the visibility is fluctuating rapidly and
the prevailing visibility cannot be determined, only the lowest visibility
should be reported, with no indication of direction.
4.30
There is no requirement to report the lowest visibility if it is 10 km or more.
4.31
When conditions exist such that ‘CAVOK’ (defined in paragraph 4)
applies, visibility shall be omitted from reports, and ‘CAVOK’ shall be
reported instead. The applicability of CAVOK shall be based on the
minimum visibility, if reported in the METAR, otherwise on the prevailing
visibility.
Range and increments
4.32
VVVV shall always appear in the METAR as four figures, expressed in
metres when the visibility is less than 10 km (coded as ‘9999’ when the
visibility is 10 km or more).
4.33
Visibility shall be recorded in metres rounded down to:
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Chapter 4: METAR structure and UK coding rules
1)
the nearest 50 m when the visibility is less than 800 m;
2)
the nearest 100 m when the visibility is 800 m or more but less than
5000 m;
3)
the nearest 1000 m when the visibility is 5000 m or more but less
than 10 km.
4.34
Any observed value which does not fit the reporting scale in use shall be
rounded down to the nearest lower step in the scale.
4.35
A visibility of less than 50 m shall be encoded as ‘0000’;
4.36
A visibility of 50 m shall be encoded ‘0050’;
4.37
A visibility of 10 km or more shall be encoded ‘9999’ (unless CAVOK
applies).
Examples of METAR visibility coding
1.
02008KT
wind zero two zero degrees, 8 knots
2.
00000KT
wind calm
3.
VRB02KT
wind variable, 2 knots (the variation in direction over the previous tenminute period has been 60 degrees or more or but less than 180 degrees
and the wind speed is 3 knots or less
4.
33022G34KT
wind three three zero degrees, 22 knots, max 34 knots
5.
16016KT 120V190
wind one six zero degrees, sixteen knots, varying between 120 degrees
and 190 degrees
6.
21015G28KT 180V270
wind two one zero degrees, 15 knots, max 28 knots varying between 180
degrees and 270 degrees
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7.
Chapter 4: METAR structure and UK coding rules
27070GP99KT
wind two seven zero degrees, 70 knots, max 100 knots or more
Runway visual range
Runway visual range coding requirements
4.38
General format: RDRDR/VRVRVRVR
Where

R is the group identifier.

DRDR is the runway designator of the threshold nearest to which the
RVR is measured, followed (if necessary) by L, C or R to distinguish
left, centre and right parallel runways respectively.

4.39
VRVRVRVR is the RVR value reported in metres.
Runway Visual Range (RVR) is a measure of the horizontal visibility along
the runway. RVR assessments are made either by human observation
(HORVR) or an Instrument RVR (IRVR).
4.40
HORVR is only reported for the touchdown zone. IRVR systems may
have sensors located at the touchdown zone, mid-point and stop end of
each runway. However, for METAR purposes, only the touchdown zone
measurement is given. If the touchdown zone value is not available then
the RVR group for that runway shall be omitted.
4.41
Where there is more than one runway available for landing, the
touchdown zone RVR should be included for all such runways (up to a
maximum of four).
4.42
When IRVR systems are used an RVR should not be computed for a light
intensity of 3 per cent or less of the maximum light intensity available on a
runway. For METAR reports the RVR should be based on the maximum
light intensity available on the runway.
4.43
The RVR group shall be reported in the METAR only when either the
minimum visibility or the runway visual range is observed to be less than
1500 m.
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NOTE:
Chapter 4: METAR structure and UK coding rules
Runway visual range reports may be provided locally in other circumstances to
support operational requirements.
4.44
VRVRVRVR shall be inserted in the METAR after a solidus (/) and always
as four figures.
4.45
50 m should be regarded as the minimum value that is possible to report.
When the actual RVR is less than this, the minimum value shall be
preceded by ‘M’.
4.46
2000 m should be regarded as the maximum value that it is possible to
report. If the actual value is greater than can be reported by the runway
visual range equipment or human observed method, the maximum value
shall be preceded by ‘P’.
NOTE:
The maximum value of runway visual range that can be reported for a particular
runway may be significantly less than 2000 m due to equipment limitations or
topography.
4.47
IRVR systems shall be sampled at a minimum of once per minute. An
averaging period of 1 minute should be used for weather reports to ATS.
An averaging period of 10 minutes for METAR reports should be used,
however where a marked discontinuity occurs only those values after the
discontinuity should be used for obtaining mean values.
NOTE:
A marked discontinuity occurs when there is an abrupt and sustained change in
runway visual range, lasting at least 2 minutes and which reaches or passes
through the criteria for the issuance of special reports to ATS as detailed in
Chapter 5 Para 5.38 and 5.39 Runway Visual Range.
Range and increments
4.48
VRVRVRVR shall always appear in the METAR as four figures expressed in
metres.
4.49
RVR is reported in metres, rounded down to:
1)
the nearest 25 m for RVR below 400 m;
2)
the nearest 50 m for RVR between 400 m and 800 m;
3)
the nearest 100 m for RVR above 800 m.
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4.50
Chapter 4: METAR structure and UK coding rules
Any observed value which does not fit the reporting scale in use shall be
rounded down to the nearest lower step in the scale.
Examples of METAR RVR coding
1.
R04/0075 RVR
for runway 04 is 75 m
2.
R27L/0650 RVR
for runway 27 left is 650 m
3.
R16R/1100 RVR
for runway 16 right is 1100 m
4.
R33/M0050
RVR for runway 33 is less than 50 m (the minimum value that is possible
to report)
5.
R24L/P1500 R24R/1100 RVR
for runway 24 left is greater than 1500 m (the maximum value that is
possible to report), whilst the RVR for runway 24 right is 1100 m
Present weather
Present weather coding requirements
4.51
General format: w’w’
Where

4.52
w’w’ is the present weather
Entries of present weather shall only relate to weather occurring at the
time of observation on or over the aerodrome, or, in the case of reporting
phenomena in the vicinity of the aerodrome, within approximately 8 km of
the Aerodrome Reference Point. Visual reference points, information from
radar, pilot reports and automatic sensors may be used to assist in this
determination.
4.53
If there is no weather of significance to aeronautical operations at the time
of the observation, either on or in the vicinity of the aerodrome, the group
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Chapter 4: METAR structure and UK coding rules
is omitted. The abbreviation ‘NSW’ (no significant weather) is not used in
the METAR.
4.54
Present weather groups consist of one or more pairs of letter
abbreviations, selected from Table 1. A group shall be constructed by
considering the need to specify intensity or proximity, a descriptor, and
finally the phenomena, in that order.
4.55
A group may contain up to nine characters, and up to three groups may
be inserted to report simultaneously occurring independent phenomena.
Where two different types of weather are observed, they should be
reported in two separate groups. However, different types of precipitation
occurring at the time of the observation should be reported as one single
group with the dominant type of precipitation reported first and preceded
by only one intensity qualifier, which refers to the intensity of the total
precipitation.
4.56
No definitions have been agreed internationally for intensities ‘light’,
‘moderate’, ‘heavy’ or ‘well developed’. However, guidance on the
assessment of intensity, incorporated into an explanation of the
descriptors and the weather phenomena significant to aviation, is provided
in paragraphs 4.71 to 4.98 below.
4.57
The intensity of weather phenomena shall only be reported if it relates to
precipitation (including that qualified by freezing, shower or thunderstorm)
and blowing snow. The intensity is indicated by the appropriate symbol
given in Table 1.
4.58
The symbol for heavy (‘+’) may also be used to qualify a ‘well developed’
funnel cloud, water spout or dust/sand whirls.
4.59
Only certain phenomena are reported if they occur within approximately 8
km of the Aerodrome Reference Point. These are blowing dust, sand and
snow, funnel cloud, waterspout, dust/sand whirls, fog, shower and
thunderstorm. In all cases the intensity of the phenomenon is not
reported. Also, in the case of shower in the vicinity and thunderstorm in
the vicinity, the precipitation type is not specified.
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4.60
Chapter 4: METAR structure and UK coding rules
The following restrictions apply to the use of the descriptors:
1)
No more than one descriptor shall be included in a w’w’ group.
2)
The following descriptors shall only be used in combination with fog:
shallow (less than 2 m above ground level), patches (fog patches
randomly covering the aerodrome) and partial (a substantial part of
the aerodrome covered by fog while the remainder is clear).
3)
The following descriptors shall only be used in combination with
dust, sand and snow: low drifting if raised by the wind to a height
less than 2 m above ground level and blowing if raised by the wind to
a height of 2 m or more above ground level.
4)
The freezing descriptor shall be used only in combination with fog,
drizzle and rain.
5)
The thunderstorm descriptor may be inserted alone if thunder is
heard with no precipitation.
6)
The descriptors for thunderstorm and shower shall be used only in
combination with rain, snow, small hail/snow pellets and hail.
4.61
The present weather codes for fog patches, fog covering a partial part of
the aerodrome and fog in the vicinity of the aerodrome may be reported
whatever the meteorological visibility reported.
4.62
The present weather code for fog shall be used only when the prevailing
visibility is less than 1000 m in all directions from the observer’s position.
If this is not the case then the observer should consider the use of one of
the descriptors given in paragraph 4.61.
4.63
The present weather code for freezing fog shall be used only when the
prevailing visibility is less than 1000 m in all directions from the observer’s
position and the air temperature is less than 0°C.
4.64
The present weather code for shallow fog and mist shall be used only
when the prevailing visibility reported is 1000 m or more. Observers
should ensure that the reduction in visibility is due to water droplets
(relative humidity should be at least 95%) and not due to smoke or haze.
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4.65
Chapter 4: METAR structure and UK coding rules
The present weather codes for mist, dust, smoke and haze shall be used
only when the prevailing visibility is 5000 m or less.
4.66
There are two present weather codes for hail. Hail is reported if
transparent, translucent or opaque balls or pieces of ice (hailstones) are
observed. Hailstones have a diameter of 5 mms or more and fall from
deep cumuliform cloud. When they fall on hard ground, they bounce with
an audible sound. Small hail and/or snow pellets on the other hand have a
diameter of less than 5 mms and take the form of translucent ice particles,
or snow pellets encased in ice. The largest stones shall determine which
element is reported and it shall be used only in combination with shower
or thunderstorm.
4.67
If a mixture of precipitation types is falling at the time of the observation,
they shall be encoded and combined into a single group in order of
dominance, prefixed by intensity (which refers to the intensity of the total
precipitation) and/or shower or thunderstorm as appropriate, e.g. ‘-DZRA’,
‘RADZ’, ‘-SHRASN’, ‘+TSSNRAGS’.
4.68
When more than one independent phenomenon is occurring
simultaneously, separate groups shall be inserted (up to a maximum of
three groups), in the order given by the columns, that is: precipitation first,
then ‘obscuration’, and finally ‘other’ (e.g. ‘+SHGS BLSN SQ’). However, if
a thunderstorm is reported, then this shall take precedence over any
precipitation in the coding of present weather groups in the METAR.
4.69
When snow falling from cloud and blowing snow are observed
simultaneously, both shall be included in the present weather report; e.g.
‘SN BLSN’. However, if due to heavy blowing snow it cannot be
ascertained that snow is falling from cloud, only ‘+BLSN’ shall be reported.
4.70
Thunderstorm, thunderstorm in the vicinity and thunderstorm with
precipitation, shall be reported as ‘present weather’ if thunder is heard
within ten minutes preceding the observation. If lightning is seen but
thunder not heard, then it is likely that the thunderstorm is not within 8 km
of the Aerodrome Reference Point and should not be reported in the
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Chapter 4: METAR structure and UK coding rules
METAR. Observers based in buildings that contain sound-reduction
materials should try to establish whether thunder is heard or if the
thunderstorm is within 8 km of the Aerodrome Reference Point by any
appropriate means.
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Chapter 4: METAR structure and UK coding rules
Table 1: Present weather abbreviations for use in the METAR
Qualifier
Intensity or proximity
Descriptor
Precipitation
Obscuration
Other
1
2
3
4
5
-
MI
shallow
DZ
drizzle
BR
mist
PO
dust/sand whirls
BC
patches
RA
rain
FG
fog
SQ
squalls
PR
partial
SN
snow
FU
smoke
FC
funnel cloud or
water spout
DR
low drifting
SG
snow grains
VA
volcanic ash
DS
dust storm
BL
blowing
PL
ice pellets
DU
dust
(widespread)
SS
sandstorm
SH
showers
GR
hail
SA
sand
TS
thunderstorm*
GS
small hail and/or
snow pellet
HZ
haze
FZ
freezing
Light
(no symbol)
Moderate
Heavy or
well
developed
+
in the
vicinity
VC
*
Weather phenomena
Although TS is categorised as a descriptor it can also be used as a weather phenomenon on its own or combined with the qualifier
VC. TS takes precedence in the METAR coding of present weather over any precipitation.
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A guide to the descriptors and weather phenomena
Blowing (BL)
4.71
A descriptor used to indicate that dust, sand or snow has been raised by
the wind to a height of 2 m or more.
Drizzle (DZ)
4.72
A fairly uniform precipitation of very fine drops of water with a diameter
less than 0.5 mms, falling from thin stratiform cloud. The impact of drizzle
droplets falling on a water surface is imperceptible, but continuous drizzle
may produce run-off from roofs and runway surfaces. Visibility is inversely
related to both the intensity of the precipitation and the number of
droplets. Also generally, the heavier the drizzle, the lower the cloud base
will be. Light drizzle corresponds to negligible run-off from roofs, whilst
moderate drizzle, typically associated with visibility less than 3000 m, will
produce some run-off. Heavy drizzle corresponds to a rate of
accumulation greater than 1 mm per hour.
Dust (DU)
4.73
A widespread suspension of small particles of dust raised from the
ground, reported when it reduces the prevailing visibility to 5000 m or less.
Rarely reported in the UK.
Dust devil (PO)
4.74
A near vertical, rapidly rotating column of air, forming a dust and/or sand
whirl as a result of very strong local convection over hot, dry ground; lifting
dust, sand, grass cuttings and other light materials picked up from the
surface. Dust devils are usually only a few metres in diameter and extend
no higher than 200-300 ft. They can be qualified by ‘well developed’
(‘+PO’). Rarely reported in the UK.
Fog (FG)
4.75
A suspension in the air of very small water droplets (or at very low
temperatures, minute ice crystals), the prevailing visibility within which is
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less than 1000 m. Relative humidity will be at or near 100% (viz. dry bulb
and dew point temperatures equal or very close to each other in value).
Freezing (FZ)
4.76
A descriptor used when it is necessary to indicate that the water droplets
in fog, drizzle or rain are in a supercooled state (viz. liquid at a
temperature below 0.0°C). On impact with the ground or an airframe,
supercooled precipitation usually forms glaze (clear ice); usually, it falls
from stratiform cloud. Supercooled fog may deposit rime (scales or grains
of ice), but rarely clear ice.
Funnel cloud (FC)
4.77
This is a rotating column of air, often a violent whirlwind, indicated by the
presence of an inverted cone-shaped cloud, extending downwards from
the base of a cumulonimbus cloud, but not necessarily reaching the
surface. The diameter can vary from a few metres to some hundreds of m.
A funnel cloud that touches the surface is described as ‘well developed’,
and known as a tornado if over ground, and a waterspout if over water.
Rare but funnel clouds are reported on average on 14 days per year in the
UK.
Hail (GR)
4.78
Transparent, translucent or opaque balls or pieces of ice (hailstones) with
a diameter of 5 mms or more, falling from deep cumuliform cloud with
vigorous updraughts within. When falling on hard ground, it bounces with
an audible sound.
Small hail (GS)
4.79
As ‘hail’, but with a diameter less than 5 mms, in the form of translucent
ice particles, or snow pellets encased in ice.
Haze (HZ)
4.80
Haze is caused by a widespread amount of extremely small dry particles
in the air, invisible to the naked eye, but sufficiently numerous to give the
air a pale, whitish or opalescent appearance. It shall be reported when it
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reduces the prevailing visibility to 5000 m or less. In this case, the relative
humidity will be less than 95% (and, as a guide, the difference between
dry bulb and dew point temperatures is usually greater than 1°C).
Ice pellets (PL)
4.81
Frozen raindrops or re-frozen melted snowflakes falling from deep
stratiform cloud, in the form of transparent or translucent ice particles,
usually having a diameter of 5 mms or less, which bounce with an audible
sound on impact and are not easily crushed. Ice pellets indicate that
supercooled precipitation may be present at higher levels, and may occur
before or after freezing rain.
Low drifting (DR)
4.82
A descriptor used to indicate that dust, sand or snow has been raised by
the wind to a height of less than 2 m.
Mist (BR)
4.83
A suspension of microscopic water droplets or wet hygroscopic particles
in the air. It should be reported when it reduces the prevailing visibility to
between 1000m and 5000m or less. In this case, the relative humidity will
be 95% or more (and, as a guide, the difference between dry bulb and
dew point temperatures is usually 1°C or less).
Partial (PR)
4.84
A descriptor used only in combination with ‘FG’ to indicate that fog, 2 m or
more deep, covers a substantial part of the aerodrome, while the
remainder is clear (e.g. a bank some hundreds of metres across). The
meteorological visibility reported will depend on the proximity of the edge
of the fog bank to the observer.
Patches (BC)
4.85
A descriptor used only in combination with ‘FG’ to indicate that fog, 2 m or
more deep, is present on the aerodrome in irregularly distributed patches.
The meteorological visibility reported will depend on the proximity of the
nearest fog patch to the observer.
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Rain (RA)
4.86
A precipitation of liquid water droplets with a diameter of 0.5 mms or
more, falling from cloud. Diameter and concentration of the drops may
vary considerably according to the intensity of the precipitation, its nature
and source (viz. light, moderate or heavy; intermittent or continuous). If
the precipitation falls from cumiliform cloud, then it is further described as
a shower (see paragraph 7.2.20). There is no internationally agreed scale
for intensity, but the following, based on rate of fall, is used in the United
Kingdom:
From stratiform cloud
Light: up to 0.5 mm/hr; Moderate: >0.5 to 4 mm/hr; Heavy: over 4 mm/hr.
From cumuliform cloud
Light: up to 2 mm/hr; Moderate: >2 to 10 mm/hr; Heavy: over 10 mm/hr.
NOTE:
As a guide to observers, light rain is precipitation of low intensity; it may consist
of scattered large drops or more numerous smaller drops. The rate of
accumulation on the ground is such that puddles form only slowly, if at all.
Moderate rain falls fast enough to form puddles rapidly, to make down-pipes flow
freely, and to give some spray on hard surfaces. Heavy rain is a downpour,
which makes a roaring noise on roofs, forms a misty spray on road surfaces etc.
In the UK, light rain makes up on average 80% of the total rainfall, moderate
15% and heavy only 5%.
Sand (SA)
4.87
In the United Kingdom, this typically would be used only in combination
with ‘BL’ or ‘DR’, to report particles of sand lifted off the ground by the
wind.
Shallow (MI)
4.88
A descriptor used only in combination with ‘FG’ when ground fog, whether
patchy or a continuous layer, is less than 2 m deep, and hence the
reported meteorological visibility will be 1000 m or more. Operationally
shallow fog may cause problems by obscuring runway markings and
lights.
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Shower (SH)
4.89
A descriptor used to identify precipitation (rain, snow, hail, snow pellets)
falling from cumuliform clouds. Showers are often short-lived, and can be
characterised by their sudden beginning and ending, and by large and
rapid changes in precipitation intensity.
Small hail (GS)
See ‘Hail’.
Smoke (FU)
4.90
A suspension in the air of small particles produced by combustion,
frequently imparting a grey or blue hue to the atmosphere. It should be
reported when it reduces the prevailing visibility to 5000 m or less; it may
even be reported when the prevailing visibility is less than 1000 m if there
are no suspended water droplets, and the relative humidity is not higher
than 90%.
Snow (SN)
4.91
A solid precipitation of aggregated ice crystals falling from cloud as
snowflakes. The shape, size and concentration of snowflakes vary
considerably according to the temperature at which they form and the
conditions in which they develop. At very low temperatures, snowflakes
are small and often show a six-rayed starlike structure. Near to freezing
point, they are agglomerated, and may have a diameter greater than 25
mms.
4.92
In the absence of an internationally agreed scale, intensity (from both
stratiform and cumiliform clouds) is assessed from the rate of
accumulation:
Light: up to 0.5 cm/hr
Moderate: more than 0.5 to 4 cm/hr
Heavy: over 4 cm/hr
4.93
As a guide to observers, snow can be regarded as light when the
snowflakes are sparse and usually small. Snow is moderate when the
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snowfall consists of usually large flakes falling sufficiently thickly to impair
visibility substantially. Heavy snow causes visibility to be reduced to a
very low value, often ‘white-out’ conditions.
Snow grains (SG)
4.94
Very small opaque white particles of ice, fairly flat or elongated with a
diameter generally less than 1 mm, that fall from a layer cloud – the frozen
equivalent of drizzle.
Snow pellets (GS)
4.95
White, opaque, approximately round ice particles falling from cumuliform
cloud, often together with snow, at temperatures near 0.0oC, and usually
having a diameter of 2 to 5 mms, resembling tiny compacted snow balls.
They rebound when falling on a hard surface, and are crisp and easily
crushed. Snow pellets result from hail being blown out of the side or top of
cumulonimbus before the manufacturing process is complete (see also
‘Small hail’).
Squall (SQ)
4.96
A strong wind that rises suddenly: that is by at least 16 knots, increasing
to 22 knots or more, and sustained for at least one minute, then dying
away quickly; distinguished from a gust by its longer duration. A squall is
associated with violent convective activity and the passage of active cold
fronts. In the latter case, typically squalls occur along the line of the front,
accompanied by a veer in wind, a sharp fall in temperature, a rise in
relative humidity and the appearance of a roll-shaped cloud with a
horizontal axis.
Thunderstorm (TS)
4.97
One or more sudden electrical discharges, manifested by a flash of light
(lightning) and a sharp crack or rumbling sound (thunder). Thunderstorms
are associated with cumulonimbus clouds. When thunder is heard with no
precipitation falling on the aerodrome, ‘TS’ alone is reported in the
METAR, or ‘VCTS’ if the source is estimated to be within 8 km of the
Aerodrome Reference Point. To report a thunderstorm with precipitation at
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the aerodrome, ‘TS’ is used as a descriptor, and combined with the
appropriate precipitation abbreviations and prefixed (where necessary) by
an intensity symbol based on the rate of fall of the precipitation.
Tornado
See ‘Funnel cloud’.
Volcanic ash (VA)
4.98
A suspension or precipitation of dust or solid particles in the atmosphere,
known to be originating from an active volcano.
Waterspout (FC)
See ‘Funnel cloud’.
Summary of present weather codes
Table 2: Present weather codes
Present weather reported
Present weather reported
Present weather that may
with intensity*
without intensity
be reported to be within 8
km of the aerodrome
reference point
- or +
VC
DZ or RA or SN or SG or
FG or BR or SA or DU or
FG or PO or FC or DS or
PL or DS or SS or TS or
HZ or FU or VA or SQ or
SS or TS or SH or BLSN or
TSRA or TSSN or TSPL or
FZFG or DRSN or DRSA or
BLSA or BLDU
TSGR or TSGS or SHRA or
DRDU or MIFG or BCFG or
SHSN or SHGR or SHGS or
PRFG
FZRA or FZDZ or BLSN or
BLSA or BLDU
PO or FC
*
NOTE 1:
No qualifier used for moderate intensity.
NOTE 2:
Funnel cloud, water spout or dust/sand whirls only to be reported as PO, FC,
+PO or +FC.
NOTE 3:
A present weather entry may contain up to nine characters and up to three
groups may be used to report simultaneously occurring phenomena.
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Cloud
Coding requirements
4.99
General format: NsNsNshshshs(CC)
or NSC or NCD
or VVhshshs
Where
NsNsNs is the cloud amount expressed as either FEW, SCT, BKN, OVC.
hshshs is the height of the cloud base in 100s of ft above aerodrome.
(CC) is the cloud type where applicable, only cumulonimbus (CB) and/or
towering cumulus (TCU) are coded in the METAR.
NSC represents no significant cloud i.e. no cloud below 5000 ft or below
the minimum sector altitude (whichever is the greater) and no towering
cumulus or cumulonimbus (TCU or CB).
NCD is not used by human observers. It represents no cloud discernible
and is ONLY reported by automatic observing systems to indicate that no
clouds are detected by the system.
VVhshshs is the vertical visibility height. VV/// indicates that vertical
visibility height is not measured, or sky obscured.
4.100
Normally up to three cloud layers may be reported (although in certain
circumstances more can be reported - see (d) below):
1)
the lowest layer, whatever the amount;
2)
the next layer above of amount 3 oktas or more (SCT, BKN or OVC);
3)
the next layer above of amount 5 oktas or more (BKN or OVC);
4)
insert any towering cumulus or cumulonimbus cloud omitted by the
above rules, whilst retaining base height order from lowest to
highest.
Range and increments
4.101
The cloud amount (NsNsNs) can be expressed as:
1)
FEW no more than quarter cover (1-2 oktas)
2)
SCT over ¼ but no more than ½ cover (3-4 oktas)
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Chapter 4: METAR structure and UK coding rules
3)
BKN over half but not complete cover (5-7 oktas)
4)
OVC complete cover (8 oktas)
The height of the cloud base (hshshs) is reported in hundreds of ft above
aerodrome level.
4.103
Where there is no cloud below 5000 ft or below the highest minimum
sector altitude (whichever is greater) and there is no towering cumulus or
cumulonimbus, ‘NSC’ (no significant cloud) is reported.
4.104
The cloud type is usually not specified. However, significant convective
cloud, indicated by ‘TCU’ (Towering Cumulus), defined as ‘cumulus with
strong vertical development’, and ‘CB’ (Cumulonimbus), shall be inserted
(without a space) after the height of the cloud base. The amount, height of
cloud base and cloud type of towering cumulus or cumulonimbus shall be
reported, irrespective of the cloud base height.
4.105
When concurrent TCU and CB cloud masses appear to have a common
base, the amount shall be encoded from the sum of the individual
amounts, and the type reported as cumulonimbus.
4.106
The height of the cloud base is rounded down to the nearest hundred ft up
to 10000 ft and to 1000 ft thereafter. A base height of less than 100 ft
above the aerodrome is encoded ‘000’.
4.107
When the term CAVOK applies (see paragraphs 4.22 to 4.24), ‘CAVOK’ is
reported in lieu of cloud information.
4.108
When the sky is obscured due to fog, falling or blowing snow, ‘VV///’ is
reported in lieu of cloud information. Note that in the UK, vertical visibility
is not assessed.
4.109
When ‘TS’ (thunderstorm) is reported as present weather, ‘CB’
(cumulonimbus) shall be reported.
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Examples of METAR cloud coding
1.
FEW010 SCT022 BKN045
1-2 oktas of cloud base 1000 ft above aerodrome level, 3-4 oktas of cloud
base 2200 ft above aerodrome level and 5-7 oktas of cloud base 4500 ft.
2.
FEW008 BKN013TCU
1-2 oktas of cloud base 800 ft above aerodrome level, 5-7 oktas of
towering cumulus cloud base 1300 ft above aerodrome level.
3.
SCT055CB
3-4 oktas of cumulonimbus cloud base 5500 ft above aerodrome level.
4.
NSC
No cloud below 5000 ft and no towering cumulus or cumulus observed at
any level (but CAVOK conditions do not exist).
Air temperature and dew point
Coding requirements
4.110
General format: T’T’ / T’dTd
Where
T’T’ = the dry bulb temperature to the nearest whole ˚C.
T’d’T’d = the dew point temperature to the nearest whole ˚C.
4.111
Two temperatures are given in the METAR; the outside air temperature
and the dew point. The dew point is not measured directly from a
thermometer; however it can be calculated from the dry bulb and wet bulb
temperatures or the relative humidity.
4.112
When the wet-bulb is frozen, the ice-bulb temperature is used to compute
dew point.
Range and increments
4.113
The dry bulb and dew point temperatures are reported to the nearest
whole degree Celsius (°C). A two-digit figure for each temperature is
given, preceded by ‘M’ if a temperature is negative.
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Chapter 4: METAR structure and UK coding rules
When the temperature value is a 0.5 reading, the value is reported to the
warmer temperature. Therefore 3.5°C is rounded up to 4°C and minus
3.5°C is rounded up to minus 3°C.
4.115
A temperature or dew point between minus 0.5°C and minus 0.1°C is
encoded as ‘M00’, whilst a temperature or dew point between 0.0°C and
plus 0.4°C is encoded as ‘00’.
4.116
Exceptionally, if a dew point temperature is unavailable, it is replaced by
additional solidii (//).
Examples of METAR temperature and dew point coding
1.
The coding 05/M00 is valid for a dry-bulb reading of plus 4.5°C to plus
5.4°C and dew point between minus 0.5°C and minus 0.1°C.
2.
A dry-bulb temperature of plus 0.4°C and a dew point calculated to be
minus 3.7°C would be encoded 00/M04.
3.
A dry-bulb temperature of plus 6.5°C but the humidity sensor is
unserviceable would be encoded 07///.
Atmospheric pressure
Coding requirements
4.117
General format: QPHPHPHPH
Where
Q is the group identifier
PHPHPHPH is the atmospheric pressure, corrected to mean sea level
(QNH)
4.118
Atmospheric pressure is used by aircraft altimeters and for this reason it is
important to provide accurate pressure measurements.
4.119
The QNH is the atmospheric pressure corrected to mean sea level (based
on International Standard Atmosphere conditions throughout the height
difference) and is reported in the METAR rounded down to the nearest
whole hectopascal.
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Chapter 4: METAR structure and UK coding rules
The SI unit of pressure is the Pascal; for reference the millibar is
equivalent to 10000 Pascals or 1 hectopascal).
Range and increments
4.121
The pressure is inserted in the METAR rounded down to the nearest
whole hectopascal, prefixed by indicator letter ‘Q’.
4.122
Pressure settings, such as QNH and QFE, are normally integer values.
Pressure settings should be available to an accuracy of one tenth of a
hectopascal on request.
4.123
If the value is less than 1000 hectopascals, a zero shall be inserted
following ‘Q’.
4.124
Some military aerodromes in the UK report the QNH pressure in the
METAR in inches of mercury. In this case the pressure group is prefixed
by ‘A’, and the QNH expressed in hundredths of inches, viz. with the
decimal point omitted between the second and third figure; e.g. ‘A3027’
would be decoded as 30.27 inches of mercury.
Examples of METAR pressure coding
1.
Q0987
Pressure reduced to mean sea level is 987 hectopascals.
2.
Q1001
Pressure reduced to mean sea level is 1001 hectopascals.
3.
Q0999
Pressure reduced to mean sea level is 999 hectopascals.
4.
Q1023
Pressure reduced to mean sea level is 1023 hectopascals.
Recent weather
Coding requirements
4.125
General format: REW’W’
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Where
w’w’ is the abbreviation for recent weather groups; always preceded by
RE.
4.126
Recent weather is defined as the weather that has ceased or decreased
in intensity since the last routine report or within the last hour, whichever
is the shorter. In the METAR, it is reported after the QNH.
4.127
Recent weather should be encoded and inserted in the METAR if, during
the period since the last routine report or in the last hour (whichever is the
shorter), one or more of the phenomena listed below was observed at the
station and has since ceased or is still on-going, but decreased in
intensity.
1)
Freezing precipitation of any intensity.
2)
Moderate or heavy: drizzle, rain, snow, ice pellets, snow pellets, hail
(any diameter), blowing snow.
3)
Funnel cloud (tornado, waterspout); volcanic ash.
4)
Thunderstorm (that occurred with or without precipitation) - however
the recent thunderstorm is reported in one group. Any associated
precipitation is reported in separate groups following rules a) and b)
above.
4.128
Up to three recent weather groups may be reported. Each group is formed
using the appropriate present weather two-letter abbreviations prefixed by
the indicator letters ‘RE’. The full list of permitted recent weather codes
are detailed in the ‘Summary of recent weather codes’ table - see below.
4.129
Not all recent weather is regarded as operationally significant.
4.130
A recent weather group is not inserted if the same phenomenon of a
similar or greater intensity is reported as present weather.
4.131
Intensity qualifiers are not used when reporting recent weather.
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Examples of METAR recent weather coding
1.
If light drizzle, rain, snow, ice pellets, small hail and/or snow pellets, hail or
blowing snow, either individually or in combination, have ceased since the
last report, no recent weather group is inserted.
2.
If a moderate snow pellet shower occurred since the last report, but then
became a heavy shower of snow and rain:

Present weather reported is +SHSNRA and recent weather is
RESHGS.
3.
If snow is currently moderate but has been occasionally heavy since the
last report:

4.
Present weather reported is SN and recent weather is also RESN.
If moderate rain has ceased since the last report, but a moderate rain
shower is falling at the time of the observation:

5.
Present weather reported is SHRA and recent weather is RERA.
If a thunderstorm with moderate or heavy rain has occurred since the last
report, but both have ceased:

No present weather is reported and recent weather is RETSRA.
Summary of recent weather codes
Thunderstorm
RETS
Thunderstorm and moderate or heavy rain
RETSRA
Thunderstorm and moderate or heavy snow
RETSSN
Thunderstorm and moderate or heavy hail
RETSGR
Thunderstorm and moderate or heavy snow pellets
RETSGS
Freezing rain
REFZRA
Freezing drizzle
REFZDZ
Moderate or heavy rain
RERA
Moderate or heavy snow
RESN
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Moderate or heavy drizzle
REDZ
Moderate or heavy ice pellets
REPL
Moderate or heavy snow grains
RESG
Moderate or heavy showers of rain
RESHRA
Moderate or heavy showers of snow
RESHSN
Moderate or heavy shower of small hail / snow pellets
RESHGS
Moderate or heavy showers of hail
RESHGR
Moderate or heavy blowing snow
REBLSN
Unidentified precipitation
REUP
Moderate or heavy shower of unidentified precipitation
RESHUP
Unidentified frozen precipitation
REFZUP
Thunderstorm with unidentified precipitation
RETSUP
Sandstorm
RESS
Dust storm
REDS
Funnel cloud
REFC
Volcanic ash
REVA
Runway states
Coding requirements
4.132
General format: RDRDR/ERCReReRBRBR
Where
RDRDR is the runway designator (RDRDR(L), RDRDR(C), RDRDR(R)),
where DRDR is the runway direction in use and the L,C,R refer to left,
centre and right respectively. After the runway designator a slash (/)
should be inserted.
ER is the runway deposits designator.
CR is the extent of runway contamination.
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eReR is the depth of deposit.
BRBR is the friction coefficient/ braking action.
4.133
If a runway inspection is not possible, it is preferable to report changes by
general assessment rather than let a clearly unrepresentative message be
repeated, for example, when:
4.134
1)
snow is judged to be turning to slush; or
2)
it is apparent that snow depth has increased/decreased.
Appropriate local arrangements should be in place to ensure that a
runway state message is available to the observer in good time for
insertion into the next METAR.
4.135
The use of the braking action / friction coefficient coding 99 which
indicates figures unreliable should not be used to indicate a missing value.
4.136
Within the UK, friction co-efficient measurements are only made on
runways contaminated by ice (gritted or un-gritted) and compacted snow.
Where contamination is caused by water, slush or wet snow then the
friction coefficient or braking action should be reported as //.
4.137
The use of the coding 99 which indicates figures unreliable should not be
used to indicate a missing value.
4.138
A full decode of the runway state group is reproduced in Table 3.
4.139
The depth of the deposit reported should be the average of the readings
for the usable length of the runway.
4.140
Should the extent of contamination be provided as values for the
Touchdown, Midpoint and End, the extent of contamination should be
reported as the worst value. i.e. if report indicates Touchdown 10%,
Midpoint 10%, End 20% then the Runway State would be coded as 2 =
11%- 25%.
4.141
If a runway is being cleared of ice, snow, slush, etc. and a Runway State
cannot be provided, as the runway(s) is non-operational due to runway
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clearance in progress the Runway State shall be reported as follows:
RDRDR///99//. E.g. R14///99//.
4.142
If contamination conditions on a single runway or on all runways at an
aerodrome have ceased to exist, the following should be reported as
follows: RDRDR/CLRD//. E.g R14/CLRD//. At airports that operate more
than one runway the code R88/CLRD// should be issued once the
contamination has been cleared from all runways.
4.143
SNOCLO shall be reported at all times when the aerodrome is closed due
to contaminated runway(s). It is coded as R/SNOCLO.
4.144
As it is impractical for the Runway State to be updated when an AUTO
METAR is being provided, no Runway State should be appended to an
AUTO METAR.
Examples of runway state coding for METAR
1.
R14/4215//
Runway 14 has 11%-25% contamination of 15 mms of dry snow, no
braking action is provided.
2.
R99/6101//
This is a repetition of the last message because no new information has
been provided. The runway has 10% or less of slush to a depth of 1 mm,
no braking action is provided.
3.
R27/759291
Runway 27 is contaminated by 10 centimetres of ice, covering 26%-50%
of the runway. A runway braking action of poor has been issued.
4.
R14///99//
Runway 14 is non operational due to runway clearance in progress.
5.
R/SNOCLO
The aerodrome is closed due to contamination of the runways.
6.
R14/CLRD//
Contamination has ceased to exist on Runway 14.
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Table 3: Runway state group coding for use in the METAR
RDRDR
Runway designator
R22/
Runway R22
R27R/
Runway 27 right
R88/
All runways
R99/
A repetition of the last message because no new information received
ER
Runway deposits
0=
Clear & dry
1=
Damp
2=
Wet or water patches
3=
Rime or frost covered (depth normally less than 1 mm)
4=
Dry snow
5=
Wet snow
6=
Slush
7=
Ice
8=
Compacted or rolled snow
9=
Frozen ruts or ridges
/=
Type of deposit not reported (e.g. due to runway clearance in progress)
CR
Extent of contamination
1=
10% or less
2=
11% to 25%
5=
26% to 50%
9=
51% to 100%
/=
Not reported due to runway clearance in progress
erer
Depth of deposit
00 =
< 1 mm
01 =
1 mm etc.
thru’ to
90 =
90 mm
91 =
not used
92 =
10 cm
93 =
15 cm
94 =
20 cm
95 =
25 cm
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96 =
30 cm
97 =
35 cm
98 =
40 cm or more
99 =
Runways non operational due to snow, slush, ice, large drifts or runway
clearance but depth is not reported
// =
Depth of deposit operationally not significant or not measurable
BRBR
Friction coefficient / Braking action
28 =
Friction coefficient 28%
35 =
Friction coefficient 25%
or
91 =
Braking action poor
92 =
Braking action medium/poor
93 =
Braking action medium
94 =
Braking action medium/good
95 =
Braking action good
99 =
Figures unreliable
// =
Friction coefficient / Braking action not reported
SNOCLO
When the aerodrome is closed due to contamination of runway(s) it is
reported as R/SNOCLO
CLRD
If contamination ceases to exist, the code CLRD is used and reported as
RDRDR/CLRD// e.g. R14/CLRD//
TREND forecast
4.145
General format: NOSIG
or BECMG [change group]
or TEMPO [change group]
4.146
Time indicators and periods are used to give additional meaning to the
above change descriptors as described in Table 4:
Table 4: Time indicators and periods used with change descriptors
Change
Time indicator
indicator and period
Meaning
NOSIG
no significant changes are forecast
BECMG
FMn1n1n1n1
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the change is
commence at n1n1n1n1 and be completed
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TLn2n2n2n2
forecast to…
by n2n2n2n2
TLnnnn
commence at the beginning of the TREND
forecast period and be completed by nnnn
FMnnnn
commence at nnnn and be completed by
the end of the TREND forecast period
Atnnnn
occur at nnnn (specified time)
-
commence at the beginning of the TREND
forecast period and be completed by the
end of the TREND forecast period
FMn1n1n1n1
commence at n1n1n1n1 and cease by
n2n2n2n2
TLn2n2n2n2
TLnnnn
TEMPO
FMnnnn
temporary
fluctuations
are forecast
to…
-
commence at the beginning of the TREND
forecast period and cease by nnnn
commence at nnnn and cease by the end
of the TREND forecast period
continue throughout the TREND forecast
period
4.147
All times quoted are UTC.
4.148
Forecasts of changes in the following elements may be included in the
TREND forecast if change criteria (published in the UKAIP GEN section)
are expected to be crossed; wind, visibility, present weather and cloud.
Range and increments
4.149
Wind: All ranges given in paragraphs 4.16 to 4.21 may be used.
4.150
Prevailing visibility: Only one value (the prevailing visibility, as defined in
paragraph 4.26) will be forecast. All ranges given in paragraphs 4.32 to
4.37 may be used, including CAVOK if applicable.
4.151
Present weather: All codes given in the first two columns of Table 2
shown earlier in this chapter may be used, including changes to intensity
where applicable. Note that weather forecast to be in the vicinity of the
aerodrome is not included in the TREND forecast, only that which is
expected to affect the aerodrome. In addition, if certain present weather is
occurring in the observation and is expected to cease during the TREND
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forecast period, the letters ‘NSW’ (no significant weather) may be used in
the TREND forecast to indicate this.
4.152
Cloud: All ranges given in paragraphs 4.101 to 4.110 of this chapter may
be used.
4.153
TREND forecasts have certain expected tolerances of accuracy and
individual elements of the forecast may vary within these limits. Further
information can be found in the UK AIP (GEN 3.5.4).
Examples of METAR TREND forecast coding
1.
TEMPO 25035G50KT
Temporary fluctuations in the wind to 250 degrees True, ten minute mean
speed 35 knots with maximum speed (gusts) 50 knots.
2.
BECMG 0500 SN VV///
Conditions are expected to become 500 m in moderate snow and sky
obscured. No timescale is stated, so it must be expected that the change
will commence at the beginning of the TREND forecast period and be
completed by the end (i.e. within 2 hours)
3.
BECMG AT 1800 9000 NSW
Conditions are expected to become 9000 m and present weather
(reported in the METAR) will cease. This is expected to happen at 1800
UTC.
AUTO METAR
4.154
AUTO METARs are reports that have been prepared by an automated
observing system without any human input or supervision. The automatic
observing system must be located on the aerodrome concerned and it
shall routinely provide the following information.

wind speed and direction;

visibility;

cloud amount and height;

temperature and dewpoint;
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4.155
Chapter 4: METAR structure and UK coding rules

pressure; and

present weather.
When the aerodrome is operational the following additional information
shall also be provided automatically when required:
4.156

RVR;

Thunderstorm and Thunderstorm in the Vicinity;

Cumulonimbus clouds;

Towering cumulus clouds.
Aerodromes that provide AUTO METARs during the Met Observers
overnight duty break are not required to provide information on
thunderstorm and cloud type.
4.157
Information on thunderstorm, thunderstorm in the vicinity and cloud type
may be included using data processed from remote sensing
instrumentation.
4.158
Where the observation is generated by an automatic observing system
without any human input or supervision, the code ‘AUTO’ shall be inserted
between the date/time of the report group and the wind group.
Automated cloud reports
4.159
Where the observation is generated by an automatic observing system
which is not operating then the report will consist of 6 slashes (//////).
4.160
Where the observation is generated by an automatic observing system
and is unable to detect towering cumulus or cumulonimbus this shall be
indicated by three slashes (///) after each cloud group of cloud amount
and height.
4.161
Where the observation is generated by an automatic observing system
the letters NCD (No Clouds Detected) shall be used to indicate that the
observing system has determined that there are no clouds below 5000 ft
present.
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Chapter 4: METAR structure and UK coding rules
When towering cumulus cloud(s) are detected by automated means, but
no information on its height or cloud amount is available. Then the
following code shall be given after the cloud amount and height
information //////TCU.
4.163
When cumulonimbus cloud(s) are detected by automated means, but no
information on its height or cloud amount is available. Then the following
code shall be given after the cloud amount and height information //////CB.
Automated visibility reports
4.164
Where the observation is generated by an automatic observing system
which is not providing visibility information then the report will consist of 4
slashes (////).
Automated present weather reports
4.165
The following present weather phenomena and their intensity, as a
minimum, are required to be provided by automated systems:
4.166

rain;

drizzle;

snow;

rain and snow;

freezing rain;

freezing drizzle;

fog;

freezing fog;

mist;

haze.
Where the observation is generated by an automatic observing system
and showers cannot be determined based upon a method that takes
account of convective cloud, precipitation should not be characterised by
SH.
4.167
Where the observation is generated by an automatic observing system
and the present weather cannot be detected due to unserviceable present
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weather sensors, the lack of present weather information should be
indicated by two slashes (//). If the present weather sensor is serviceable
but not detecting any present weather then no present weather group
should be reported.
4.168
If the present weather sensor is unable to determine the state or form of
the precipitation ‘UP’ (unidentified precipitation) or ‘FZUP’ (freezing
unidentified precipitation), together with intensity qualifications, should be
reported as appropriate. Recent unidentified precipitation (‘REUP’,
‘RETSUP’ or ‘REFZUP’) shall be reported if moderate or heavy
unidentified precipitation has ceased or decreased in intensity since being
reported the last routine report or within the last hour, whichever is the
shorter.
Examples of AUTO METAR coding:
1.
METAR EGZZ 282220Z AUTO 29010KT 6000 FEW010/// BKN025///
17/12 Q0996=
2.
METAR EGZZ 290450Z AUTO VRB02KT 3000 BR NCD 10/09 Q1002=
3.
METAR EGZZ 301150Z AUTO 18025G35KT 8000 TSRA FEW010///
SCT028/// //////CB 18/15 Q1009=
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Chapter 5: Weather reports to air traffic services
Chapter 5
Weather reports to air traffic services
Introduction
5.1
Weather reports to Air Traffic Services are issued half-hourly
(exceptionally hourly) and are used by the ATS unit to provide weather
information to operators, aerodrome users and pilots at or in the vicinity of
the aerodrome. Whilst these reports are very similar to the METAR, there
are slight differences in content and coding; these are highlighted below.
Further information can also be found in CAP 493 Manual of Air Traffic
Services, Part 1.
5.2
During any period that weather reports are being provided to ATS, special
reports also shall be produced and made available to pilots, operators and
aerodrome users, if conditions warrant. Unless otherwise agreed by the
UK Meteorological Authority, the criteria for the production of a special
report shall be those given in paragraphs 5.29 to 5.44 of this chapter.
5.3
By agreement between the Aerodrome Meteorological Observation
Service Provider and ATS Provider, the format of weather observations
provided to ATS may be varied (e.g. the report may be provided in the
METAR code).
5.4
Dynamic meteorological information may be provided by ATS units to
aircraft for take-off and landing. Equipment used to provide dynamic
meteorological information to ATS is subject to requirements specified in
CAP 670 ATS Safety Requirements.
5.5
Weather reports to ATS may be provided from automated systems without
any human input or supervision when an aerodrome is non operational.
Permission from the CAA is required to enable the use of automated local
and special weather reports when used during operational hours of an
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Chapter 5: Weather reports to air traffic services
aerodrome. Where information is used from automated systems to
compile ATIS reports then this shall be indicated on the transmission.
Surface wind
5.6
The surface wind information provided should be representative of the
conditions along the runway. Since, in practice, the surface wind cannot
be measured directly on the runway, surface wind observations for takeoff and landing should be sited to give the best practicable indication of
conditions along the runway, e.g. lift-off and touchdown zones.
5.7
In reports to aircraft for take-off and landing, direction is to be expressed
in degrees Magnetic, and the reading is averaged over the previous 2
minutes; in addition, the extremes in direction and speed (gust and lull)
during the past 10 minutes shall be provided. The instantaneous surface
wind should be available to give to pilots on request. Further information
can be found in CAP 670 ATS Safety Requirements.
5.8
As well as 2-minute mean wind speeds, maximum (gust) and minimum
(lull) wind speeds shall be provided when the difference is 10 knots or
more from the 2-minute mean wind speed.
5.9
Variations in wind direction shall be reported when the total variation in
direction over the previous ten-minute period is 60 degrees or more.
Variations are reported in clockwise order (e.g. 290V090 or 170V250).
5.10
The mean wind direction shall not be included for variable winds when the
total variation in direction over the previous ten-minute period is 60
degrees or more or but less than 180 degrees and the wind speed is 3
knots or less; the wind in this case shall be reported as variable; however
the two extreme directions between which the wind has varied should be
included. When a wind speed of less than 1 knot is observed it should be
reported as calm.
5.11
The mean wind direction shall not be indicated for variable winds when
the total variation in direction over the previous ten-minute period is 180
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Chapter 5: Weather reports to air traffic services
degrees or more or where it is not possible to report a mean direction e.g.
when a thunderstorm passes over the aerodrome. The wind should be
reported as variable and no reference should be made to the two extreme
directions between which the wind has varied.
Visibility
5.12
In reports to Air Traffic Services, the prevailing visibility shall be reported.
If, however, the visibility in one direction, which is not the prevailing
visibility, is less than 1500 m or less than 50% of the prevailing visibility,
then additionally the minimum visibility observed shall be reported. If
CAVOK conditions exist, ‘CAVOK’ is still reported in lieu of a visibility
value.
5.13
The visibility shall be reported as stated in Chapter 4 paragraphs 4.32 to
4.37. However, when the visibility is 10 km or more it shall be given as 10
km, unless CAVOK applies.
5.14
Information on how to report prevailing visibility is given in Appendix B.
RVR
5.15
The report to ATS includes the RVR reading for the runway in use at the
time of the observation. This data may be displayed at the observer’s
workplace or may be provided by the ATS unit.
5.16
Equipment used to determine and display dynamic Instrumented Runway
Visual Range (IRVR) information to ATS is subject to requirements
specified in CAP 670 ATS Safety Requirements.
5.17
The procedure for the calibration of Human Observer RVR reference
tables is described in Appendix C, Human Observed RVR conversion
table.
5.18
Operational procedures for Human Observer RVR reporting are described
in CAP 168 Licensing of Aerodromes, CAP 493 Manual of Air Traffic
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Chapter 5: Weather reports to air traffic services
Services Part 1 and, where appropriate, in the ATC unit Manual of Air
Traffic Services Part 2.
Present weather
5.19
In reports to Air Traffic Services, up to three present weather codes, with
a maximum of nine characters, may be reported when simultaneously
occurring phenomena is observed. Additionally, the word ‘NIL’ is inserted
into the report if there is no weather significant to aviation. When CAVOK
conditions exist, ‘CAVOK’ is still reported in lieu of present weather.
Cloud
5.20
In reports to Air Traffic Services, cloud layers above 5000 ft shall be
omitted, but CB and TCU shall always be reported whatever the height of
the cloud base.
5.21
‘No significant cloud’ or ‘NSC’ shall be reported when no cloud below
5000 ft and no CB or TCU are present.
Temperature
5.22
In reports to Air Traffic Services, these temperatures should be prefixed
‘PS’ when the temperature is positive and ‘MS’ when negative.
Pressure
5.23
In reports to Air Traffic Services, aerodrome QNH, QFE / threshold QFE
and, if appropriate, threshold QFE(s) shall be reported.
5.24
At aerodromes that have an instrument runway with a threshold elevation
7ft or more below the aerodrome elevation, a threshold QFE will be
provided during periods that the runway is in use.
5.25
Correction tables can be generated for a particular datum using a formula
available from the UK Meteorological Authority on request.
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Chapter 5: Weather reports to air traffic services
Recent weather
This is not required for reports to Air Traffic Services.
Supplementary information
5.26
In reports to Air Traffic Services, extra data of use to pilots may be
reported in the form of ‘supplementary information’. This may include but
is not limited to:
1)
Marked variations in visibility.
2)
Aircraft reports (Air-reports) of severe turbulence and icing, but only
after notification to the meteorological forecast office.
3)
Fog in the vicinity, with a direction to indicate the location, and
patches or banks of fog with their location.
4)
Distance and direction of showers in the vicinity.
5)
Aircraft reports (Air-reports) of wind shear.
6)
Significant meteorological conditions, particularly those in the
approach and climb-out areas. These may include cumulonimbus
clouds, thunderstorms, hail, severe squall lines, freezing
precipitation, severe mountain waves and blowing snow.
5.27
Where directions of phenomena are given, one of the eight main compass
points shall be used.
5.28
Supplementary information should not be used for relaying forecasts. In
this context, ‘fog to the west’ may be used but ‘fog thickening’ may not.
5.29
Aircraft reports (Air-reports) of severe turbulence and icing are to be
notified to the meteorological forecast office. Unless otherwise advised by
the meteorological forecast office, or locally agreed between the ATS
Provider and the Aerodrome Meteorological Observation Service
Provider, the observer should then include this information as part of the
report to Air Traffic Services. If reported, the entry will be repeated on the
following observation, and thereafter removed unless otherwise advised of
the continuation of such conditions. The meteorological forecaster may
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Chapter 5: Weather reports to air traffic services
issue a ‘SIGMET’ warning as a result of Aircraft Reports (Air-reports) of
severe turbulence and icing.
Special reports for air traffic services purposes
5.30
Special reports to the ATS Provider shall be made if a deterioration or
improvement in a weather element passes through pre-determined criteria
but not at the time of the routine weather observation. At civil aerodromes,
special reports need only be sent to the ATS Provider.
5.31
Special reports to the ATS Provider shall use the following change criteria,
unless otherwise agreed with the UK Meteorological Authority:
Surface wind
5.32
Special reports for wind shall be issued only when the Air Traffic Services
Provider has no serviceable wind indicator. Criteria are to be agreed
locally, based on changes of operational significance at the aerodrome;
otherwise,
1)
A change in the mean direction of 60 degrees or more, the mean
speed before or after the change being 10 knots or more, but a
change of 30 degrees when 20 knots or more.
2)
A change in the mean speed of 10kt or more.
3)
A change in gust speed of 10kts or more, the mean speed before or
after the change being 15kt or more.
5.33
ATC operations without real-time surface wind information shall either be
conducted in accordance with existing approved procedures or must be
notified to the relevant Manager ATS Safety Regulation Operations as
soon as practicable and the operational procedures agreed.
Surface visibility
5.34
Issued when the prevailing visibility changes from one of the following
ranges to another:

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
5000 m to 9 km

3000 m to 4900 m

2000 m to 2900 m

1500 m to 1900 m

800 m to 1400 m

750 m or less
5.35
At the onset or cessation of the requirement to report minimum visibility.
5.36
If the minimum visibility is being reported, when the minimum visibility
changes from one of the ranges, given in a) above, to another.
5.37
Additional change groups of 100 m or less, 150 to 300 m, 350 to 550 m
and 600 to 750 m are used where an RVR is not available, either
permanently or during temporary unserviceability. These criteria will apply
by local arrangement.
5.38
Additional change groups may be used to meet local operational needs.
Details of any additional change groups used at aerodromes shall be
published in the UK AIP.
Runway visual range
5.39
Special reports for RVR are only made by local arrangement.
5.40
Issued when the visibility changes from one of the following ranges to
another:

800 m or more

550 m to 750 m

300 m to 500 m

275 m to 175 m

175 m to 50 m

50 m or less
Present weather
5.41
The onset, cessation or change in intensity of any of the following weather
phenomena or combinations:
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Chapter 5: Weather reports to air traffic services

freezing rain or freezing drizzle of any intensity

freezing fog

moderate or heavy precipitation (including showers)

thunderstorm (with or without precipitation)

squall, funnel cloud

low drifting or blowing: snow, sand or dust
Cloud
5.42
Issued when the lowest cloud of over 4 oktas (BKN or OVC) changes from
one of the following ranges to another:

2000 ft or more

1500 ft to 1900 ft

1000 ft to 1400 ft

700 ft to 900 ft

500 ft to 600 ft

300 ft to 400 ft

200 ft

100 ft

Less than 100 ft
(including sky obscured)
5.43
When the amount of cloud below 1500ft changes from 4 oktas or less (nil,
FEW or SCT) to more than 4 oktas (BKN or OVC), and vice versa.
Temperature
5.44
When the air temperature changes by 2.0 degrees from that given in the
last report.
Pressure
5.45
When the ‘as read’ pressure changes by 1.0 hectopascal or more since
the last report.
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Chapter 6: General requirements for observing equipment
Chapter 6
General requirements for observing equipment
Introduction
6.1
Meteorological observing equipment shall provide a timely and accurate
source and display of meteorological information to aid in the safe and
expeditious flow of civil air traffic.
6.2
The purpose of this chapter is to provide requirements and
recommendations covering all meteorological instruments and systems
installed at UK aerodromes. It covers the performance criteria and
safeguarding of meteorological equipment installed at the aerodrome and
intended to be used for the origin of aerodrome weather reports.
6.3
The Aerodrome Meteorological Observing Service Provider should ensure
that appropriate consideration and provision for service continuity of
observing equipment has been made, including any necessary support
facilities, such as backup power supply etc.
6.4
It is recommended that, where appropriate, a formal agreement such as a
Service Level Agreement, be reached between the Aerodrome
Meteorological Observing Service Provider and the (agency(s) providing
meteorological equipment maintenance and calibration services.
General requirements
6.5
Equipment installed shall have been designed following design practices
as described below and in Chapter 7, Design Requirements for
Meteorological Equipment.
6.6
These shall include:
1)
The existence of appropriate technical specifications for the
equipment.
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Chapter 6: General requirements for observing equipment
2)
Calibration standards traceable to a recognised national or
international standard.
6.7
Wherever possible the observing equipment should be designed in such a
manner that a) the system alerts the user to a failure of part or all of the
equipment or power supply or b) that such faults should be obvious to the
user.
6.8
Equipment shall operate within and recover to the tolerance values
specified in each element’s requirements from the ranges given for each
element in Chapter 7, Design Requirements for Meteorological
Equipment.
6.9
The instrument housing shall be designed to prevent atmospheric
influences and radiation errors from affecting the parameters measured by
the installed sensor(s), whilst allowing a free flow of air across the
sensor(s), to enable the sensor to represent the ambient environment.
6.10
Sensors are required to be positioned in such a manner that allows them
to measure meteorological elements free of other influences e.g. jetengine wash.
Operation and maintenance requirements of
meteorological equipment
6.11
Equipment should be installed in accordance with the manufacturer’s or
supplier’s instructions and shall be tested to confirm correct and reliable
operation.
6.12
The frequency of calibration checks, replacement and servicing intervals
shall be specified and based on manufacturers’ recommendations, or if
operational experience indicates a need, more frequently. Additional
guidance on calibration requirements for wind and pressure measuring
equipment is detailed in Appendix I.
6.13
Re-calibration shall be traceable to national and international standards.
This may be achieved through a recognised accreditation scheme such as
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Chapter 6: General requirements for observing equipment
the National Accreditation of Measurement and Sampling (NAMAS).
NAMAS is a service of the United Kingdom Accreditation Service (UKAS).
6.14
UKAS can be contacted at 2 Pine Trees, Chertsey Lane, Staines-uponThames, TW18 3HR. Tel: 01784 429000 or Email: info@ukas.com, or see
www.ukas.org.
6.15
Routine care and maintenance must be scheduled to ensure that
equipment continues to operate effectively within calibrated limits. This
may include tasks such as cleaning sensor heads, clearing sensor ports
of debris and checking for water leaks from rain.
6.16
Atmospheric pressure measuring equipment shall be checked daily for
signs of sensor drift by comparison with other instrumentation on the
aerodrome or readings from nearby aerodromes. Appendix D, Daily
Atmospheric Pressure Equipment QNH Check, provides an example of
the type of form that may be used to assist in the monitoring process.
NOTE:
Semi-automated observing systems comprising duplicate or triplicate sensors
that carry out automatic comparison between individual sensors would be
considered to be an acceptable means of compliance with this paragraph.
6.17
Maintenance activity that will render the equipment unable to provide
information or liable to produce inaccurate information should be
scheduled at times during which the loss of data is not operationally
significant (ideally outside aerodrome operating hours). Alternatively, the
equipment shall be withdrawn from service until the maintenance is
completed.
Installation of meteorological observing equipment
6.18
Guidance should be sought, as required, from the CAA’s aerodrome
section on the suitability of location of new sensors and observing
systems for safeguarding purposes.
6.19
Siting and performance requirements of instrumentation are given in
Chapter 7, Design Requirements for Meteorological Equipment.
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6.20
Chapter 6: General requirements for observing equipment
Aerodromes that are certified under the SES Common Requirements as
an Air Navigation Service Provider are required to ensure that all required
meteorological equipment is included within the unit safety case or the
unit’s safety assurance documentation whichever is applicable.
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Chapter 7: Design requirements for meteorological equipment
Chapter 7
Design requirements for meteorological equipment
Introduction
7.1
The purpose of this chapter is to provide minimum standards for
meteorological equipment at UK aerodromes producing METAR reports
and at other aerodromes as specified in Chapter 1, Paragraph 1.5.
7.2
The Aerodrome Meteorological Observing Service Provider should ensure
that appropriate consideration and provision for service continuity of
observing equipment has been made, including any necessary support
facilities, such as backup power supply etc.
7.3
If backup sensors are available, the procedures for use, operational
limitations and maximum period of use must be documented by the
Aerodrome Meteorological Observing Service Provider.
7.4
Sensor siting shall not encroach the obstacle limitation surface or obstacle
free zones. However, consideration may be given by the Safety
Regulation Group to the collocation of sensors on existing structures on
the aerodrome.
7.5
Notwithstanding the constraints listed in paragraph 7.4, the exposure of
the sensor should minimise the effects of all obstructions. The tower used
to mount the wind sensor is not considered an obstruction to the sensor
collection system but, with the exception of the temperature, dew point,
and pressure sensors, it should be at least 3 m away from all other
meteorological sensors. Sensors should be located as far as practicable
from any source likely to significantly affect the quality of the data.
7.6
The display shall present a clear and unambiguous indication of the
operational status of the sensor system to the user, in a format applicable
to the proposed installation.
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7.7
Chapter 7: Design requirements for meteorological equipment
Where possible, the equipment should be self-monitoring and provide a
suitable indication of equipment status and serviceability.
7.8
Where equipment is not self-monitoring, a failure of the equipment should
be obvious.
Meteorological displays
Performance
7.9
Meteorological displays that present dynamic meteorological information
to ATS is subject to the requirements contained in CAP 670 ATS Safety
Requirements.
7.10
Where separate display systems are used to source data for the
preparation of METAR reports and for the presentation of dynamic
meteorological information to ATS, the display device for the METAR shall
be designed in such a way as to draw the attention of the operator to
significant changes in the displayed meteorological information. (A
significant change is defined in Chapter 5, Weather Reports to Air Traffic
Services, paragraphs 5.29 to 5.44.)
7.11
The wind sensor display shall indicate whether the direction is referenced
to True North or Magnetic North.
7.12
On aerodromes with more than one wind sensor, the display shall clearly
indicate the sensor or location from which the information is derived.
7.13
Numeric or graphical displays used to display other forms of
meteorological information shall provide a clear indication of function and
assignment.
7.14
Equipment that displays times and records of observations (or sampled
data) shall reflect UTC and be accurate to ± 15 seconds.
7.15
On semi-automated meteorological observing systems, an area of the
screen may be made available for the display of supplementary
information.
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Chapter 7: Design requirements for meteorological equipment
Equipment used to provide dynamic meteorological information solely to
the Meteorological Observer should be refreshed and updated regularly
and at least every 5 minutes.
Backup
7.17
Appropriate contingency arrangements should be in place in the event
that any display or display system fails; where this occurs with a semiautomated meteorological observing system, the contingency should take
into account any limitations of observers’ certification.
Surface wind speed and direction equipment
7.18
This section details the performance criteria and siting of surface wind
measurement equipment installed at UK aerodromes for METAR
purposes.
7.19
Reports disseminated off the airport in METAR code shall be in
compliance with the requirements of Chapter 4, METAR Structure and UK
Coding Rules and the information given to ATS Providers shall comply
with the requirements in Chapter 5, Weather Reports to Air Traffic
Services.
7.20
Where separate sensor and display systems are used to source data for
the preparation of METAR reports and for the presentation of real-time
reports to ATS, the equipment used to provide real-time information to
ATS is subject to the requirements contained in CAP 670 ATS Safety
Requirements.
Siting
7.21
For METAR reports, sensors shall be positioned to represent the wind
flow at 10 m above the surface. Measurements from sensors positioned
between 8 and 13 m high need no corrections; measurements from
sensors positioned between 5 and 7 m or 14 and 15 m high should be
increased by 10% or decreased by 10% respectively. The minimum
acceptable height for the primary wind sensor is 5 m.
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7.22
Chapter 7: Design requirements for meteorological equipment
For reports to Air Traffic Services, sensors shall be positioned to
represent the wind flow at 10 m above the surface. Measurements from
sensors positioned between 8 and 13 m high need no corrections;
measurements from sensors positioned between 5 and 7 m or 14 and 15
m high should be increased by 10% or decreased by 10% respectively.
The minimum acceptable height for the primary wind sensor is 8 m.
Performance
7.23
The wind speed and direction measuring equipment shall provide an
accurate and representative measurement of wind speed and direction.
7.24
Wind direction data should be oriented with respect to True North.
7.25
The wind speed measurement shall be to an accuracy of within plus or
minus 1 knot, or plus or minus 10 per cent for wind speeds in excess of 10
knots, of the actual wind speed (whichever is the greater), over the
following range:
Table 5: Tolerance values of sensors and equipment
Variable
In tolerance operating range
Recoverable range
Windspeed
0 to 100 knots
0 to 130 knots
7.26
Aerodromes that are certificated under the EASA Common Requirements
(EC REG 1035/2011) as an Air Navigation Service Provider but do not
routinely disseminate MET information beyond the aerodrome and are
without published or airport owned instrument approach procedures shall
provide wind speed measurements to an accuracy of at least ±2kt or 5%
whichever is greater.
7.27
With wind speeds in excess of 2 knots, the wind direction system shall be
capable of producing an overall accuracy better than plus or minus 10
degrees.
7.28
The sensor shall be sampled at a minimum rate of four times every
second. Where wind systems measure the gust, the equipment shall
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calculate the 3 second gust as a rolling average of the wind speed
samples.
7.29
The equipment shall be capable of producing 2 and 10 minute rolling
averages of the wind speed and direction. The algorithms used for the
production of such averages shall be defined.
7.30
The average direction displayed shall take regard of the numerical
discontinuity at North.
7.31
The information reported shall be compliant with Chapter 4, METAR
Structure and UK Coding Rules and Chapter 5, Weather Reports to Air
Traffic Services, with respect to marked discontinuity of the wind, wind
variation and maximum wind speed (gusts).
Backup equipment
7.32
Alternative anemometry, meeting the above requirements and located on
the aerodrome may be used as a backup to the wind information.
7.33
Exceptionally, a hand-held anemometer may be used for a METAR report.
Such instrumentation should be used in accordance with manufacturer’s
recommended procedures. Readings taken at ground level should be
corrected to a height of 10 m.
Pressure measurement equipment
7.34
This section details the performance criteria and siting of ground based
pressure measuring equipment installed at UK aerodromes for METAR
purposes.
7.35
Pressure sensors can accurately measure atmospheric pressure and will
provide representative data for the aerodrome weather report provided the
sensors are correctly located and maintained in accordance with this
document.
7.36
The following requirements relate to the basic measurement of pressure
and any derived values such as QNH and QFE.
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Chapter 7: Design requirements for meteorological equipment
Pressure sensor derived values are of critical importance to aviation
safety and operations. Great care shall be taken to ensure that pressure
sensor siting is suitable and provides accurate data.
Siting
7.38
The equipment shall be installed so that the sensor measurements are
suitable for the operational purpose and free of external influences.
7.39
If the equipment is not installed at the same level as the notified
aerodrome elevation, it shall be given a correction factor, in order to
produce values with respect to the reference point.
7.40
Where required, the manufacturer’s recommended venting method shall
be employed to isolate the sensor from the internal environment.
7.41
The pressure sensor shall be installed on the aerodrome, usually in a
weatherproof facility (building, shelter, enclosure, etc.).
7.42
In most cases, internal venting of the pressure sensors will be
satisfactory. However, if it is determined that internal venting may affect
the altimeter setting value to the extent that it is no longer within the
accuracy limits given below, outside venting should be used. When the
pressure sensor is vented to the outside, a vent header shall be used.
Siting that will cause pressure variations due to air flow over the venting
interface should be avoided. The venting interface will be designed to
avoid and dampen pressure variations and oscillations due to “pumping”
or “breathing” of the pressure sensor venting equipment.
7.43
The sensors should also be located in an area free of jarring, vibration,
and rapid temperature fluctuations (i.e. avoid locations exposed to direct
sunlight, drafts from open windows, and out of the direct path of air
currents from heating or cooling systems). If the pressure sensors are
sited outdoors, the height of the vent header shall not be less than one 1
metre above ground level. Regular inspections of the vent header shall be
carried out to ensure that the header does not become obstructed by dust
etc.
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Performance
7.44
No observing system, that determines pressure automatically, shall be
dependent upon a single sensor for pressure measurement. A minimum
of 2 colocated sensors shall be used. The pressure sensors shall be
accurate to within 0.5 hectopascals of each other.
7.45
In the event of failure of one or more individual pressure sensors, or
where pressure sensors are not accurate to within 0.5 hectopascals of
each other, the system shall not provide any pressure reading to the user.
7.46
Automatic sensors shall be sampled at a minimum rate of once per minute
in order to detect significant changes.
7.47
For manual systems, a single, calibrated, barometer may be used for
pressure measurement by an observer.
7.48
The measurement system shall provide a pressure reading to an accuracy
of ±0.5 hectopascals, or better over the following range:
Table 6: Tolerance values of sensors and equipment
Variable
In tolerance operating range
Recoverable range
Pressure
900 to 1050 hPa
850 to 1200 hPa
7.49
The sensor shall provide an output with a minimum system resolution of
0.1 hPa.
7.50
Aerodromes that are certified under the EASA Common Requirements
(EC REG 1035/2011 as an Air Navigation Service Provider but do not
routinely disseminate MET information beyond the aerodrome and are
without published or airport owned instrument approach procedures shall
provide pressure information to an accuracy of at least ± 1hPa, and to a
resolution of at least 1 hPa.
7.51
Observing systems that determine pressure automatically shall be
designed in such a way as to draw the attention of the operator to a
change of 1.0 hectopascal or more in the ‘as read’ pressure from the
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previous reading to 1 decimal place, (e.g. 998.4 hPa to 997.4
hectopascal).
7.52
Primary and backup atmospheric pressure measuring equipment should
be checked daily for signs of sensor drift by comparison with other
pressure instrumentation on the aerodrome or other suitable location.
Appendix D, Daily Atmospheric Pressure Equipment QNH Check,
provides an example of the type of form that may be used to assist in the
monitoring process.
7.53
Observing systems comprising of more than one pressure sensor that
employ documented internal cross-checking processes, do not require
additional checks for pressure sensor drift.
NOTE:
Although observing systems comprising of more than one pressure sensor that
employ documented internal cross-checking processes do not themselves
require additional checks, these sensors if correctly maintained, would be
considered to be acceptable instrumentation against which to carry out daily
checks of backup sensors.
Backup
7.54
Suitable backup instrumentation:

Precision aneroid barometers

Digital Precision Pressure Indicators
Temperature and dew point measurement
7.55
This section details the performance criteria and siting of temperature and
dew point measuring equipment installed at UK aerodromes.
Siting
7.56
The sensors should be mounted at a height of 1.25 to 2 m, above an earth
or grass surface away from buildings and other structures likely to
influence the reading. The sensors shall be exposed in an instrument
housing, which provides protection from atmospheric radiation and water
droplets either as precipitation or fog. Consideration should be given to
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Chapter 7: Design requirements for meteorological equipment
siting sensors away from exhausts of building heating and equipment
cooling systems.
Performance
7.57
The equipment shall be capable of measurement to an accuracy better
than plus or minus 1.0 degrees Celsius for air temperature and dew point,
over the following range:
Table 7: Tolerance values of sensors and equipment
Variable
In tolerance operating range
Recoverable range
Temperature
minus 25°C to plus 50°C
minus 30°C to plus 70°C
Humidity
5 to 100% RH condensing
0 to 100% RH condensing
7.58
Dew point shall be displayed for temperatures below zero; frost point
should not be displayed.
7.59
Temperature and dew point measurements shall be measured to a
resolution of 0.1 degrees Celsius.
7.60
Electronic sensor(s) shall be sampled at minimum of once per minute.
Backup
7.61
Alternative sensors shall be provided with an accuracy better than plus or
minus 1.0 degrees Celsius for air temperature and dew point. Sensors or
readouts should be easily accessible to the duty observer. Backup
sensors mounted in the vicinity of the VCR should be contained in an
instrument housing which is appropriately exposed and protected from the
bulding heating and cooling systems.
Cloud base recorder systems
7.62
This section details the performance criteria and siting of cloud base
measuring equipment installed at UK aerodromes, and requirements
regarding the display of information from the equipment. Installation and
use of such equipment is mandatory for licensed aerodromes with
precision approach runways, and also for aerodromes providing
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Chapter 7: Design requirements for meteorological equipment
automated observations. At any other aerodromes, a cloud base recorder
is optional.
7.63
When automated equipment is used for the measurement of the height of
cloud base, height of cloud base display(s) should be located in the
appropriate aerodrome control tower. The display(s) in the aerodrome
control tower and the display(s) used by the aerodrome meteorological
observers should relate to the same sensor, and where separate sensors
are required, the displays should clearly identify the area monitored by
each sensor.
7.64
A cloud base recorder can provide valid measurements of the height of
cloud elements within the view of the sensor, but this is limited by the
detection system’s coverage of the sky. For this reason, sensors used to
measure cloud are considered to be an aid to an observer rather than a
direct source of information for the METAR. The accredited observer
needs to assess the output of the sensors and give a final judgement on
the variation of the element over the aerodrome that should be included in
the observation.
Siting
7.65
The sensor shall be positioned in accordance with the manufacturer’s
specifications and is normally mounted on a platform or pedestal. The
sensor should be located as far as practicable from other light sources
that might affect the measurement.
Performance
7.66
The performance of the cloud base recorder is limited by the view of the
sensor. However, the equipment shall be capable of measurement to the
following accuracy limits from the surface to 5000 ft above ground level:

Cloud height ± 30 ft up to and including 300 ft; ± 10% above 300 ft
7.67
The cloud base recorder shall measure to a resolution of 100 ft.
7.68
The sensor(s) shall be sampled at a minimum of once per minute.
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Chapter 7: Design requirements for meteorological equipment
Where appropriate software is utilised, cloud base detection systems may
also provide an indication of the cloud amount. A cloud cover algorithm
unit calculates the cloud amounts and the heights of different cloud layers,
in order to construct an approximation of the entire sky. Such an
approximation is limited by the detection system’s coverage of the sky and
shall not be used in the METAR report unless validated by the accredited
observer.
Backup
7.70
Unless further cloud detection systems are available, the accredited
observer should assess the amount of cloud by eye and estimate the
height, assisted by reference material as appropriate. Human estimates of
cloud height, without reference to any form of measuring equipment
(particularly at night), may not meet the accuracy requirements stated
above.
Visibility measuring systems
7.71
This section details the performance criteria and siting of visibility
measuring equipment installed at UK aerodromes. Installation and use of
such equipment is mandatory for licensed aerodromes with CAT ll and
CAT lll instrument approach and landing operations, and also for
aerodromes providing automated observations. At other aerodromes,
such equipment is optional.
7.72
A visibility measuring system can provide valid measurements of the
visibility within the range of the sensor. For this reason, sensors used to
measure visibility are considered to be an aid to an observer rather than a
direct source of information for the METAR. The accredited observer
needs to assess the output of the sensors and give a final judgement on
the variation of the element over the aerodrome that should be included in
the observation.
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Chapter 7: Design requirements for meteorological equipment
Siting
7.73
The sensor shall be positioned in accordance with the manufacturer’s
specifications and is normally mounted on a mast at a height of
approximately 2.5 m. The sensor should be located as far as practicable
from other light sources that might affect the measurement.
Performance
7.74
The performance of the measuring system is limited by the view of the
sensor. However, the equipment shall be capable of measurement to the
following accuracy limits to a range of 15 km:

Visibility ± 50 m up to and including 550 m; ± 10% between 600 m
and 1500 m; ± 20% above 1500 m.
7.75
The visibility measuring system shall measure to a resolution of 50 m.
7.76
The sensor(s) shall be sampled at a minimum of once per minute. An
averaging period of 1 minute should be used for weather reports to ATS.
An averaging period of 10 minutes for METAR reports should be used,
however where a marked discontinuity occurs only those values after the
discontinuity should be used for obtaining mean values.
NOTE:
A marked discontinuity occurs when there is an abrupt and sustained change in
visibility, lasting at least 2 minutes and which reaches or passes through the
criteria for the issuance of special reports to ATS as detailed in Chapter 5
paragraphs 5.33 to 5.37 Surface Visibility.
Backup
7.77
Unless further visibility measuring systems are available, the accredited
observer should assess the visibility by eye. This should be assisted by
the availability of visibility reference points annotated on to a 360°
panoramic photograph. At locations where this is possible, suitable
reference points that are illuminated at night should also be marked.
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Chapter 7: Design requirements for meteorological equipment
Present weather detectors
7.78
This section details the performance criteria and siting of present weather
detector equipment installed at UK aerodromes. Installation and use of
such equipment is mandatory for licensed aerodromes providing
automated observations. At other aerodromes, such equipment is
optional.
7.79
The purpose of a present weather detector is to detect precipitation and,
in some cases, discriminate type and intensity. Guidance given by
weather sensors can be misleading such as when light drizzle and mist
occurs; without observing from outside the building, it is often difficult for
an observer to determine whether drizzle is present or merely dampness
caused by mist or fog.
7.80
Sensors used to detect and measure present weather are considered to
be an aid to an observer rather than a direct source of information for the
METAR.
Siting
7.81
The sensor shall be positioned in accordance with the manufacturer’s
specifications. The sensor should be located as far as practicable from the
shielding effects of structures, buildings and other obstacles.
Performance
7.82
The sensor should be capable of detecting precipitation of rate greater
than or equal to 0.05 mm per hour, within 10 minutes of the precipitation
commencing.
7.83
Where intensity is measured, the sensor should be capable of measuring
the range of intensity from 0.00 mm per hour to 100 mm per hour and
resolve this to 0.1 mm in the range 0-10 mm per hour, 0.5 mm in the
range 10.5 to 50 mm per hour and 1 mm in the range 51 to 100 mm per
hour. The sensor should be accurate to within ±30% in the range 0.5 to 20
mm per hour.
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7.84
Chapter 7: Design requirements for meteorological equipment
The sensor should discriminate the following present weather phenomena
and their intensity as a minimum rain, drizzle, snow, rain and snow,
freezing rain, freezing drizzle, fog, freezing fog, mist and haze.
7.85
If the present weather sensor is unable to determine the state or form of
the precipitation ‘UP’ (unidentified precipitation) or ‘FZUP’ (freezing
unidentified precipitation), together with intensity qualifications, should be
reported as appropriate.
Backup
7.86
Unless further precipitation systems are available, the accredited observer
should assess present weather manually, assisted by reference material
as appropriate.
Integrated Met measurement systems
7.87
At aerodromes with runways intended for Category II and III instrument
approach and landing operations, automated equipment for measuring
and for monitoring surface wind, visibility, runway visual range, height of
cloud base, air and dew-point temperatures and atmospheric pressure
shall be installed.
7.88
These systems enable all the Met sensor data to be acquired, measured,
processed and made available for subsequent display and use in other
systems in real time. In addition these systems enable the METAR to be
coded and disseminated on the AFTN. As well as routing information to
other systems and displays, the Integrated Met Measurement System
enables the production and dissemination of the weather reports if
required in the METAR format. Typically measurements of wind, pressure,
runway visual range, air and dew-point temperature are used directly with
the ability for the human observer to modify or accept the cloud height and
amount, visibility and present weather reports.
7.89
The meteorological parameters that are received by the Integrated
Measurement System often require processing before they can be used
by the Met Observer or ATS staff. It should be noted that there are
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different processing, display and averaging requirements for Met
information used for ATS purposes from that being used to compile
meteorological reports.
7.90
Where an Integrated Met Measurement System is used to provide
information to ATS displays or ATS systems e.g. ATIS it is required to
comply with the SES Interoperability (IOP) Regulation (EC REG
522/2004). Where the Integrated Met Measurement System solely
provides information to the Met Observers display IOP approval is not
required.
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Chapter 8: Dissemination of weather reports
Chapter 8
Dissemination of weather reports
Introduction
8.1
Meteorological reports should be disseminated beyond the aerodrome in
a manner agreed between the ATS Provider and the UK Meteorological
Authority.
8.2
This is normally achieved by the transmission of routine reports to the UK
Civil Aviation Communications Centre by the Aeronautical Fixed Service.
8.3
CAP 493 Manual of Air Traffic Services Part 1 describes the elements of a
meteorological report that are routinely required to be passed to pilots by
ATC.
8.4
Where the passing of meteorological information increases ATC workload
to the extent that the provision of the ATC service is affected, the Provider
of ATC should consider the broadcast of meteorological reports on ATIS.
8.5
Meteorological reports included on ATIS is an ATS function and is
covered in full detail in CAP 493 Manual of Air Traffic Services Part 1 and
CAP 670 ATS Safety Requirements.
8.6
CAP 797 Flight Information Service Officer Manual provides further detail
regarding meteorological information passed to pilots by Aerodrome Flight
Information Service Officers.
Timing requirements
8.7
In the UK, METARs are transmitted every half hour or exceptionally every
hour. Typically observations are made at 20 minutes past the hour (where
half-hourly observations are provided) and at 10 minutes to the hour (for
both half-hourly and hourly observations). Since aerodromes have predetermined designated places in meteorological bulletins and on VOLMET
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Chapter 8: Dissemination of weather reports
etc., it is essential that observations are completed in accordance with
normal observing practice and are transmitted within 5 minutes of normal
dispatch time.
8.8
The Meteorological Forecast Office also requires timely observations to
ensure that amendments to Aerodrome Forecasts are issued quickly, and
to assist in the timely issue of aerodrome warnings.
METAR
8.9
Once a METAR has been transmitted it will be collected at the Civil
Aviation Communications Centre and assembled into pre-determined
‘bulletins’. The bulletins are then disseminated via Aeronautical Fixed
Service (AFS) channels. Selections of the AFS data will be available on
the Aeronautical Fixed Telecommunications Network (AFTN) or via the
Satellite Distribution System (SADIS), as agreed between the UK
Meteorological Authority, the UK Civil Aviation Communications Centre
and ICAO Regional Planning Groups.
8.10
A small selection of METARs are sent for broadcast on VOLMET.
VOLMET is a voice broadcast of a set of METARs broadcast on four
frequencies covering different regions of the UK with each frequency
transmitting a different METAR set. The four regions are London VOLMET
(Main), London VOLMET (South), London VOLMET (North) and Scottish
VOLMET. Details can be found in the UK AIP GEN 3.5.8.
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Chapter 9: Reliability and availability of reporting
Chapter 9
Reliability and availability of reporting
Completeness of reports
9.1
The purpose of a weather observation is to provide a complete picture of
the conditions at the aerodrome to a variety of recipients.
9.2
The meteorological forecaster is required to take account of all
meteorological variables when preparing an Aerodrome Forecast;
METARs are used to verify base conditions before forecasting how these
elements will change with time. Missing information in the METAR may
lead to greater inaccuracies in the forecast which may impact on tactical
planning by pilots, operators and other aerodrome service providers.
Contingency arrangements for the failure of
meteorological observing sensors and systems
9.3
Contingency arrangements shall be developed and followed in the event
of the failure or non-availability of equipment used to derive the content of
an observation.
9.4
Contingency arrangements may include the use of alternative or stand-by
equipment, the inclusion in a report of an estimate of the meteorological
phenomenon that cannot be sensed or measured, or reporting the
element affected as ‘not available’. It should be noted that different
contingency arrangements may be appropriate when a report to Air Traffic
Services is compiled and when a METAR is compiled.
9.5
Contingency arrangements shall be documented.
9.6
If a semi-automatic observing system fails an alternative means of
recording shall be maintained in order to log all reports.
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Missing meteorological aerodrome reports
9.7
If the routine supply of METARs ceases, for whatever reason, there may
be an impact on users of the meteorological information (e.g. selection of
alternate aerodromes and fuel upload planning) and the provision of an
Air Traffic Service. Wherever practical, suitable contingency measures
shall be identified and associated operational procedures documented.
9.8
In accordance with ICAO Annex 3, if a regular supply of METARs ceases
or is incomplete, the Aerodrome Forecast may be cancelled, as the
meteorological forecaster loses the site-specific information on which to
confirm the forecast. The forecaster will not issue further Aerodrome
Forecasts until the transmission of METARs re-commences.
9.9
In practical terms this means that if a gap of two hours between METAR
reports occurs (i.e. more than three half hourly METARs are not received
or a second hourly METAR is not received) or if an element is missing
from more than three consecutive half-hourly reports then the Aerodrome
Forecast will be cancelled. The Aerodrome Forecast will not be re-issued
until two complete METARs have been received.
9.10
Accredited observers at some H24 aerodromes take a duty break
overnight, of maximum two hours duration. A supply of AUTO METARs
will be provided during this period. If the duty observer has not
recommenced observations after two hours (i.e. when more than three
AUTO METARs are issued) the Aerodrome Forecast may be cancelled.
9.11
The Meteorological Authority will be notified if the meteorological
forecaster is unable to produce an Aerodrome Forecast or has to cancel
an Aerodrome Forecast because of missing or erroneous information in a
METAR.
Timeliness
9.12
The Aerodrome Meteorological Observing Service Provider shall allocate
sufficient time resources to the aerodrome meteorological observing staff
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to enable them to carry out observing duties. The observer may need to
assess certain elements of the weather from an outside observing position
that is close to ground level. All reports should be checked before issue.
9.13
Data collection for observations made at 20 minutes past the hour (where
halfhourly observations are provided) must begin no earlier than 10
minutes past the hour. Data collection for observations made at 10
minutes to the hour (for both half-hourly and hourly observations) must
begin no earlier than 20 minutes to the hour. The pressure should be the
meteorological element that is read as close to the nominal observation
time (20 minutes past the hour and 10 minutes to the hour) as possible.
Pressure readings shall not be read earlier than 5 minutes before the
nominal observation time.
9.14
METAR reports made at 20 minutes past the hour (where half-hourly
observations are provided) should be transmitted between 20 and 25
minutes past the hour and METAR reports made at 10 minutes to the hour
(for both half-hourly and hourly observations) should be transmitted
between 10 and 5 minutes to the hour.
9.15
Specials reports, when applicable, should be made without delay and
recorded in the appropriate manner.
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Chapter 10: Records and archives
Chapter 10
Records and archives
Introduction
10.1
A continuous log shall be maintained of all METAR and special reports
produced at the aerodrome. These data may be required in the event of
an official enquiry relating to an aircraft accident or incident either at, or in
the vicinity of the aerodrome and can be useful for planning future
aerodrome services.
10.2
When the observations are made using a semi-automated observing
system, the system should be arranged either so that a printout of the
METAR and special reports is made for retention, or so that the METAR
and special reports are stored on a disk, which can be retained for future
reference.
10.3
For manual observations, a record shall be maintained to log all the
readings and reports.
10.4
In the event of a mistake being discovered in the METAR report, a
corrected METAR shall be issued. Following an erroneous log entry, the
original value and corrected value shall be clearly indicated, especially
when a report has been used by the Air Traffic Service Provider or coded
in the METAR. An original erroneous figure must not be deleted and
replaced at a later time with a corrected figure. Thus the log will show both
the original erroneous report and the subsequent correction with the time
at which the correction was made.
10.5
A corrected METAR shall only be issued ahead of the subsequent
METAR, typically 30 minutes later. Thereafter, any mistake should be
logged only.
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10.6
Chapter 10: Records and archives
A method shall be established to enable identification of the individual
who originates an observation (including any follow-up corrections). Any
such record should be retained for at least 30 days.
10.7
Compliance with this requirement does not necessarily require the identity
of the observer to be recorded on the equipment used to produce the
observation.
Meteorological information records
10.8
Where observing systems are in use, the equipment shall be capable of
producing a printed record of all observation reports (METAR, special
reports and any non-routine observation at the time of an aircraft accident
on or in the vicinity of the aerodrome) produced during the preceding 30
days. Where manual observations are produced, each instrument reading
should be recorded in a book or other suitable log, in accordance with
World Meteorological Organisation guidance material, and retained for a
period of at least 30 days. Similarly where continuous analogue
recordings are made of meteorological elements, any charts or other
recordings should be retained for at least 30 days.
10.9
Air Traffic Control units should examine the requirements for producing a
printed post-incident meteorological report given in CAP 493 Manual of Air
Traffic Services Part 1, Section 6, Chapter 1, Paragraph 5.
10.10
METAR and TAF reports distributed via the AFS are stored by the UK Met
Office for a minimum of one year. Such data may be accessed on
request. The UK Met Office may make a charge for this service.
10.11
Where observing systems sample conditions more frequently than is
required for the production of routine or special reports, it is recommended
that a facility exists for the system to store the previous 60 minutes of data
from each sensor, on command from the observer (for example following
the completion of a nonroutine observation). Data should be retained for a
period of at least 30 days.
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10.12
Chapter 10: Records and archives
All records and data should be available for examination by the CAA or
the UK Air Accidents Investigation Branch (AAIB) on request. The
aerodrome shall agree that any data or record provided to the CAA or
AAIB may be quoted or used as part of an investigation, legal enquiry or
legal proceedings.
10.13
Any records and data of which copies are requested by the CAA or AAIB
to assist in the investigation of an incident are to be retained until
specifically released by the CAA or AAIB.
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Chapter 11: Definitions, abbreviations and bibliography
Chapter 11
Definitions, abbreviations and bibliography
Glossary
11.1
This Glossary contains terms that have a specific meaning in civil aviation,
safety, or regulatory matters.
Accuracy
A degree if conformance between the estimated or measured
value and the true value.
Aerodrome
Any area of land or water designed, equipped, set apart or
commonly used for affording facilities for the landing and departure
of aircraft.
Aerodrome control
service
An air traffic control service to aerodrome traffic.
Aerodrome
Meteorological
Observing Units
A unit on an aerodrome that produces METAR observations or is
responsible for the receipt (and onward transmission around the
aerodrome, where appropriate) of aerodrome meteorological
warnings.
Aerodrome
Reference Point
The designated geographical location of an aerodrome.
Aeronautical fixed
service
A telecommunication service between specified fixed points
provided primarily for the safety of air navigation and for the
regular, efficient and economical operation of air services. (ICAO
Annex 11, Chapter 1)
Aeronautical Fixed A worldwide system of aeronautical fixed circuits provided as part
Telecommunication of the aeronautical fixed service, for the exchange of messages
Network
and/or digital data between aeronautical fixed stations having the
same or compatible communications characteristics. (ICAO Annex
11, Chapter 1)
Aeronautical
Information
Publication (AIP)
A publication issued by or with the authority of a State and
containing aeronautical information of a lasting character essential
to air navigation.
Aeronautical
Information Service
(AIS)
Publisher of Notices to Airmen (NOTAM) and United Kingdom
Aeronautical Information Publication.
Aerodrome
Licensee
In relation to any aerodrome, the person in charge of the
aerodrome.
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Air-report
A report from an aircraft in flight prepared in conformity with
requirements for position, and operational and/or meteorological
reporting
Air Traffic
All aircraft in flight or operating on the manoeuvring area of an
aerodrome.
Air Traffic Control
Centre
An air traffic control unit established to provide an area control
service to aircraft flying within a notified flight information region
which are not receiving an aerodrome control service or an
approach control service. (CAP 393 Air Navigation: The Order and
the Regulations)
Air Traffic Control
Service
A service provided for the purpose of preventing collisions
between aircraft, and, on the manoeuvring area, between aircraft
and obstructions, and expediting and maintaining an orderly flow
or air traffic.
Air Traffic Control
Unit
A unit of air traffic controllers established by a person appointed by
a person maintaining an aerodrome or other place in order to
provide an area control service, an aerodrome control service or
an approach control service.
Air Traffic Service
A generic term meaning air traffic control service, flight information
service and air-ground communication.
Alternate
aerodrome
An aerodrome to which an aircraft may proceed when it becomes
either impossible or inadvisable to proceed or to land at the
aerodrome of intended landing.
Altitude
The vertical distance of a level, a point or object considered as a
point, measured from mean sea level. (ICAO Annex 3, Chapter 1)
Availability
The ability of a system to perform within specified limits, a required
function under given conditions, at a given time.
CAA
This means the UK Civil Aviation Authority, comprising Directorate
of Airspace Policy (in which the UK Met Authority resides), Safety
Regulation Group, Economic Regulation Group and Consumer
Protection Group.
CAVOK
The visibility, cloud and weather groups are replaced by the term
CAVOK (Cloud And Visibility OK) when the following conditions
exist simultaneously:
1)
2)
3)
4)
5)
March 2017
Visibility is 10 km or more.
No minimum visibility is reported.
No cloud below 5000 ft or Minimum Sector Altitude
(whichever is the greater).
No cumulonimbus (CB) or towering cumulus cloud (TCU)
at any level.
No significant weather at or in the vicinity of the
aerodrome.
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Cloud of
operational
significance
A cloud with the height base below 1500 m (5000 ft) or below the
highest minimum sector altitude, whichever is the greater, or a
cumulonimbus cloud or a towering cumulus cloud at any height.
Displayed Gust
This is a wind speed, averaged over a 3 second sample, that has
increased from the 2 or 10 minute mean wind speed by 10 knots
or more.
Equipment Failure
The inability of equipment to fulfil its operational requirements.
Failure may be systematic or due to a physical change.
Gust
This is the peak wind speed averaged over a 3 second period.
Height
The vertical displacement of a level, point, or object considered as
a point measured from a specified datum.
Maintenance
The preservation or restoration of the required system
performance over the system lifecycle.
May
Used to indicate that the following clause is optional, alternative, or
permissive.
METAR
A meteorological report in the format prescribed by the World
Meteorological Organisation for worldwide dissemination of
aeronautical meteorological information.
Meteorological
Bulletin
A text comprising meteorological information preceded by an
appropriate heading.
Meteorological
observation
A snap-shot of the state of the atmosphere and environment at a
given time; comprising one or more meteorological elements,
carried out either by visual estimation or with the aid of appropriate
instrumentation.
Meteorological
report
A statement of the observed meteorological conditions, related to
a specific time and location, and prepared according to a
prescribed format for subsequent issue to users.
Mitigation
Steps taken to control or prevent a hazard from causing harm and
reduce risk to a tolerable or acceptable level.
Observation
(Meteorological)
The evaluation of one or more meteorological events.
Observing system
A machine that receives data from one or more instruments
measuring meteorological elements and may performs tasks such
as data logging, processing and display.
OPMET
Aeronautical meteorological data, describing METAR, TAF,
SIGMET, GAMET, Volcanic Ash Graphics.
Precision approach
runway
An instrument runway intended for the operation of aircraft using
precision approach aids that meet the Facility Performance
requirements defined in ICAO Annex 10 appropriate to the
Category of Operations.
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Prevailing Visibility
Chapter 11: Definitions, abbreviations and bibliography
The greatest visibility value, observed in accordance with the
definition of “visibility”, which is reached within at least half the
horizon circle or within at least half of the surface of the
aerodrome. These areas could comprise contiguous or noncontiguous sectors.
NOTE: This value may be assessed by human observation
and/or instrumented systems. When instruments are
installed, they are used to obtain the best estimate of the
prevailing visibility.
Primary Sensor
The sensor normally used to determine the element that is being
measured.
QFE
QFE is the atmospheric pressure corrected to a specific elevation
e.g. the official aerodrome elevation or the runway threshold
elevation.
QNH
QNH is the atmospheric pressure corrected to mean sea level,
assuming International Standard Atmosphere conditions across
the height difference.
Reliability
The ability of a system to perform a required function under given
conditions for a given time interval.
Requirement
A requirement is an expressed or implied need that is satisfied
through appropriate compliance action. A requirement may call for
compliance to such standards, codes of practice, or specifications
as considered appropriate by the regulator.
Routine
Maintenance
Maintenance at regular periodic intervals, identified at the systems
design stage of equipment, functions, components etc., which are
known to cause or potentially cause degradation to the required
system performance.
Runway
A defined rectangular area on a land aerodrome prepared for the
landing and take-off run of aircraft along its length.
Runway visual
range (RVR)
The range over which the pilot of an aircraft on the centre line of a
runway can see the runway surface markings or the lights
delineating the runway or identifying its centre line.
Shall (is to, are to,
and must)
Means that the requirement or instruction is mandatory.
Should
Means that it is strongly advisable that an instruction or action is
carried out, it is recommended or discretionary. It is applied where
the more positive ‘shall’ is unreasonable but nevertheless a
provider would need good reason for not complying.
SIGMET
Information
Information issued by a meteorological watch office concerning the
occurrence or expected occurrence of specified en-route weather
phenomena which may affect the safety of aircraft operations.
Specification
A precise technical definition of the required parameters or
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Chapter 11: Definitions, abbreviations and bibliography
performance to be achieved.
Special
observation
A weather report made following a deterioration or improvement in
a weather element through pre-determined criteria, that does not
occur at the time of the routine weather observation.
Standard
Characteristics, methods, principles and practices that can be
used to satisfy a requirement.
Threshold QFE
QFE is the atmospheric pressure corrected to the runway
threshold elevation.
Touchdown zone
The portion of a runway, beyond the threshold, where it is intended
landing aeroplanes first contact the runway.
VOLMET
A Very High Frequency broadcast, typically providing METAR
reports for a maximum of nine aerodromes on each channel in a
continuous loop. Intended for aircraft in flight, it has a range of
around 300 nautical miles at FL300.
Visibility
Visibility for aeronautical purposes is the greater of:
1)
2)
the greatest distance at which a black object of suitable
dimensions, situated near the ground, can be seen and
recognized when observed against a bright background;
the greatest distance at which lights in the vicinity of 1
000 candelas can be seen and identified against an unlit
background.
NOTE: The two distances have different values in air of a given
extinction coefficient, and the latter b) varies with the
background illumination. The former a) is represented by
the meteorological optical range (MOR).
Abbreviations
AAIB
Air Accident Investigation Branch
AFS
Aeronautical Fixed Service
AFTN
A
C
Aeronautical Fixed Telecommunication Network
AIP
Aeronautical Information Publication
ANO
Air Navigation Order
ATC
Air Traffic Control
ATIS
Automatic Terminal Information Service
ATS
Air Traffic Service
CAA
Civil Aviation Authority
CAP
Civil Aviation Publication
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Chapter 11: Definitions, abbreviations and bibliography
CAT
D
E
CB
Cumulonimbus
DfT
Department for Transport
EASA
EC
F
ft
H
hPa
I
Category
European Aviation Safety Agency
European Commission
Foot (feet)
Hectopascal
ICAO
International Civil Aviation Organisation
IRVR
Instrumented Runway Visual Range
km
Instrumented Runway Visual Range
kt
Knots
K
Met
M
METAR
m
N
R
S
T
U
W
Meteorology/Meteorological
Aviation routine weather report
Metre(s)
mm
Millimetres
NSC
No Significant Clouds
NCD
No Cloud Discernible (reported by automatic observing systems only)
REG
Regulation
ROP
Runway Observation Point
RVR
Runway Visual Range
SADIS
Satellite Distribution System
SARP
Standards and Recommended Practice
TCU
UK
Towering Cumulus
United Kingdom
UTC
Universal Co-ordinated Time
WMO
World Meteorological Organisation
Bibliography
ICAO Annex 3, Meteorological Service for International Air Navigation, International
Civil Aviation Organization
ICAO Annex 11, ATC/FIS Alerting Service, International Civil Aviation Organization
March 2017
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Chapter 11: Definitions, abbreviations and bibliography
ICAO Doc 4444, Procedures for Air Navigation Services Rules of the Air & Air Traffic
Services (PANS-RAC), International Civil Aviation Organization
ICAO Doc 8896, Manual of Aeronautical Meteorological Practice, International Civil
Aviation Organization
ICAO Doc 9328, Manual of RVR Observing and Reporting Practices, International
Civil Aviation Organization
EASA Common Requirements Regulation (EC REG 1035/2011)
SES Service Provision Regulation (EC REG 550/2004)
SES Interoperability (IOP) Regulation (EC REG 552/2004)
WMO Document No. 306, Manual on Codes, Volume 1, Part A; FM15-X Ext.
METAR, World Meteorological Organization, Geneva
WMO Document No. 731, Guide on Meteorological Observation and Information
Distribution Systems at Aerodromes, World Meteorological Organization, Geneva
WMO Document No. 732, Guide to Practices for Meteorological Offices Serving
Aviation, World Meteorological Organization, Geneva
WMO Document No. 782, Aerodrome Reports and Forecasts - User’s Handbook to
the Codes, World Meteorological Organization, Geneva
UK Aeronautical Information Publication, National Air Traffic Services Ltd.
CAA CAP 168 Licensing of Aerodromes, UK Civil Aviation Authority
CAA CAP 493 Manual of Air Traffic Services (Part 1), UK Civil Aviation Authority
CAA CAP 393 Air Navigation: The Order and the Regulations, UK Civil Aviation
Authority
CAA CAP 782 Regulation of Aeronautical Meteorological Services, UK Civil Aviation
Authority
CAA CAP 797 Flight Information Service Officer Manual, UK Civil Aviation Authority
CAA CAP 670 ATS Safety Requirements, UK Civil Aviation Authority
March 2017
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Met Office Document No. 2612, Register of Observations (METAR), UK Met Office
Met Office Document No. 796, Observing Codes for Use at UK Civil Aeronautical
Stations, UK Met Office
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Purpose of the aerodrome meteorological regulatory oversight audit
Appendix A
Purpose of the aerodrome meteorological
regulatory oversight audit
Introduction
A.1
In order to comply with ICAO SARPs, the UK Meteorological Authority
arranges for aerodrome meteorological observing units to be visited at
regular intervals. The aim of these visits is to ensure that a high standard
of observations is maintained, instruments and their displays are
functioning correctly and to check the exposure of instruments.
A.2
The UK Meteorological Authority requires regulatory oversight audits to
take place at aerodromes that are Designated Meteorological or Air Traffic
Service (ATS) Air Navigation Service Providers (ANSPs). All ANSPs that
are certificated under the EASA Common Requirements Regulation (EC
REG 1035/2011) as a Meteorological ANSP (i.e. those that provide
METARs which are disseminated beyond the aerodrome) are required to
be visited annually. Additionally, aerodromes that publish instrument
runway procedures are required to have annual audits. Those
aerodromes that are Designated as ATS ANSPs and do not provide
METARs are required to have audits every 2 years.
A.3
The Met Authority may vary the frequency of oversight audits based on an
assessment of the risks associated with the operations of each
organisation.
A.4
The audit will check local meteorological procedures and inspect the
standard of weather reports, instrumentation and, if applicable,
meteorological flight briefing documentation provided. The inspector may
also offer help and advice regarding the provision of meteorological
services to both the airport management, observers and users.
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A.5
Purpose of the aerodrome meteorological regulatory oversight audit
The visit may be used to send completed Register of Observations
(Metform 2612) to the Public Record archive and to request new
Registers.
A.6
The Met Office will supply Registers of Observations (Metform 2612) free
of charge, on the basis that upon completion, these registers may be
archived by the Met Office. Archived data and registers may be used in
generating climatological data, as noted in the UK AIP (GEN table
3.5.3.2).
Conduct of meteorological regulatory oversight audit
A.7
Meteorological instrumentation will usually be checked during the visit.
The exposure and accuracy of the instrumentation will be reviewed, as
well as procedures for use of the instrumentation. Backup sensors and
contingency arrangements will also be reviewed.
A.8
The Aerodrome Meteorological Observing Service Provider’s
arrangements for ensuring that all accredited Aerodrome Meteorological
Observers maintain their observing competence will be reviewed, and a
check will be made of the Service Provider’s annual assessments of
accredited observers.
A.9
A review of various regulatory documents will be carried out and to assist
with the review the Aerodrome Meteorological Observing Service Provider
is asked to make relevant documentation readily available for inspection
during the audit, or if appropriate, to provide documents prior to the audit,
which may include:
i) An inventory of meteorological observing equipment;
ii) Procedures for maintenance and calibration of meteorological
observing equipment;
iii) Maintenance and calibration records;
iv) The unit Manual of Air Traffic Services Part 2;
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Purpose of the aerodrome meteorological regulatory oversight audit
v) Records of accredited Aerodrome Meteorological Observers
A.10
The quality and regularity of METAR observations will be checked prior to
the visit and any issues raised on the day.
A.11
The aerodrome entry in UK AIP GEN table 3.5.3.2, AD 2.11 and any
weather-related warnings contained within AD 2.20 will be reviewed
during the visit and details of any amendments taken.
A.12
Compliance with EASA Common Requirements Regulation (EC REG
1035/2011), in particular Annex 3, Meteorological Provision, will be
audited. Two specific aspects will be reviewed, in accordance with the
Regulation; firstly technical and operational competence and capability,
secondly working methods and operational practices.
A.13
Following the visit, a copy of the report detailing any findings arising from
the audit shall be sent to the Manager of the Aerodrome Meteorological
Observing Service Provider. The unit management will be invited to
respond to the findings prior to the target date specified in the report.
A.14
Time during the visit will also be made for meetings with users of
meteorological information on the aerodrome, e.g. airlines, flying clubs,
handling agents, to discuss the meteorological services provided for their
operations. A summary of the discussions will be included in the report.
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Frequently asked questions on the compilation of the METAR
Appendix B
Frequently asked questions on the compilation of
the METAR
Introduction
B.1
There are a number of common queries that are often raised when
completing the METAR. Full details can be found in the earlier sections of
Chapter 6 but the most frequently asked questions are given below to
assist the observer.
FAQs
Wind
B.2
Which anemometer is used for the METAR report?
Whatever runway is in use, the wind velocity for the METAR is normally
taken from one designated anemometer, and usually is averaged over 10
minutes. It may include a gust speed recorded in that 10 minute period
when appropriate.
B.3
How is the wind direction reported and when is a variation group
included?
Wind direction shall be recorded in degrees true, and shall include a
variation group, if, during the previous 10 minutes the direction has varied
though an arc of 60 degrees or more but less than 180 degrees and the
mean speed during the previous 10 minutes is more than 3 knots.
Visibility
B.4
What visibility is reported in the METAR?
In the METAR, the prevailing visibility shall be reported. If the visibility in
one direction, which is not the prevailing visibility, is less than 1500 m or
less than 50% of the prevailing visibility, the lowest visibility observed shall
also be reported and its general direction in relation to the aerodrome
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indicated by reference to one of the eight points of the compass. If the
lowest visibility is observed in more than one direction, then the most
operationally significant direction should be reported. When the visibility is
fluctuating rapidly and the prevailing visibility cannot be determined, only
the lowest visibility should be reported, with no indication of direction.
Examples of how to observe and report prevailing visibility are given in
Annex A to this Appendix.
Present weather
B.5
When is mist reported?
Mist shall be reported when the prevailing visibility is between 1000m or
more and 5000m or less and the relative humidity will be 95% or more,
and as a guide, the difference between the dry bulb and the dew point
temperature is usually 1 0C or less.
NOTE: If the prevailing visibility is 5000m or less, relative humidity is less than 95% and the
difference between the dry bulb and the dew point temperatures is greater than 1
0
C you should consider whether the reduced visibility is caused by dust, smoke
or haze.
B.6
How often does heavy rain occur on average?
Care should be taken to avoid over-estimating the intensity of
precipitation. Statistically, in the United Kingdom, light rain falls on 80% of
occasions and heavy rain falls on less than 5% of occasions.
B.7
When should separate weather groups be reported?
Although up to three weather types may be reported, they should be
occurring independently; e.g. a mist or fog group shall not inserted if the
reduction in visibility is due wholly to falling precipitation.
B.8
How is 10 km visibility and broken cloud at 5500 ft reported?
CAVOK should be used in this instance.
Cloud
B.9
How is 8 km visibility and broken cloud at 5500 ft reported?
When CAVOK does not apply, the visibility, present weather (if applicable)
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Frequently asked questions on the compilation of the METAR
and cloud groups shall be reported. In this instance, this should be coded
as 8000 NSC, provided that the cloud layers are not TCU or CB.
B.10
When should significant convective cloud types be reported?
There are two types of significant convective cloud that is reported in the
METAR, towering cumulus and cumulonimbus (TCU and CB). They are
associated typically with moderate or heavy showers, but hail and thunder
are associated only with cumulonimbus. Towering cumulus and
cumulonimbus should be reported in the METAR whenever visible to the
observer. There is no intentionally agreed criteria for reporting towering
cumulus cloud but as a guide, the cloud will be at least 10000 ft tall from
base to top.
B.11
How should I assess the amount of cloud to report in the METAR, if
there is no blue sky visible but it is clearly sunny through gaps in the
cumulus / towering cumulus / cumulonimbus cloud cover?
It is very easy to significantly overestimate (sometimes by as much as 4
oktas) the amount of large convective cloud present, by erroneously
including the sides of such cloud. Cloud amounts in the METAR refer only
to the amount of cloud base present. This automatically discounts cloud
sides from the amount to be reported when considering convective cloud.
Observers should take care to report only convective cloud bases in
METAR reports.
B.12
Is there an easy way to estimate the height of ‘cotton wool’ clouds?
A rough estimate of the height of fair weather convective cumulus cloud
base in hundreds of ft may be obtained by multiplying the difference in
dry-bulb and dew point temperatures by four hundred.
B.13
When should ‘Sky Obscured’, VV/// be reported?
When the sky is obscured due to fog, falling or blowing snow**, ‘VV/// is
reported in lieu of cloud information. If during the day, cloud or blue sky
can be seen through the fog or blowing snow or, at night, stars are visible,
then the sky is said to be visible and cloud would be reported as
appropriate. If, due to thick fog or heavy precipitation (usually snowfall),
the sky, stars or cloud cannot be seen then Sky Obscured, VV/// would be
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Frequently asked questions on the compilation of the METAR
reported. As a general guide you should consider reporting VV///
whenever the visibility is less than 200 metres unless there is definitive
visible evidence of a cloud layer.
**NOTE:
Other phenomena that could cause the sky to be obscured are blowing dust,
blowing sand, volcanic ash and in offshore environments, sea spray.
B.14
Our automated met system is recording OVC002, a prevailing
visibility of 250 metres and fog is present at the airfield; is it correct
to report these conditions?
With conditions as stated there is a high probability that the sky will be
obscured and VV/// should be reported. Automated sensors are to be
considered as an aid to the observer rather than a direct source of
information. Given the limitations of sensors observers should always
verify automated readings and in this example should confirm the
accuracy of the readings by making their own visual assessment of
conditions before submitting the report.
Temperature
B.15
Can the dry bulb temperature be colder than the dew point
temperature?
No. A check should be made that the dry bulb temperature is equal to or
warmer than the dew point temperature, particularly when both are
negative.
Pressure
B.16
Which pressure is reported in the METAR – the airfield threshold
pressure or the mean sea level pressure?
Care should be taken to report the aerodrome QNH (mean sea level
pressure) in the METAR.
Recent significant weather
B.17
When should recent weather be reported?
If since the last routine report, precipitation or blowing snow has been
moderate or heavy and has now ceased or decreased in intensity, a
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recent weather group will be required. This also applies if light freezing
precipitation, a thunderstorm or funnel cloud has recently ceased.
Intensity symbols are not used when reporting recent weather.
Runway state group
B.18
When should a runway state group be reported?
These will normally be inserted when the runways are contaminated with
snow, slush or other contaminants to the extent that the runway
characteristics are affected. A repeat of the previously reported group
should be indicated in the METAR by changing the first two digits of the
runway state group to ‘99’.
Observing from outside the building
B.19
Is it necessary to carry out observations from outside the building?
Yes, in certain weather conditions the observer may need to assess
elements of the weather from an outside observing position that is close to
ground level.
Final checks
B.20
Are there any other actions to take before transmission of the
METAR?
If time allows, the completed message should be checked through and
compared with previous reports for consistency; typographical errors are
not uncommon (for example typing 300 m visibility instead of 3000 m).
Restricted meteorological observer’s (RMO) certificate
B.21
The aerodrome’s SAMOS is not capable of coding the METAR. I am
an accredited Restricted Met Observer – can I manually code and
disseminate the METAR?
No, if there is a system problem which means that the semi-automated
observing system is unable to code the METAR an accredited observer
holding a restricted certificate will be limited to providing Official local
weather reports.
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B.22
Frequently asked questions on the compilation of the METAR
The aerodrome’s SAMOS is not receiving data from the
wind/temperature/pressure sensor but the system is otherwise fully
operational. I am an accredited Restricted Met Observer – can I
manually enter details read from the contingency sensor(s) allowing
the system to automatically code and disseminate the METAR?
Yes, if the semi-automated observing system is able to code the METAR
an accredited observer holding a restricted certificate can manually enter
the relevant weather elements so that the system can code and
disseminate the METAR.
Annex A to Appendix B
Assessing prevailing visibility
1.
Mentally divide the horizon circle into as many sectors of equal visibility as
needed to measure the differing visibilities, as in the following example:
Figure 1: Assessing prevailing visibility
2.
Prevailing visibility is the greatest visibility value which is reached within at
least half the horizon circle or within at least half of the surface of the
aerodrome; in the above example, the visibility value which is reached
within at least half the horizon circle is 8 kilometres (the area covered by
the 8km and 12km visibility sectors).
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3.
Frequently asked questions on the compilation of the METAR
The lowest visibility is also reported in the METAR when it is less than
1500 metres (providing it is not the prevailing visibility) or less than 50% of
the prevailing visibility, along with its general direction. In the above case,
the lowest visibility (4 kilometres) is not reported, as it is not less than 50%
of the prevailing visibility.
4.
Important points to note:

RVR is included in the METAR whenever the lowest visibility is less
than 1500 metres.

Present weather phenomena such as smoke, mist and fog should be
included in the METAR report if the prevailing visibility (or minimum
visibility, if reported) reaches the criteria for inclusion of that
particular weather group (i.e. 5000 metres or less for smoke, mist,
haze and less than 1000 metres for fog and freezing fog). Note that
present weather codes for fog patches (BCFG), fog covering a
partial part of the aerodrome (PRFG) and fog in the vicinity of the
aerodrome (VCFG) may be reported whatever the visibility reported.
Figure 2: Examples of reporting prevailing visibility in the METAR code
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Human observed RVR conversion table
Appendix C
Human observed RVR conversion table
Foreword
C.1
This Appendix sets out the requirements for producing the Human
Observed RVR Conversion Table using a distance based methodology.
C.2
The UK Meteorological Authority is responsible for the policy on the
production of the Human Observed Runway Visual Range Conversion
Table.
C.3
A distance based methodology shall be used for creating this table.
C.4
Aerodromes shall issue an RVR Conversion Table every 3 years or
following any changes to the runway lighting system.
C.5
Aerodromes shall ensure that all relevant runway lights are illuminated at
the correct intensity in order to use the Human Observed RVR Conversion
Tables.
C.6
Details on the use of the Human Observed RVR Conversion Table are
found in CAP 168 Licensing of Aerodromes Appendix 2A.
Distance based method
C.7
This method establishes the distance from the ROP to each of the runway
edge lights on the opposite side runway from the ROP. Using the far side
lights provides the observer with a better assessment of the individual
lights along the runway than would be achieved using the same side
lights. With this method the straight line distance from the ROP to each
light is derived from the aerodrome survey (detailed in CAP 232
Aerodrome Survey Information Chapter 5) and this becomes the reported
RVR, noting the UK standard reporting steps:
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Human observed RVR conversion table
0 to 400 m in 25 m steps
400 to 800 m in 50 m steps
800 to 1500 m in 100 m steps
C.8
The aerodrome is responsible for producing a Human Observer RVR
Conversion Table (an example is provided at Annex B).
Reference copies
C.9
HORVR conversion tables should be retained and managed by the
aerodrome as part of the unit document management procedure.
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Human observed RVR conversion table
Annex A to Appendix C
Distance based method example
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Daily atmospheric pressure equipment QNH check
Appendix D
Daily atmospheric pressure equipment QNH
check
D.1
Atmospheric pressure measuring equipment shall be checked daily for
signs of sensor drift by comparison with other pressure instrumentation on
the aerodrome. However, the check should not take place if the mean
wind speed exceeds 25kts, or when the pressure change is greater than 1
hectopascal per hour, as this may adversely affect the comparison.
D.2
The Aerodrome QNH (to the nearest tenth of a hectopascal) should be
used in the comparison.
D.3
The use of the table below to record the daily pressure check may
indicate sensor calibration drift before the sensor reaches the limits of
allowed accuracy. However, if differences consistently reach 0.5hPa then
calibration of the barometer should be brought forward.
Date
Primary sensor
Backup sensor
Difference
…
…
…
…
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Theoretical observer training requirements
Appendix E
Theoretical observer training requirements
Introduction
E.1
The objective of the theoretical training is to provide tuition in the skills
required to accurately and reliably produce weather reports to ATS and to
issue reports in the METAR format.
E.2
The theoretical training is intended to be classroom based. On successful
completion of the theoretical training, the trainee will be eligible to go
forward for practical observer training session and, on suitable completion
of this, will be awarded a meteorological observing certificate.
Training programme
E.3
The purpose of the theoretical training is to instruct the trainee observers
on observing techniques and how to correctly encode and decode
weather reports. The trainee will also be given instruction on how to carry
out simple care and maintenance of instruments.
E.4
Details of approved training courses are included in the UK AIP Section
GEN 3.5.
Syllabus
E.5
The training shall provide instruction on the following:

The process of compiling and preparing weather reports, especially
using the METAR code.

The reporting of the surface wind measurements, including backup
facilities.

The observing and reporting of visibility by day and by night.

Runway visual range (RVR) - familiarisation with the code and when
to report.
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Theoretical observer training requirements

The observing and reporting of “present weather” in its various
forms, including the relationship between humidity and reporting of
Mist/Haze.

The observing and reporting of cloud base and height, including
when to report CAVOK, no significant cloud and sky obscured.
Identification of convective clouds and their reporting in the METAR
code.

The reporting of temperature and determination of dew point values,
especially the attention and care required when reporting sub-zero
temperatures.

The observation of atmospheric pressure and the reporting of QNH
and QFE.

Runway State - familiarisation with the code and when to report.

When and how to report “Recent Weather”.

The criteria and process for issuing special weather reports.

Familiarisation with OPMET bulletins containing METAR reports.

Interpretation of standard weather charts to self-brief on the
prevailing weather
Examinations
E.6
At the end of the theoretical training, the trainee will demonstrate their
understanding of the theory by sitting a written examination. The trainee is
expected to obtain a pass in order to go on to practical training.
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Appendix F
Practical observer training requirements for a
meteorological observer’s certificate (manual
observed weather reports)
Introduction
F.1
The purpose of the practical training is to apply the theoretical knowledge
gained in the context of an operational environment and to enable the
practical competence of the observer to be assessed.
F.2
The practical training will concentrate on the correct application of
meteorological observing techniques, use of a range of meteorological
instrumentation including, but not limited to, semi-automated observing
systems.
F.3
Whenever possible, practical training should follow immediately after the
theory training. This will allow the student to gain confidence in using
instrumentation, as well as allowing an assessment of the observing
competence of the student to be carried out.
F.4
It is recommended that the formal practical observer training should last
between 4 days and 2 weeks.
Training organisations
F.5
The training programme, examinations and competency of aerodrome
meteorological observers shall be approved by the UK Meteorological
Authority. This is to ensure that the standard of observing training is
uniform for all students. Details of the approval process are available from
the UK Meteorological Authority on request.
F.6
Practical training shall be supplied by suitably experienced observers,
having at least 5 consecutive years’ experience of aviation meteorological
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observing. Ideally the experience will have been gained from working at
more than one aerodrome and also have included experience of
producing both fully manual observations and observations assisted by
automated meteorological weather observing systems. Thus observers
providing practical training will have a wide perspective on different
observing methodologies used in the UK.
F.7
A training supervisor (who may also be one of the observing staff
discussed in Appendix H, paragraph H5) should be assigned to the
student to ensure that all aspects of the training programme are covered
and to act as a mentor for the student. The training supervisor will also
have at least 5 consecutive years’ experience of aviation meteorological
observing and preferably some experience of synoptic weather observing.
The examination and/or report may be delegated to another member of
the observing staff, if required.
F.8
Copies of the following publications should be held at the observing office
for the use of trainees during training:

Metform 716 Cloud Types For Observers

Metform 796 Meteorological Observing Codes For Use In METARs

Metform 2612 METAR Register Of Observations, or equivalent form
for manually recording observations.
Training programme
F.9
The practical training will cover the following aspects:

Manual observing techniques

Observing during daylight

Observing during darkness (including the transition between day /
night)
F.10
During the practical training, continuous assessment shall be made of the
trainee’s ability to observe and correctly record the weather. The trainee
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shall also undertake an oral examination during the course of the second
week.
Assessment details during the practical training.
F.11
The assessment during the second week of training will entail a written
paper on observing and coding, and oral questions. Part of the
assessment will also involve a competency assessment of the student
based on aspects of aviation observing.
F.12
At any point if the required standard is not being met, the student must be
notified of deficiencies and of what standard is required. The student
should also be encouraged to seek guidance from the mentor or member
of the observing staff if they are concerned about meeting any
competency.
F.13
To complete this course successfully the student must:

Achieve a pass mark of at least 75% on the practical exam
(consisting of at least 75% in the following aspects – ‘Observing and
use of instruments’ & ‘Oral Questions’).

Achieve a pass mark of at least 75% on the written exam.

Pass all of the competencies and have them signed off by the end of
the training.
F.14
Anyone who fails to complete ALL the competencies or if a pass mark is
not attained in any of the examinations, will not receive accreditation.
F.15
Under some circumstances such as extremely benign weather or illness, it
may not be possible to complete all competencies in the allocated time. In
this case, assessment should be made by additional oral questions that
explore weather scenarios that have not been met during the practical
training.
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Meteorological observers competence-based document
F.16
A competence is a normal task completed in the course of making
weather reports. Part of the routine duties of ATS personnel at airports will
be to observe, code and transmit METAR reports.
F.17
In order to complete an observation successfully, a number of tasks must
be carried out. This document sets out the standard that the student must
achieve in each task. He or she must aim to reach this level of
achievement during the practical training, and maintain this standard
through to the end of the course.
F.18
Problems may arise when assessing competence in certain aspects of
weather observing if the weather situation remains constant through much
of the assessment period. In this case, the comments box should be fully
utilised by the examiner. Aspects such as overall theory knowledge may
be taken into account.
F.19
Details regarding completion of the competency document and a copy of
a form are reproduced as Annex A to this Appendix.
Operational competence
F.20
Following the award of the meteorological observing certificate the trainee
observer should undertake a period of observing “on the job” under
supervision (see Appendix H, Competency of observers). This is to
ensure that the observer is fully confident and capable of providing reports
under operational conditions.
F.21
No observer will be immediately fully experienced in all types of weather
conditions. For this reason, even when an observer starts operational
observing without supervision, there may be occasions when some
guidance is necessary from a more experienced observer when a weather
event is encountered for the first time.
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Continuous assessment
F.22
Every accredited observer should be assessed on an annual basis by the
Manager, or other nominated person, of the Aerodrome Met Observing
Service Provider to ensure the observer’s ongoing competence (see
Appendix H, Competency of observers).
Annex A to Appendix F
Table 8: Observing competencies to be assessed during the practical simulations
Elements
Cloud
(see note)
Tasks
Criteria
Recognise and name cloud types
relevant to METAR reports
Correctly identify all cloud types
observed relevant to METARs
Estimate cloud amount, total and
layers
Accurately estimate cloud amount
in each layer ±1 okta
Estimate cloud heights
Accurately estimate cloud height
±30% if not using the LCBR
Encode cloud data
Correctly encode cloud data in the
METAR register
Estimate the visibility
Accurately estimate visibility using
visibility points ±20%
Encode visibility data
Correctly encode visibility data in
the METAR register
Read the various types of
thermometers available
Accurately read all types of
thermometers to ±0.1°C
Encode temperature data
Correctly encode temperature data
in the METAR register
Estimate mean wind speed and
direction
Estimate wind speed to the nearest
force on the Beaufort scale and
estimate wind direction using 16
point compass, ±1 compass point
Assess mean and gust wind
speeds and direction from wind
display systems including
significant variations
Correctly obtain mean and extreme
values from wind dials
Encode wind data
Correctly encode wind data in the
METAR register
Recognise and record the weather
Identify variations in weather types,
Visibility
Temperature
Wind
Weather
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(see note)
Pressure
Night
observations
types and intensities that make up
the Present and Recent weather
codes used in observations
intensities and persistence. This
will be checked using exercises
and where possible observing
simulations
Encode weather data
Correctly encode weather data in
the METAR register
Read Precision Aneroid
Barometers and apply instrument
and pressure level corrections for
QNH and QFE
Correctly read Precision Aneroid
Barometers and apply instrument
and pressure level corrections
Demonstrate the ability to observe
during hours of darkness
Estimate cloud detail, weather
conditions and visibility to an
acceptable level of accuracy
To have all the routine observations
coded up in METAR format on time
Ensure routine observations are
completed within the specified time
limits of:
T+50: begin no earlier than &+40
and complete METAR by T+55
To complete
all
observations, To report and record all applicable
special reports
and on time
Demonstrate the ability to record
and disseminate observations
using local backup procedures
NOTE:
T+20: begin no earlier than T+10
and complete METAR by T+25
Ensure specials are made when
applicable without delay and record
appropriately
Maintain a suitable level of
accuracy and regularity of
observations using appropriate
contingency observing equipment
and procedures
Clouds and weather – if little variety is seen during the week, overall theory
knowledge on the subject may be taken into account for assessment purposes.
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Training requirements for a restricted meteorological observer’s certificate
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Appendix G
Training requirements for a restricted
meteorological observer’s certificate
Introduction
G.1
The training aims to provide tuition in the theory and practical skills
required to make semi-automated weather reports to the standard
necessary to ensure the safety of aircraft. On completion the trainee is
awarded a restricted meteorological observing certificate. The certificate
will only apply to observing duties where the Aerodrome Meteorological
Observing Service Provider uses approved semi-automated systems.
G.2
Weather reports to ATS may be compiled using an approved semiautomated observing system but the dissemination of a METAR requires
validation of the visual elements (visibility, present weather and cloud)
before transmission.
Training programme
G.3
Training shall be carried out to enable the observer to visually evaluate
the visibility, present weather, cloud amounts, height of cloud bases, and
presence of significant convective clouds. This will allow either these
parameters to be added to the weather report, or if sensors are available
on the semi-automated system for recording these that they are validated
before the weather report is disseminated.
G.4
The theory training should last two days followed by a period of
supervised observing of at least three days.
G.5
Aerodrome weather reports may be compiled using an approved semiautomated observing system. This allows the non-visual elements to be
generated from sensor measurements, and the visual elements to be
assessed by a qualified observer. Training is necessary to enable the
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observer to visually evaluate the visibility, present weather, cloud
amounts/bases, and presence of significant convective clouds (TCU and
CB) so that the sensor readings can be qualified.
G.6
The observer should be aware of the operational requirements for METAR
reports at the aerodrome. At aerodromes where TRENDs are attached to
the METAR reports, the observer should understand the procedures for
obtaining the TREND message from the meteorological forecasting office.
G.7
The observer should understand the different requirements between
weather reports provided for ATS and those disseminated beyond an
aerodrome, in particular the METAR.
G.8
The observer should be able to interpret the self-briefing weather
documentation available on an aerodrome.
Syllabus
G.9
The syllabus should cover the following aspects, in line with the
requirements specified above:

The METAR and introduction to ICAO Annex 3 Chapter 4.

The purpose of the METAR to flight operations and forecasting.

Differing requirements for surface wind reports ATC/METAR.

Estimating visibility - differences between visibility reported in the
METAR, slant visibility and when reported, RVR.

Atmospheric conditions which result in reduced visibility.

Observing present weather/recording past weather - limitations of
sensors.

Explanation of freezing precipitation and implications for observing.

Association of precipitation with synoptic weather patterns.

Identification of convective clouds.

Operational significance of TCU/CB clouds.

Estimating cloud amounts and cloud bases - limitation of sensors.

CAVOK.

Special reports and reports in the case of an aircraft incident.

Procedures for attaching a TREND to a METAR.
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
Supplementary groups on METAR reports.

Self-briefing information.
Practical training for a restricted meteorological observer’s
certificate
G.10
Following the classroom instruction a period of supervised observer
training shall be arranged. At the completion of the training programme
trainees shall be assessed by a written test, on successful completion of
which they will be awarded a restricted meteorological observer’s
certificate. This will differ from the certificate awarded to those observers
who attend the full standard training programme and will only apply to
observing using semi-automated observing systems.
Operational competence
G.11
Following the award of the meteorological observing certificate the trainee
observer should undertake a period of observing “on the job” under
supervision. This is to ensure that the observer is fully confident and
capable of providing reports under all types of meteorological conditions.
G.12
No observer will be immediately fully experienced in all types of weather
conditions. For this reason, even when an observer starts operational
observing without supervision, there may be occasions when some
guidance is necessary from a more experienced observer when a weather
event is encountered for the first time.
Continuous assessment
G.13
Every accredited observer should be assessed on an annual basis by the
Manager, or other nominated representative, of the Aerodrome Met
Observing Service Provider to ensure the observer’s ongoing competence
(see Appendix H, Competency of observers).
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Relocation training
G.14
In the event that an observer is transferred from one ATS unit to another a
short period of local familiarisation training shall be given before
commencing unsupervised operational meteorological observing duties at
the new unit.
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Competency of observers
Appendix H
Competency of observers
H.1
The observer shall be required to demonstrate competence in all aspects
of meteorological observing under normal working conditions.
H.2
Table 8 shows all of the competencies that an observer may be required
to demonstrate. The specific competencies required will be dependent on
the aerodrome, type of meteorological observing equipment used and
level of instrument equipage.
H.3
Consideration shall be given, in particular, to the observing and coding
and any supplementary information required to be provided with relatively
rare weather events, such as thunderstorms or snow, as well as observing
backup procedures.
H.4
Table 10 provides additional areas that it may be desirable for an
observer to have knowledge or awareness of; however this does not form
part of the competence assessment.
H.5
Every accredited observer should be assessed on an annual basis by the
Manager, or other nominated person, of the Aerodrome Met Observing
Service Provider to ensure the observer’s ongoing competence.
H.6
A record of observer competency checks shall be kept for a minimum of
12 months including the date that the check was carried out.
Table 9: Meteorological observing competencies
Element
Tasks
Competence assessment
Wind
Assess mean and gust wind
speeds and direction from wind
display systems including
significant variations
Correctly obtain mean and extreme
wind speed and direction values as
required
Encode wind data
Correctly encode wind data as
required
Estimate the visibility
Correctly encode visibility using
Visibility
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Competency of observers
visibility points
Runway
visual range
Weather
Cloud
Temperature
Pressure
Night
observations
To complete
all
observations,
and on time
Encode visibility data
Correctly encode visibility data as
required
Obtain runway visual range from
either human observed RVR
conversion tables or using IRVR
system
Correctly obtain runway visual
range information as required
Encode runway visual data
Correctly encode runway visual data
as required
Recognise and record the
weather types and intensities that
make up the Present and Recent
weather codes used in advance
Identify variations in weather types,
intensities and persistence. This
may be checked using exercises
and observing simulations
Encode weather data
Correctly encode weather data
required
Recognise and name cloud types
relevant to METAR reports
Correctly identify all cloud types
observed relevant to METARs
Estimate cloud amount, total and
layers
Estimate cloud amount in each layer
±1 okta
Estimate cloud heights
Estimate cloud height to within
±30% if not using a ceilometer
Encode cloud data
Correctly encode cloud data as
required
Read the various types of
thermometers available
Accurately read thermometry to
±0.1°C
Encode temperature data
Correctly encode temperature data
as required
Read pressure sensors and apply
instrument and pressure level
corrections for QNH and QFE
Correctly read pressure sensors
and apply instrument and pressure
level corrections as required
Demonstrate the ability to
observe during hours of darkness
Estimate cloud detail, weather
conditions and visibility to an
acceptable level of accuracy
To have all the routine
observations coded up in METAR
format on time
Ensure routine observations are
completed within the specified time
limits of:
T+50: begin no earlier than &+40
and complete METAR by T+55
T+20: begin no earlier than T+10
and complete METAR by T+25
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Competency of observers
To report and record all
applicable special reports
Ensure specials are made when
applicable without delay and
recorded appropriately
Demonstrate the ability to record
and disseminate observations
using local backup procedures
Maintain a suitable level of accuracy
and regularity of observations using
appropriate contingency observing
equipment and procedures
Table 10: Desirable additional areas of awareness and knowledge for the observer
Area
Detail
Knowledge of definitions
Observation, aeronautical meteorological report, visibility
(prevailing and minimum), runway visual ranges, altitude,
height, aerodrome elevation, landing forecast (trend),
aerodrome forecast, SIGMET.
Meteorological observing
systems
Limitations of sensors and algorithms used to determine the
‘visual’ elements of visibility, present weather, cloud amount
and cloud base height.
Dissemination of weather
information
Knowledge of procedures for dissemination of weather
information at the aerodrome. Elementary understanding of
the general organisation of aeronautical telecommunications.
Supplementary
information provided in
METAR and local reports
Runway state report, aircraft icing and turbulence, wind
shear.
Meteorological aspects of
flight planning
Meteorological basis for pressure-pattern flying, weather and
aerodrome forecasts; interpretation of area, route and
terminal forecasts.
ATS
Familiarity with special requirements relating to Category II
and III operations particularly in respect of runway visual
range, cloud base information, and any other specific local
requirements by aeronautical users for meteorological
information.
Operation of aircraft
Flight planning, altimeter setting procedures, standard
atmosphere, effects of various weather phenomena on
aeronautical operations and on aerodrome ground services.
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Calibration requirements for wind and pressure measuring equipment
Appendix I
Calibration requirements for wind and pressure
measuring equipment
Wind measuring equipment
Analogue cup or vane systems
I.1
Calibration of analogue cup or vane anemometers must be carried out in
a wind tunnel. Every anemometer in use on the aerodrome whether for
use solely for ATS purposes or use in Met reports must be calibrated at a
minimum of every 2 years. It should be noted that if operational
experience indicates a need, the calibration should be carried out more
frequently.
I.2
It is recommended that while the cup or vane system is being calibrated it
is serviced in order that the bearings and other moving parts are replaced.
This prevents the system from seizing up or providing inaccurate wind
readings due to the cup or vane system requiring a greater starting speed
before providing a valid wind speed reading. Each cup or vane system
should have a calibration certificate that details the cup and vane sensor
serial number(s), the date of calibration, the company or organisation that
carried out the calibration and the calibration source (detailing the
standard to which it is traceable).
Digital cup or vane systems
I.3
Servicing of digital cup or vane anemometers should be carried out on an
annual basis. Where the sensors are refurbished to an as new condition
as part of the service, wind tunnel calibration is not required; otherwise
calibration in a wind tunnel is required at least every 2 years.
NOTE: Sensors serviced using manufacturers’ replacement parts would be considered to
have been refurbished to an as new condition. If manufacturers’ replacement
parts are not used calibration in a wind tunnel is required.
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Calibration requirements for wind and pressure measuring equipment
All cup or vane systems
I.4
Where cup or vane systems are stored as a replacement for those
systems undergoing calibration these should also have been calibrated no
more than 3 years previously before use.
I.5
Where required, maintenance checks may be carried out on each cup and
vane anemometer system in use, these checks should include ensuring
that the vane direction system is aligned with magnetic north by ±3
degrees. It should be noted that the requirement for the actual reporting of
the wind direction is ± 10 degrees.
I.6
Where a comparison is made with a hand held anemograph this should
only be used to indicate that the instrument is functioning correctly, it
should not be considered a calibration.
Ultrasonic wind sensors
I.7
Ultrasonic wind sensors should be calibrated in accordance with the
manufacturer’s recommendation. It should be noted that as these systems
do not use bearings or have moving parts and consequently do not need
as frequent maintenance as the cup and vane systems. Therefore
recalibration in a wind tunnel is not usually required. During any
maintenance inspection a speed zero check and an alignment check of all
the vertical transducers should be performed. An orientation check should
also be carried out to ensure the sensor is aligned with magnetic north by
±3 degrees.
I.8
For each anemometer and wind vane (direction sensor) a record should
be kept which details the serial number(s) and date of installation as well
as the location on the aerodrome.
Pressure measuring equipment
I.9
All pressure sensors that are used at an aerodrome as primary or as
contingency devices are required to be calibrated on an annual basis.
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I.10
Calibration requirements for wind and pressure measuring equipment
The calibration should be against traceable national or international
standards. It should be noted that where a pressure sensing device is
consists of 2 or more pressure sensors these individual sensors are
required to be calibrated in order to ensure the corrections used within the
device are accurate.
I.11
The calibration should ensure that a range of pressure values are tested,
these should be between 900 hPa and 1050 hPa, it is recommended that
as a minimum 5 values are used. Any difference between the barometer
under test and the check barometer should be less than ±0.5 hPa.
Therefore the barometer that is used as the checking device should have
an accuracy greater than ±0.2 hPa.
I.12
The calibration certificate should detail the barometer’s serial number, the
date of calibration, the company or organisation that carried out the
calibration and the calibration source (detailing the standard to which it is
traceable). It should also provide a copy of the calibration report that
shows the pressure values that were tested and the resulting values from
the barometer being calibrated.
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