Everything you ever wanted to know about weather

Everything you ever
wanted to know about
weather instruments
Stephen Burt FRMetS
CoCoRaHS webinar, 17 October 2013
CoCoRaHS
This presentation is Copyright © Stephen Burt 2013. All rights
reserved. Reproduction and distribution is permitted for noncommercial purposes only provided the material is reproduced in its
original format. Photographic copyrights remain with the original
photographer as shown. www.measuringtheweather.com
Your presenter: Stephen Burt
• I’m 55, married with two grown-up daughters,
and I live in central southern England, about
50 miles west of London
• I’ve kept my own weather observations for 42 years, initially with basic instruments, almost
fully computerised last 20+ years
• I’m Chairman of the largest UK group of amateur observers, the Climatological Observers Link
www.colweather.org.uk
• I’m a fellow of the Royal Meteorological Society and a Member of the American Meteorological
Society and the Irish Meteorological Society
• My early working years were with the UK Met
Office, then 25 years as a marketing director in
the computer industry
• In 2012 I published my third book, The Weather
Observer’s Handbook (Cambridge University
Press)
• I’m currently completing a Masters degree (MSc) in meteorology at the University of
Reading, UK
Topics
• Basic principles
• Why measure the weather?
• Instrument siting and exposure
• Measuring precipitation
• Measuring air temperature
• Measuring humidity and dew point
• Measuring barometric pressure
• Measuring wind speed and direction
• Keeping metadata
• Making the most of your
observations
Why measure the weather?
• A global habit
• Many different reasons
– Input to weather and climate forecasting models
– Aviation and transport needs
– Statutory records
– Hobby/interest
– Education – 8 to 80
– And many more!
Ev-2K-CNR COMMITTEE
– Climatology and climate change
AWS on Mt Everest, at 8000 m
• Well-kept weather records – by organisations and individuals alike –
contribute to scientific evaluation of all types of weather and climate
phenomena, on scales from seconds to millennia
Audience survey - 1
Q1. Do you make instrumental weather observations yourself
currently, or have done so within the last year or two?
› Yes
94%
› No
6%
› No, but made weather observations some years ago
If yes to Q1, do you make these weather observations › as part of your job?
1%
› for your own interest or hobby purposes?
91%
› both?
8%
If yes to Q1, how long have you made weather observations yourself?
› less than a year?
24%
› More than 1 year?
76%
Audience survey - 2
If yes to Q1, do you make instrumental weather observations using › fully manual instruments (such as a thermometer, raingauge)?
51%
› fully automatic instruments – automatic weather station?
6%
› a mix of both methods?
43%
If yes to Q1, how do you keep your records?
› Manually (manuscript, in a logbook or similar)
23%
› Mostly or completely on computer (spreadsheet or similar)
44%
› A mix of both
33%
Q2. Which weather elements are of most interest to you?
› Precipitation
56%
› Air temperature
10%
› Humidity and dew point
3%
› Barometric pressure
8%
› Wind speed and direction
23%
Site and exposure ... the basics
• Site – the area or enclosure where the instruments are exposed
• Exposure – the manner in which the sensor or sensor housing is
exposed to the weather it is measuring
– ‘Representative and comparable’
Preferable characteristics
Avoid
Open and well-exposed - well away from
trees, hedges, buildings and other
obstructions
Sheltered locations
Ground-level, on flat ground
On sloping ground or in hollows
Rooftop sites (except wind, sunshine)
Above short grass
Artificial surfaces – concrete, tarmac etc
Safe and secure access
Insecure or unsafe locations
• Budget instruments correctly exposed on a good site will give better
results than poorly-located expensive instruments
Precipitation
What are we attempting to measure?
• Rain, drizzle, snow, rain/snow mixed, hail – also dew, frost or fog
Credit: World Meteorological Organization, Geneva
• Highly variable in space and time
– Obstructions minimum distance
2 x their height away
– But – very open sites may need
some shielding, especially in snowfall
• Many different types of gauge
– International and climatic variations
– Differing standards worldwide
WORLD METEOROLOGICAL ORGANIZATION
• Very sensitive to exposure –
especially wind effects
Raingauge intercomparison at Vigna di Valle, Italy
Measuring precipitation: daily-read gauges
• National standards vary
– Rim height US 3-4 feet (90-120 cm), UK/Ireland 1 foot
(30 cm)
• Round, deep funnel to minimise turbulence
and outsplash
• Calibrated measuring cylinder – resolution
0.1 mm or 0.01 in
• Capacity for at least! 100 year 24 h event
– Consider siting of gauge – will it flood?
• Time of reading – usually morning,
7-9 a.m. Local Time
– Essential for comparability
HENRY REGES, CoCoRaHS
– Minimum capacity US 500 mm / 20 in
Measuring snowfall
• Snow depth
– Graduated stick held vertically
– Average several readings
• Precipitation measurements
– Standard rain gauges prone to wind errors up to 80 per cent
STEPHEN BURT
– Relationship snow depth:water equivalent
very variable, average 10-12 : 1, varies 5:1 to 20:1
– Wind shields can help
– Recording gauges usually useless
in snowfall – except vibrating wire types
Nipher and Alter wind shields
Measuring precipitation: recording gauges
• Used for timing and intensity only
– Adjacent ‘standard’ raingauge should always be used for reference total
• Main types
– Tipping-bucket
– Vibrating wire
• Can be remotely logged or telemetered
– Resolution 0.1 mm or 0.01 in
IAN STRANGEWAYS
– 1 mm / 0.04 in too coarse – miss small events
Tipping bucket raingauge
Air temperature
What are we attempting to measure?
• ‘True air temperature’: need protection from –
– Solar radiation (direct or reflected sunshine)
– Terrestrial radiation (from Earth and Sky)
– Precipitation
– BUT: need unobstructed ventilation … !
• Sensor exposure critical for representative value
– Not unduly influenced by exposure or siting
– Exposure ‘errors’ are much greater than calibration errors
• Standard housings and exposure essential for comparability
– Open, level site with minimum obstructions/shelter from trees or buildings
– Typical sensor height 1.25 – 2 m (4-5 ft) above short grass, away from tarmac etc
Measuring air temperature: thermometer screens
 Almost any screen is better than a bare sensor
Basic principles
– Shelter from radiation (solar and terrestrial) and precipitation
– Even temperature environment
– Respond quickly to changes in air temperature (small mass, optimum ventilation)
› Response time may be much greater than sensor response time
Thermometer screens
– ‘Traditional’ louvred screens – usually white-painted woodwork, steel stand
such as Stevenson Screen, Cotton Region Shelter
› Designed for ‘liquid in glass’ thermometers
– Automatic Weather Station (AWS) screens
› Smaller, ‘multi-plate’ plastic units, usually white – highly variable quality
› Designed for smaller electronic sensors - faster response time
– Aspirated screens
› Constant airflow over sensor
Thermometer screens – ‘traditional’ louvred
St James’s Park, London
Cannes, France
Granger, Utah
Agoium, Morocco
Rothera Base,
Antarctica
Asheville, NC
Hohenpeißenberg Observatory, Germany
PICTURE CREDITS: TOP ROW, L TO R: STEPHEN BURT; NOAA ARCHIVES; GRANT GOODGE
BOTTOM ROW: STEPHEN BURT; STEPHEN BURT; TAMSIN GRAY, BRITISH ANTARCTIC SURVEY; STEFAN GILGE, DEUTSCHER WETTERDIENST
Thermometer screens – ‘modern/AWS’
Mt Everest 8000 m
Fort Collins, Colorado
Birmingham University, England
PICTURE CREDITS: TOP ROW, L TO R: Ev-2K-CNR COMMITTEE; GRANT GOODGE; STEPHEN BURT
BOTTOM ROW: STEPHEN BURT; STEPHEN BURT; GRANT GOODGE
Berkshire, England
Fort Collins, Colorado
Thermometer screens – aspirated
• Principle: forced advection
of ambient air over sensor
– Improves both conduction
and response
Fan
– Minimises radiation
and wetting errors
– Closest to true
air temperature (probably!)
• BUT – non-homogeneous
with existing louvred
screens
RM YOUNG
www.ncdc.noaa.gov/crn/
Rain shield
STEPHEN BURT
• Used in US Climate
Reference Network,
USCRN
Sensor
RM Young aspirated screen
Concentric
air intakes
Temperature sensors
• ‘Traditional’ liquid-in-glass
– Current temperature, dry- and wet-bulb,
maximum and minimum
– Continuity – stable – but calibration must be checked
– Expensive, fragile, bulky – require louvred screen
– Must be manually read (and reset as necessary)
• Electronic sensors
– Resistance temperature devices (RTDs)
› Platinum Resistance Thermometer (PRT) –
more accurate and repeatable, easy to calibrate
– One sensor for current and extreme temperatures
– Smaller – fit in AWS screen, faster response
STEPHEN BURT
› Thermistor – cheaper, less accurate
PRTs and liquid-in-glass thermometers
– Physically and electrically robust - but calibration must be checked
– Capable of automatic logging, and so steadily replacing ‘glass’ thermometers
Response time - sampling and logging intervals
15
Stevenson screen vs
Aspirated screen
°C
14
1 minute averages
5 October 2013
13
12
Stevenson screen
Aspirated screen
11
10
1800
60 minutes
1815
1830
1845
5
1900
1915
Time, UTC
1930
1945
2000
2015
2030
2045
• Sampling interval – 5-15 sec ideal for air temperatures
• Logging interval – WMO recommend 60 sec running average
• Many consumer AWSs provide only ‘spot’ values
2100
°F
59
58
57
56
55
54
53
52
51
50
Humidity and dew point
What are we attempting to measure?
• The amount of water vapour
in the air
– Warmer air can ‘hold’ more water vapour:
at 30 °C, 7 x as much as at 0 °C
› Expressed in relative terms –
‘Relative Humidity’
“17 °C, 50% RH”
17 °C, SVP ~ 20 hPa
50% RH  VP ~ 10 hPa
VP 10 hPa  dew point 7°C
› Or, as an absolute measure –
humidity mixing ratio, g/kg
› Or, as the ‘dew point temperature’
› The dew point temperature is
the temperature at which the amount
of water vapour in the air just equals
the maximum amount that the air can hold
at that temperature
› The larger the difference between
dew point and the current temperature, the lower the humidity
Measuring humidity and dew point
• Dry- and wet-bulb thermometers – ‘traditional’ method
– The wet bulb is cooled by evaporation
– Liquid-in-glass thermometers or electronic sensors
– RH, dew point (etc) calculated from tables, by logger or by PC/smartphone apps
– Disadvantages: water supply needed!
• Capacitative sensors – used by most AWSs
– Polymer film between electrodes, impedance varies directly with RH
– Advantages: small, low power, remote logging, inexpensive, works below 0°C, no
water supply required
– Disadvantages: slow response near saturation, calibration drift
• Usually exposed in thermometer screen alongside temperature
sensors
– Accuracy at best typically 3% RH or 0.5 degC / 1 degF dew point
Barometric pressure
What are we attempting to measure?
• ‘Air pressure’ – force per unit area exerted
by a vertical column of air
• Easiest of all elements to measure
– You can do this one indoors!
– But locate away from sources of heat,
or draughts, or A/C
STEPHEN BURT
• ‘Pressure trend’ useful for short-term
local forecasting
Household aneroid barometer
• Most AWSs now use a small solid-state sensor
– To compare readings, they must be reduced to a common or standard level,
usually Mean Sea Level, or MSL
– Beware of calibration drift – check 2-3 times per year
Wind speed and direction
What are we attempting to measure?
• Challenging for instruments, logging equipment
and site/exposure
• Wind is a vector quantity – it has both direction
and speed
STEPHEN BURT
• The most variable element of all!
Cup anemometer
• Wind direction is where the wind is coming from
– Measured by electrical wind vane
– Referenced to True North, not Magnetic North
• Mean wind speeds refer to 10 minute means, gust speeds refer to
3 second means
– Measured by cup anemometer, rotor anemometer or sonic anemometer
Wind speed and direction
• Greater level of difficulty and
complexity to obtain representative
readings
– No obstacles within 10 x their height in any
direction
– Rooftop sites are not ideal, but are often all
that is available
– Planning or zoning restrictions may apply
• Most vulnerable to weather exposure
• Instruments will need occasional
maintenance – ensure safe access
SWISS INSTITUTE FOR SNOW AND AVALANCHE RESEARCH
• Ideal exposure is at 10 m (33 ft) above
ground in open site
Vallée de la Sionne, Canton Valais – 2696 m
Metadata
• Literally ‘data about data’
• Description of
– Site (latitude, longitude, altitude) and surroundings
– Instruments in use and changes over time
– Where the records are kept, and what format they are in
– Units in use and any changes
• Essential for any future users of the records
– You may not be around to tell them!
• Document in simple structured text file, and keep updated annually
• Add photographs – and take new ones every so often
STEPHEN BURT
Metadata – simple example
Radcliffe Observatory, Oxford, England
Making the most of your observations
A means to an end
• Keep your observations in spreadsheets
– ‘Export’ function from AWS, ‘Import’ into spreadsheet
– Better analysis and presentation facilities than AWS software
– Develop a format and structure that work, then stick to it
– Hourly, daily and/or monthly files
– Don’t forget metadata!
• Files quickly build into useful datasets
• Try simple plots of (say) daily maximum and minimum, then try more
advanced plots and analyses
– Augment records with longer-period nearby sites if available
› Be careful of inhomogeneities
– Practice builds experience – go ahead, try it!
Daily maximum and minimum temperatures
Daily max and min temperatures and rainfall
mm
Daily maximum and minimum temperatures
More advanced analyses using Excel
1. Hourly pressure means
Averages over 10 years 2001-10
More advanced analyses using Excel
2. Hourly sunshine means
Year
(Mult iple Items)
Average of Sunshine hr Column Labels
Row Labels
0000 0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 Grand Total
1
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.30 0.34 0.36 0.38 0.35 0.30 0.24 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.10
2
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.26 0.34 0.35 0.33 0.34 0.33 0.33 0.32 0.23 0.02 0.00 0.00 0.00 0.00 0.00 0.00
0.12
3
0.00 0.00 0.00 0.00 0.00 0.00 0.09 0.33 0.38 0.44 0.45 0.42 0.40 0.40 0.40 0.39 0.37 0.21 0.00 0.00 0.00 0.00 0.00 0.00
0.18
4
0.00 0.00 0.00 0.00 0.00 0.16 0.43 0.48 0.50 0.49 0.49 0.46 0.45 0.45 0.47 0.48 0.49 0.43 0.17 0.00 0.00 0.00 0.00 0.00
0.25
5
0.00 0.00 0.00 0.00 0.12 0.38 0.43 0.45 0.44 0.42 0.39 0.35 0.35 0.37 0.37 0.38 0.42 0.43 0.40 0.11 0.00 0.00 0.00 0.00
0.24
6
0.00 0.00 0.00 0.00 0.21 0.43 0.47 0.51 0.52 0.48 0.46 0.42 0.41 0.39 0.42 0.44 0.47 0.47 0.47 0.29 0.00 0.00 0.00 0.00
0.29
7
0.00 0.00 0.00 0.00 0.13 0.36 0.42 0.45 0.43 0.40 0.37 0.36 0.36 0.36 0.42 0.43 0.41 0.44 0.41 0.25 0.00 0.00 0.00 0.00
0.25
8
0.00 0.00 0.00 0.00 0.01 0.25 0.43 0.45 0.45 0.44 0.42 0.40 0.41 0.40 0.40 0.41 0.45 0.43 0.29 0.03 0.00 0.00 0.00 0.00
0.24
9
0.00 0.00 0.00 0.00 0.00 0.02 0.33 0.43 0.47 0.52 0.48 0.47 0.47 0.46 0.45 0.43 0.46 0.32 0.02 0.00 0.00 0.00 0.00 0.00
0.22
10
0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.28 0.38 0.42 0.40 0.40 0.41 0.40 0.40 0.38 0.25 0.01 0.00 0.00 0.00 0.00 0.00 0.00
0.16
11
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.09 0.30 0.35 0.38 0.38 0.40 0.37 0.33 0.23 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.12
12
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.27 0.33 0.34 0.33 0.32 0.29 0.14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.09
Grand Total
0.00 0.00 0.00 0.00 0.04 0.13 0.22 0.29 0.37 0.40 0.40 0.39 0.39 0.38 0.38 0.35 0.29 0.22 0.14 0.06 0.00 0.00 0.00 0.00
0.19
Averages over 10 years 2001-10
More advanced analyses using Excel
2. Hourly sunshine means – conditional formatting
Year
(Mult iple Items)
Average of Sunshine
Column
hr Labels
Row Labels 0000 0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 ANNUAL
January
0
0
0
0
0
0
0 0.00 0.17 0.30 0.34 0.36 0.38 0.35 0.30 0.24 0.03
0
0
0
0
0
0
0
0.10
February
0
0
0
0
0
0 0.00 0.10 0.26 0.34 0.35 0.33 0.34 0.33 0.33 0.32 0.23 0.02
0
0
0
0
0
0
0.12
March
0
0
0
0
0
0 0.09 0.33 0.38 0.44 0.45 0.42 0.40 0.40 0.40 0.39 0.37 0.21 0.00
0
0
0
0
0
0.18
April
0
0
0
0 0.00 0.16 0.43 0.48 0.50 0.49 0.49 0.46 0.45 0.45 0.47 0.48 0.49 0.43 0.17
0
0
0
0
0
0.25
May
0
0
0
0 0.12 0.38 0.43 0.45 0.44 0.42 0.39 0.35 0.35 0.37 0.37 0.38 0.42 0.43 0.40 0.11
0
0
0
0
0.24
June
0
0
0
0 0.21 0.43 0.47 0.51 0.52 0.48 0.46 0.42 0.41 0.39 0.42 0.44 0.47 0.47 0.47 0.29
0
0
0
0
0.29
July
0
0
0
0 0.13 0.36 0.42 0.45 0.43 0.40 0.37 0.36 0.36 0.36 0.42 0.43 0.41 0.44 0.41 0.25
0
0
0
0
0.25
August
0
0
0
0 0.01 0.25 0.43 0.45 0.45 0.44 0.42 0.40 0.41 0.40 0.40 0.41 0.45 0.43 0.29 0.03
0
0
0
0
0.24
September
0
0
0
0
0 0.02 0.33 0.43 0.47 0.52 0.48 0.47 0.47 0.46 0.45 0.43 0.46 0.32 0.02
0
0
0
0
0
0.22
October
0
0
0
0
0
0 0.06 0.28 0.38 0.42 0.40 0.40 0.41 0.40 0.40 0.38 0.25 0.01
0
0
0
0
0
0
0.16
November
0
0
0
0
0
0
0 0.09 0.30 0.35 0.38 0.38 0.40 0.37 0.33 0.23 0.02
0
0
0
0
0
0
0
0.12
December
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.09
ANNUAL
0
0
0
0 0.04 0.13 0.22 0.29 0.37 0.40 0.40 0.39 0.39 0.38 0.38 0.35 0.29 0.22 0.14 0.06
0
0
0
0
0.19
0 0.17 0.27 0.33 0.34 0.33 0.32 0.29 0.14
0
Averages over 10 years 2001-10
More advanced analyses using Excel
3. Rainfall amounts by wind direction
Total rainfall (mm) over 17 years 1994-2010
More advanced analyses using Excel
4. Rainfall intensity by wind direction
Mean rainfall intensity (mm/hr)
over 17 years 1994-2010
More advanced analyses using Excel
5. Is it windier when it’s raining?
Averages over 10 years 2001-10
Mean rainfall intensity (mm/hr)
over 17 years 1994-2010
More advanced analyses using Excel
5. Is it windier when it’s raining?
Averages over 10 years 2001-10
More advanced analyses
6. Wind roses
Prepared using WindRose PRO software from Enviroware
www.enviroware.com
Topics covered
• Basic principles
• Why measure the weather?
• Instrument siting and exposure
• Measuring precipitation
• Measuring air temperature
• Measuring humidity and dew point
• Measuring barometric pressure
• Measuring wind speed and direction
• Keeping metadata
• Using your data
Additional topics
• Choosing a weather station
• Buying a weather station
• Earth and grass temperatures
• Sunshine and solar radiation
• Observing hours and time standards
• Dataloggers and AWS software
• Non-instrumental weather observing
• Calibration
• Collecting and storing data
• Sharing your observations
The Weather Observer’s Handbook by Stephen Burt
Published by Cambridge University Press
www.cambridge.org/9781107662285
http://www.amazon.com/Weather-Observers-Handbook-StephenBurt/dp/1107662281/ref=sr_1_1?s=books&ie=UTF8&qid=1380660845&sr=11&keywords=weather+observers+handbook
Paperback $40 (Amazon $26), Hardback $99
ISBN: Paperback 978-1-107-66228-5, Hardback 978-1-107-02681-0
456 pp., 20 chapters, 4 Appendices, Index
128 b/w illus. 2 maps 50 tables, 228 x 152 mm
“I would highly recommend this comprehensive weather-observing
guide to hobbyists, professionals, teachers, and college instructors. The
author has done an outstanding job making the book accessible to
anyone interested in observing the weather, even if they do not have a
technical background. At the same time, there is plenty of useful
information for those of us who have been professionally involved in
observing the weather for quite some time.”
Bulletin of the American Meteorological Society, May 2013
Questions