Campbell ET101 Instruction manual

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CAMPBELL SCIENTIFIC, INC. warrants that the installation media on which
the accompanying computer software is recorded and the documentation
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under normal use. CAMPBELL SCIENTIFIC, INC. warrants that the
computer software itself will perform substantially in accordance with the
specifications set forth in the instruction manual published by CAMPBELL
SCIENTIFIC, INC. The recommended minimum hardware for Visual Weather
is a 300 MHz Pentium II processor with 64 megabytes of RAM and a screen
resolution of at least 800x600. Visual Weather uses the features of Windows
NT, 2000, or XP that maximize the reliability of unattended scheduled data
collection and multitasking application programs. Visual Weather may be run
successfully on Windows 95, 98, or ME if the user limits the number of screens
open at any one time.
CAMPBELL SCIENTIFIC, INC. will either replace or correct any software
that does not perform substantially according to the specifications set forth in
the instruction manual with a corrected copy of the software or corrective code.
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computer hardware, computer operating systems or the use of CAMPBELL
815 W. 1800 N.
Logan, UT 84321-1784
Phone (435) 753-2342
FAX (435) 750-9540
Campbell Scientific Canada Corp.
11564 -149th Street
Edmonton, Alberta T5M 1W7
Phone (780) 454-2505
FAX (780) 454-2655
Campbell Scientific Ltd.
Campbell Park
80 Hathern Road
Shepshed, Loughborough
LE12 9GX, U.K.
Phone +44 (0) 1509 601141
FAX +44 (0) 1509 601091
Visual WeatherTM Software
Table of Contents
1. Introduction.................................................................1
1.1 Background...............................................................................................2
2. System Requirements and Installation .....................2
2.1 Configuration of TCP/IP Services ............................................................2
3. Weather Station Configuration ..................................4
Before You Begin .....................................................................................4
Getting Help..............................................................................................4
Example Weather Station Configuration ..................................................4
After Configuring a Weather Station......................................................16
4. Manually Connecting to a Weather Station.............17
Retrieving Data Manually.......................................................................17
Setting a Weather Station Clock .............................................................18
Manually Sending a Program to a Weather Station................................18
Monitoring Weather Station Parameters and Status ...............................18
5. Generating Reports ..................................................19
5.1 Manually Generated Reports ..................................................................19
5.2 Batch Reports..........................................................................................20
6. Exporting Data to a File............................................20
7. Viewing All Stations in a Network ...........................20
8. Log Files for Troubleshooting Communication
Problems ................................................................21
A. Evapotranspiration, Vapor Pressure Deficit,
and Crop Water Needs ........................................ A-1
A.1 Evapotranspiration .............................................................................. A-1
A.2 Vapor Pressure Deficit of the Air ....................................................... A-9
A.3 Crop Water Needs, Crop Coefficients .............................................. A-10
Visual Weather
Software Table of Contents
B. Growing Degree Days (GDD) and
Pest Emergence................................................... B-1
B.1 Growing Degree Days ......................................................................... B-1
B.1.1 Method....................................................................................... B-1
C. Dew Point and Web Bulb Temperatures............... C-1
C.1 Dew Point ............................................................................................ C-1
C.1.1 Method....................................................................................... C-1
C.2 Wet Bulb Temperature ........................................................................ C-1
C.2.1 Method....................................................................................... C-2
D. Wind Chill................................................................ D-1
D.1 Wind Chill........................................................................................... D-1
D.1.1 Method ...................................................................................... D-1
D.2 References ........................................................................................... D-1
E. Chill Hours .............................................................. E-1
E.1 Chill Hours ...........................................................................................E-1
E.1.1 Method........................................................................................E-1
Visual Weather Software
1. Introduction
Visual Weather software is designed to work with Campbell Scientific's preconfigured weather station models ET101, ET106, and MetData1. This
software will allow you to:
Create and send configurations to one or more weather stations.
Select a mode of communication (e.g., phone modem or RF) for each
weather station.
Define a data retrieval schedule for each station.
Connect to any weather station that has been configured with Visual
View a graphical display of a weather station’s current weather conditions.
Print a summary of each weather station’s configuration.
Monitor the "health" (e.g., battery voltage, internal temperature) of each
weather station on a single screen.
View and print preformatted reports (tables and graphs) from the data
retrieved from each weather station.
Perform post-processing on retrieved data to produce special
meteorological and agricultural reports (e.g., evapotranspiration,
Set up a schedule to automatically print reports (Batch Reports).
Export retrieved data to an ASCII file (fixed column width) so it can be
imported into a spreadsheet or other software package for further data
These tasks can be accomplished simply by clicking a few buttons and
following the on-screen instructions. It requires no computer experience or indepth technical knowledge of the weather station equipment. We hope all
users—novices or experts—will find Visual Weather a very easy to use, yet
powerful, tool for managing a weather station network.
This software is primarily designed to calculate quantities, such
as evapotranspiration (ETo), crop water needs, and growingdegree-days, which are important to the agricultural community.
This software expects that your weather station has at least a ‘full
set’ of sensors, namely sensors that measure temperature,
relative humidity, precipitation, solar radiation, and wind
speed/wind direction. If your weather station does not measure
these weather conditions, then you may not be able to utilize the
full potential of this software. For example, if your station does
not have a humidity sensor, then you will not be able to get
reports on ETo, crop water needs, or wet bulb temperatures.
It is best to consult the CSI applications engineers who will
advise you on sensors required for your needs.
Visual Weather Software
1.1 Background
Each weather station consists of a datalogger and a group of sensors. The
sensors are designed to measure environmental factors such as air temperature
or solar radiation. The datalogger measures the sensors and processes and
stores the results. The stored results (final storage data) can be retrieved with a
PC using communication software.
Dataloggers must be "told" (that is, configured) when and how to measure the
attached sensors, process these measurements, and store the information in
final storage. Datalogger configuration can be a formidable task for many
users. However, with Visual Weather you simply select the sensors connected
to the weather station and the program is generated for you!
2. System Requirements and Installation
The recommended minimum hardware for Visual Weather is a 300 MHz
Pentium II processor with 64 megabytes of RAM and a screen resolution of at
least 800x600. Visual Weather uses the features of Windows NT, 2000, or XP
that maximize the reliability of unattended scheduled data collection and
multitasking application programs. Visual Weather may be run successfully on
Windows 95, 98, or ME if the user limits the number of screens open at any
one time.
If you need to contact weather station(s) in remote locations,
then you may also need RF modems and radios or an analog
phone line and a phone modem. Visual Weather can detect
modems installed on your computer provided they were
configured in your computer’s Window environment. During
configuration of a new weather station you will find it
convenient to select the detected modem.
Some newer computers have USB ports instead of serial ports
for communication with other devices. Communication using
Visual Weather requires serial communication. If your computer
has only a USB port, you will need a USB to serial port adapter.
To install the software, insert the Visual Weather CD into the CD ROM drive
of your computer. The installation process should begin automatically. If it
does not begin, access your CD ROM drive and double-click the SETUP.EXE
file. Follow the on-screen instructions to complete the installation.
2.1 Configuration of TCP/IP Services
TCP/IP services must be running on the computer for Visual Weather to run.
Following are the procedures for enabling TCP/IP communication on a
Windows 95, 98, or NT system. For Windows 2000 the same things need to be
set up, but they are accessed in different ways. See the documentation and
help for Windows 2000 to add a dial-up connection and associate it with
Visual Weather Software
Before beginning this procedure make sure that you have your
Windows installation CD-ROM (or floppy disks as appropriate)
As you install these options you may be prompted to insert various disks or the
CDROM to complete the installation.
Click on the Start button and select Settings | Control Panel.
When the Control Panel window comes up double click on the
Add/Remove Programs icon.
Select the Windows Setup tab.
Select Communications and click on the Details button.
On the Communications options screen click the box by “ Dial-Up
Networking” (Win 98/95) or “ Phone Dialer” (NT). If already checked,
click cancel and skip to step 9.
Click OK on the Communications Options screen and on the Windows
Setup screen.
Provide the Windows installation software as prompted and then follow
the directions.
When you are prompted to reboot the computer choose Yes.
After the computer boots, go to the Windows Control Panel and double
click on the Network icon.
10. In the list box on the Configuration tab (Win95/98) or Protocols tab (NT)
of the Network window which comes up, see if there is an entry TCP/IP ->
Dial-Up Adapter or TCP/IP protocol. If this entry exists, cancel and skip
the next steps.
11. Click on the Add button. In the Select Network Component Type window
which comes up select Protocol or TCP/IP protocol and click on the Add
or OK button.
12. When the Select Network Protocol window comes up select Microsoft
under Manufacturers:, and TCP/IP under Network Protocols:. Click OK.
Visual Weather Software
3. Weather Station Configuration
3.1 Before You Begin
This section assumes that:
You have installed Visual Weather on your computer.
You have installed a weather station and the desired sensors are connected
properly to the station. (Please refer to the Weather Station manual.)
The battery supplying power to the station (12 Volts DC) is charged.
Communication hardware (e.g. modem, phone line, radio, etc.) is
connected properly and functioning.
3.2 Getting Help
Visual Weather has a complete on-line help system. Most screens have a Help
button at the bottom right that, when pressed, will bring up help about the
screen. In addition, help for any field, button, or option can be invoked by
selecting it and pressing the F1 key on your computer. A Help file Table of
Contents, an Index, and a Find function can be invoked by selecting Help from
Visual Weather's main menu.
3.3 Example Weather Station Configuration
An ET106 Weather Station is used in this example to demonstrate how a
weather station is configured using Visual Weather. Configuration for an
ET101 or MetData1 weather station is similar.
From the Windows Start menu, select Programs | Visual Weather | Visual
Weather. This opens Visual Weather's main screen.
Visual Weather Software
VISUAL WEATHER MAIN SCREEN. From the list on the main screen select
Configure a Weather Station (or from the menu select File | Configure
WEATHER STATION MODEL SELECTION. Select the option button for your
weather station model. In the example below, ET106 has been chosen.
To proceed to the next step, click Next.
Visual Weather Software
WEATHER STATION INFORMATION. This screen allows you to enter
information about the weather station that may be helpful later, and that
Visual Weather will use when saving a weather station's configuration
A unique name must be given to each weather station. Visual Weather
will not allow you to enter the name of an existing station. Station notes
are optional, but they may be useful in identifying the weather station.
Make sure to select the appropriate battery type for your station. When
monitoring a weather station, the low battery alarm limit will be different
depending upon the option chosen.
If you would like to have a custom image or company logo appear on
your Visual Weather screens and printed reports, you can select it by
using the respective Browse button on this screen. If you do not choose a
custom image, then the default image you see on this screen will be used.
If you do not add a custom logo, then a logo will not appear on any of
your printed reports.
Click Next to proceed.
Visual Weather Software
TYPICAL OR CUSTOM STATION. An ET106 weather station is most often
sold with the following sensors:
CS500 air temperature and relative humidity sensor
LI200X solar radiation sensor
TE525 tipping rain gage
034A wind speed and direction sensor
This is the default configuration, but Visual Weather allows you to select
of other sensors by selecting the Custom option.
Typical Weather Station
When a Typical station is selected, the sensors noted above will be
measured every 5 seconds. The measured values are processed every 15
minutes, hourly, and daily. For example, with a scan rate of 5 seconds,
720 measurements are performed in one hour (3600 ÷ 5). The sensor
measurements are processed for this time period and stored for later use.
Similarly, measurements are processed for the time spans of 24 hours and
15 minutes. When data is retrieved from the weather station, all of these
values are saved on your computer associated with the unique station
name you provided in Step 4.
Visual Weather Software
If a Typical Weather Station setup is desired, select Next to proceed to the
Communications section (and you may skip to step 8). If a Custom
Weather Station setup is desired, additional setup screens will be
Custom Weather Station
Select the Custom option if you are using sensors other than those
mentioned above, if you want to use a different scan rate (10, 30, or 60
seconds), or if you want to set the shortest data storage interval to
something other than 15 minutes (1 - 59 minutes). Select Next to proceed.
SENSOR SELECTION. This screen is used to select the sensors that are being
measured by your custom weather station and the frequency at which they
will be measured.
Each of the seven fields on the left corresponds to the picture of the
connectors on the right (this diagram represents the actual physical
connectors of the weather station). Select the arrow button to the right of a
field to invoke a list of valid sensors for that connector. The sensors
selected for each field and the physical connection of the sensors to the
weather station must match or you will receive incorrect data from the
weather station.
Visual Weather Software
Refer to the weather station's user manual for additional
information on sensor connection.
After the sensors have been selected for the weather station, select a scan
interval (5, 10, 30, or 60 seconds). Select Next to proceed.
FINAL STORAGE. This screen is used to select the values that will be stored
and the time span for the shortest data storage interval. By default, the
time span for this interval is 15 minutes; you can select from 1 to 59
The check marks appearing in the Hourly Data and Daily Data columns
(which are colored red in the software) indicate values that are
automatically stored for the hourly and daily storage intervals (provided
the sensors are connected to the weather station).
To set the time for the Custom Data interval, click the up or down arrow
that appears to the right of the Custom Data field and increase or decrease
the value. In the example above, the custom data interval is 20 minutes.
To set the data values that will be stored at this interval, select the check
box for a measurement. In the example above, Wind Speed and Direction
is selected.
Visual Weather Software
Click Next to configure the communications link for the weather station.
COMMUNICATIONS. The Communications screen is where you define the
type of link you will be using to communicate with your weather station.
For all communication modes, you must indicate the appropriate COM
port on your computer to which the cable or modem is attached. Select the
arrow to the right of the COM Port field to display a list of available
COM ports. Visual Weather can only use COM ports that are already
installed on your computer.
Click on the arrow to the right of the field Windows Modem/Com Port. A
list consisting of a modem, if one was configured in Windows on your
computer, and COM ports will be displayed. If the station you are
configuring is to be contacted via phone modem or phone-to-RF then
select the modem displayed in the list. Select/Enter rest of the applicable
information such as mode of communication, station's contact phone
If you are using Direct or Short haul or RF mode of communication then
you must select the appropriate COM port from the displayed list of COM
ports. Visual Weather expects a dedicated (reserved) COM port for these
modes of communication. Note that only the COM ports available for
connection are listed. For example, if another weather station is already
connected directly to COM Port 1 of your computer then you may not
connect another station on the same port. In this case you will not find
COM1 in the list of COM Ports.
Visual Weather Software
Next, select the mode of communication that will be used to contact the
weather station by selecting the appropriate option button. Visual Weather
supports five modes of communication (see Figure 1):
Direct communication
Cable SC25PS
Cable SC12
RS 232 (SC32A)
Short Haul
Short haul modem
Cable Twisted Pair
Short haul modem
Phone modem
Phone modem
CSI Remote modem
(such as COM 300)
Telephone line
Analog wall phone jack
Analog wall phone jack
Radio Frequency (RF)
Phone modem
Remote modem
RF Remote modem
Base RF
Radio Frequency (RF)
Cable SC25PS
RF (base modem)
Radio Frequency (RF)
RF Remote modem
FIGURE 1. Modes of Communication Supported by Visual Weather
Direct - The weather station is connected directly to a computer's
COM port using a serial cable. Direct communication is used when
the weather station and computer are in close proximity (typically
less than 25 feet).
Short Haul - The weather station is connected to a computer via a two
twisted pair cable and short haul modems. Short haul modems are
used in instances where the weather station and computer are a few
hundred feet away from each other.
Phone Modem - A phone modem is installed at the computer, and
another phone modem is installed at the weather station site. Phone
modem communication is used when a telephone line (or cellular
service) is available at the weather station site.
Visual Weather Software
RF - Two RF modems, one connected to the computer (called the
base RF) and another at the weather station (called the remote RF),
communicate via radio frequency (RF). Both modems have numeric
addresses (referred to as RF switch IDs) to identify them. The base
RF ID is assumed to be 255 and must be configured as such for
communication to work. You need to know and enter the RF switch
ID of the remote RF modem only.
Phone-to-RF - A combination of communication modes is used.
Weather stations located in remote areas are sometimes contacted in
this manner. You will need to know the phone number and remote
RF switch ID.
Visual Weather supports most commercially available internal
and external phone modems. A list of modems is provided from
which to choose when setting up the communications link for a
station in Visual Weather. If you do not find your modem listed,
select the <default modem>. This should work in most instances.
If the default modem does not work, contact your Campbell
Scientific representative for assistance. You will need to provide
the command set included in the user's manual for your modem.
Depending on the mode of communication you select, additional fields
will be available under Communication Device Settings to enter baud
rate, phone numbers, RF IDs, etc.
Before continuing, check that the information entered
into all the fields on this screen is correct. If you have
entered any incorrect information, you will not be able
to contact the weather station.
Select Next to continue.
Visual Weather Software
DATA COLLECTION SCHEDULE. This screen is used to define the schedule
on which the weather station will be called and data will be retrieved.
The Next Time to Call fields determine when the first call will be made to
the weather station. If the date and time entered have already passed, then
a call will be made at the Calling Interval. The Calling Interval field is
how often the weather station will be called. If you do not change any
information in these three fields, Visual Weather will contact the station at
the top of the next hour and every hour thereafter.
The calling interval can be as frequent or infrequent as you desire.
However, calling the station hourly will ensure that you have up-to-date
information for your reports. Also, you will be more quickly alerted to
problems that might occur with communication, the "health" of the
datalogger, or the data itself.
Retries are used to define a different schedule on which the weather
station will be contacted if a scheduled attempt to contact the station fails.
The Primary Retry Interval is the first interval that will be used. This
interval will be used the number of times specified in the Number of
Primary Retries field. If communication is successful on one of the
primary retries, then data collection will revert back to the original calling
interval. However, if all of the primary retries fail, then Visual Weather
will call the station at the interval entered into the Secondary Retry field
Visual Weather Software
(provided the checkbox next to this field is checked). Visual Weather will
continue calling on this interval until a successful call is made, at which
time it will revert back to the original calling interval.
For example, in the screen above, the weather station will be contacted
each hour. If a call fails, Visual Weather will wait 5 minutes and call
again. This 5 minute interval will be attempted twice, then Visual
Weather will begin calling every 24 hours. At any time during this
process if communication is successful, Visual Weather will resume
contacting the weather station on an hourly basis.
The defined schedule can be turned on or off by selecting the appropriate
option button. Schedule On is selected by default. This means that Visual
Weather will attempt to contact the station based on the information
entered on this screen. The schedule may be paused at any time by
selecting the Off option. Pausing the schedule can be useful, such as when
you are performing maintenance on the weather station, but in most
instances you will want to set the Schedule option to On.
Click Next to proceed.
Visual Weather Software
10. WEATHER STATION SUMMARY. The Summary screen displays all of the
information you entered when configuring the weather station. After
verifying that the information is correct the page can be printed and saved
for your records.
If you want to change a selection (for example, you noticed that you
selected a wrong model for the Rain Gage sensor), click the Previous
button and navigate through the screens until you reach the screen where
you wish to make a different selection. After making the change(s) click
the Next button on each screen to return to the Summary screen.
Finally, click the Finish button to complete the weather station
configuration. This configuration is saved under the name you gave the
weather station in the Weather Station Information screen.
The summary information can be reviewed at any time by selecting
View | Summary from the menu. This page will also include information
on the number of final storage locations used per day.
Visual Weather Software
completion of the weather station configuration, the following dialog box
If you answer Yes, the software will attempt to connect to the weather
station. The weather station program created during configuration will be
transferred once communication is established. A message will be
returned when the program is compiled by the weather station, or if an
error occurs during the process, the message "Station could not compile
program" is displayed.
To verify that the weather station configuration was successful, go to the
main screen (Home) and select View | Collection Schedule. If you see the
weather station name and collection schedule information, the station has
been created successfully and it will be contacted for data retrieval per the
There are times when you should not send, or cannot send, a program to
the station:
If you are not a principal user or manager of the weather station(s),
and the station manager has already configured the weather station(s)
for data collection and the station is actively collecting data. If this is
the case, you should contact the station manager and ask what
sensors, scan rate, and outputs were selected during the weather
station's configuration. Simply re-create the same configuration in
Visual Weather and answer No when prompted to send the
configuration to the station. This will associate your configuration
with the one running on the station, and create a station locally on
your computer. You may create reports, monitor the real-time
weather, etc., from the weather station once it is set up.
If security is in place for the weather station. Enabled security
prohibits anyone besides the station manager from sending a new
program to the weather station.
3.4 After Configuring a Weather Station
When the weather station configuration has been created and the resulting
program transferred, Visual Weather can be left to run unattended. Note,
however, that Visual Weather must be running for scheduled data retrieval and
Visual Weather Software
batch reports to be performed. If for some reason you must close Visual
Weather, these tasks will resume once the software is restarted. Data will not
be lost unless the software has been closed a sufficient amount of time for the
weather station to begin overwriting its oldest data with new data. This can be
anywhere from 10 to 30 days depending upon the number and frequency of
sensor measurements being stored to final storage and the memory capacity of
the datalogger. It is prudent to retrieve data at least daily to protect against loss
of data. An indication of the number of final storage locations used per day is
provided on the summary information screen (View | Summary).
Data collection from the stations can be verified by choosing View | Collection
Schedule from the Visual Weather menu. The resulting window lists all
stations and data collection status. The next time Visual Weather will attempt
to collect data is displayed in the Next Data Collection column. The Collection
State column indicates if data collection is Normal, or if collection is in a
Primary or Secondary retry state.
4. Manually Connecting to a Weather Station
In addition to performing automatic scheduled data retrieval, you can connect
to a weather station manually and perform system maintenance, monitor
current weather parameters, or retrieve data.
To connect to a weather station, select Connect from the main screen. All
weather stations that you have configured using Visual Weather will be listed
on the left side of the screen under the Station Selection field. Select the
weather station with which you want to communicate and press the Connect
button. Visual Weather will attempt to establish communication with the
weather station; once connected, the Connection Status will read Connected.
The amount of time that your computer has been in communication with the
weather station is indicated by the Time Elapsed Since Connection field. When
communication is initiated manually, it will be maintained until you select the
Disconnect button, you return to the Home screen, or you close Visual
Weather. Note that whenever you are connected to a weather station, the
weather station uses more power, the phone connection remains active (thus,
phone costs may be incurred), no other station on that COM port can be
contacted for data, and no other users may connect to the weather station.
4.1 Retrieving Data Manually
To manually retrieve data from a weather station, click the Start button in the
section of the screen titled Manual Data Collection. You will see an hourglass
to indicate that Visual Weather is retrieving data. A number will be
incremented in the Records Received field as records are received from the
weather station. You will be notified by a message when the retrieval process
is complete. You can stop data retrieval at any time by selecting the Stop
Visual Weather Software
4.2 Setting a Weather Station Clock
When a successful connection is established with a weather station, the weather
station time, your computer time, and the difference between these two times
will be displayed in the lower right portion of the screen (Synchronize Clocks).
A difference of a few seconds can be ignored, but if times differ by several
minutes you may wish to set the weather station clock by selecting the Set
Station Clock button. Note that it is nearly impossible to synchronize the two
clocks exactly, since it takes time to transfer that clock setting to the weather
4.3 Manually Sending a Program to a Weather Station
Another program may be transferred to the weather station provided that:
The program you want to transfer was created using Visual Weather
software, and it is for the type of weather station you will be transferring it
to (that is, a program transferred to an ET106 must be one that was created
for an ET106). Real-time monitoring, special calculations, and report
generation assume that the program in the datalogger is in a certain format.
If this format is not used, then there will be numerous errors. Therefore,
programs created with other tools (such as Campbell Scientific’s SCWIN
or Edlog) should not be sent to a weather station if you wish to manage the
weather station using Visual Weather.
Security has not been set in the weather station to restrict program transfer,
or you have administrative rights to the security settings for the weather
Select the Send Configuration button to send a new program file. You can
also retrieve a program from a weather station by selecting Retrieve
Configuration. (Note that programs cannot be reliably retrieved over some
communications links, such as RF or slow telephone connections.)
4.4 Monitoring Weather Station Parameters and Status
When you have a connection established with a weather station, you can view
current weather parameters and the status of the weather station. To view
weather parameters, select the Monitor Current Weather button at the upper
right of the screen. This invokes a window with three tabs that displays
information for:
Air Temperature, Relative Humidity, Wind Speed/Direction
Sunshine, Rain, ETo
Moisture, Temperature, Leaf Wetness, Snow
Note that the tabs available will depend upon the sensors that are actually
being measured by the weather station.
To view datalogger status parameters (or the "health" of the datalogger), select
View Station Status. A screen will appear that displays battery voltage (main
Visual Weather Software
power source and backup), datalogger errors, datalogger internal temperature,
and enclosure relative humidity. If any of these parameters are outside the
acceptable range, the cells in which the values are displayed will be red.
5. Generating Reports
Reports can be generated from retrieved data for the last 24 hours, daily
(midnight-to-midnight), any 7 day period, or any 30 day period. Reports can be
created for an existing weather station or for a weather station that was once in
the network but has been deleted (in which case, reports will contain historical
data only).
There are four types of reports that can be selected. STATION STATUS REPORTS
contain information on the "health" of the weather station—main and backup
battery voltages, enclosure RH, and datalogger internal temperature.
STANDARD WEATHER REPORTS consist of air temperature, solar radiation,
rainfall, relative humidity, wind speed and direction, and wind rose data. By
default, the Station Status and Standard Weather reports are selected; therefore,
these reports will be generated when a report is initiated. However, you can
choose not to generate these reports if desired.
ADDITIONAL REPORTS include sensors that are not part of a typical weather
station, but can be added to a custom weather station. These include leaf
wetness, snow depth, soil or fuel temperature, and soil or fuel water content.
SPECIAL REPORTS consist of calculations for evapotranspiration (ETo),
growing-degree-days, dewpoint, wind chill factor, chill hours, and wet bulb
temperature. Most of these calculations require additional user input (such as
anemometer height and weather station elevation, longitude, and latitude when
computing ETo).
The formulas used in deriving the values for the Special Reports
are presented in the Appendices of this manual.
5.1 Manually Generated Reports
To create one or more reports, select Reports from the Main screen. Highlight
the weather station for which to generate the report(s), and select the report
period, report beginning and ending dates, and system of units (metric or nonmetric). If you would like a special image or company logo on the report(s),
select the images to be used by pressing the Browse buttons for the Station
Image and Company Logo fields. Select or clear the check boxes in the far
right column of the screen to determine which reports to generate, and click the
Create Reports button. Station Status Reports, Standard Weather Reports, and
Additional Reports will be generated immediately. If any Special Reports have
been selected, another screen will appear that has a tab for each report type.
You will need to provide additional information for most of the reports before
they can be generated. Once the information is typed in, press the Finish
button and the reports will be created.
Visual Weather Software
5.2 Batch Reports
Batch Reports let you define and save the settings so that it can be run again
and again. A Batch Report can be set to run "on demand" (that is, when
selected from a menu) or to run automatically on a defined schedule without
any interaction from the user.
To create a Batch Report choose Reports | Batch Reports | Create New Batch
from the menu on Visual Weather's Home screen. Batch Reports can be
created only for current weather stations (that is, you cannot create Batch
Reports for historical data of deleted weather stations). Configuring a Batch
Report is very similar to manual reports, except that you have additional fields
in which you determine when the report should be generated (on a schedule or
on demand) and in what format it should be output (to the printer, as a BMP, or
as a JPG).
To run reports on demand, select the Generate on Demand option, define the
report types to be generated, and press the Create Batch button. Any time you
want to run the report, select Reports | Batch Reports | Run Batch and the name
of the report you want to run.
To run reports automatically, select the Generate According to Schedule
option, set up the schedule, define the report types to be generated, and press
the Create Batch button. As long as Visual Weather is running, the report will
be run.
6. Exporting Data to a File
Data retrieved from a weather station can be exported to a file for further data
processing. From the Main screen click Data Export. Highlight the desired
weather station name from the Available Stations list. The names of the data
tables in the weather station will be displayed. The tables are named HourlyT
(one hour data), DailyT (24 hour data), and CustomT (the shorter interval
table—by default, 15 minute data).
Select one or more of the tables to be exported and choose the beginning and
ending times for the data. Click Start Export to begin. By default, exported
data will be saved in the c:\campbellsci\VisualWeather\Export folder with a
name of weatherstation.dat (where weatherstation is the name that was
provided when configuring the weather station). A different directory in which
to store the file may be specified by pressing the Browse button.
Data is exported in an ASCII comma-separated format with field names. It can
be imported into a spreadsheet or other software for further data processing.
7. Viewing All Stations in a Network
All weather stations in a network can be monitored on a single screen using
Visual Weather (provided all the weather stations were configured with the
computer system you are using to monitor them).
Visual Weather Software
Select Weather Station Network from the Main screen. This screen shows
weather stations currently in the network. You can view the current battery
voltage, enclosure relative humidity, air temperature, rain, wind speed and
direction, or solar radiation by highlighting the desired station image, selecting
the parameter you want to view from the Displayed Station Information list,
and clicking the Update Measurements button. Visual Weather will attempt
to establish communication with the weather station and retrieve the requested
information. This screen can also be used to remove a configured station from
the network, edit the communication path for an existing station, or edit the
schedule for an existing station.
8. Log Files for Troubleshooting Communication
If you experience difficulties in communicating with a weather station set up
with Visual Weather, contact your Campbell Scientific representative. You
may be asked to provide log files for your communication attempts.
Log files are enabled by selecting File | Advanced | Low Level Logs from
Visual Weather's main menu. When the log files are enabled, a check mark
will appear beside the menu item. The log files can be toggled off by selecting
the menu item a second time.
These logs contain low level communication information for all devices in the
network. They are useful in determining where the problem may lie in the
communications link. If the default directories are accepted during the
installation of Visual Weather, the logs files will be stored in
C:\CampbellSci\Visual Weather\VWComms\Logs.
Unless you are experiencing problems in communication, the log
files should be disabled. Log files can consume a substantial
amount of hard disk space since up to five 1.2 M files can be
Visual Weather Software
This is a blank page.
Appendix A. Evapotranspiration, Vapor
Pressure Deficit, and Crop Water Needs
A.1 Evapotranspiration
This appendix explains the process of evapotranspiration (ET) and the methods
Visual Weather uses to calculate hourly ET values. It also explains the
calculations for vapor pressure deficit of the air and crop irrigation based on
Evapotranspiration is the process of water loss in vapor form from a unit
surface of land both directly (evaporation) and from leaf surfaces
The energy required for vaporization comes mainly from the solar radiation
and the ambient air temperature. Higher values of solar radiation and ambient
air temperature increase ET. As the air near the soil surface saturates with
water vapor, the rate of water loss due to evaporation decreases. Therefore,
with increasing humidity ET decreases. The wind displaces saturated air with
relatively drier air. Thus, at higher wind speeds ET increases. Other factors,
such as soil composition, irrigation frequency, rain, and crop type also play
important roles that effect the process.
Given the diversity of crops, their heights, ability to absorb solar radiation,
foliage resistance to wind, and variability in wind speeds at various altitudes
some standards have to be predefined as a basis for comparisons of ET under
variable climatic conditions.
For the purpose of standardization the following definition has been adopted
for a reference crop surface.
'A hypothetical reference crop with an assumed crop height of 0.12
meters, fixed surface resistance of 70 S/m and an albedo of 0.23'.
This hypothetical reference crop surface closely resembles an adequately
watered, extensive surface of green grass of uniform height, completely
shading the ground.
The (bulk) surface resistance, rs, consists of resistance to vapor flow through
the soil surface and crop leaves. The latter is called stomatal resistance.
Stomata are small openings on leaves through which the vapor is lost. For the
reference crop the value of rs is fixed at 70 S/m.
Albedo is a crop's ability to reflect the incident radiation. Also known as
canopy reflection coefficient, its value for the reference crop is 0.23 (unitless
number). Reference crop surface albedo = (radiation energy reflected by
reference crop surface) / (solar radiation incident on the reference crop surface)
= 0.23.
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Over the years the Food and Agriculture Organization of the United Nations
(FAO) has published several documents related to evapotranspiration and other
agriculture related issues. The above standard definition is recommended in the
recent publication (FAO 56, 1998) on the subject of evapotranspiration.
The FAO 56 Penman-Montieth (PM) equation is now accepted as a method to
calculate ET from the reference crop surface, in all regions and climates. ETo
represents the value of evapotranspiration (in units of mm/hour) from the
reference crop surface.
The ETo values are calculated using the following FAO56PMETo equation.
ETo =
0.408 ∆ (R n - G) + γ (900 / (T + 273)) u 2 (e s - e a )
∆ + γ (1 + 0.34∗ u 2 )
ETo = reference evapotranspiration (mm/day);
∆ = slope of the vapor pressure versus temperature curve (kPa/°C)
Rn = net radiation at the crop surface (MJ/m²/day)
G = soil heat flux density (MJ/m²/day)
γ = psychrometric constant (kPa/°C)
T = air temperature at 2 m height (°C)
u2 = wind speed at 2 m height (m/s)
es = saturation vapor pressure (kPa)
ea = actual vapor pressure (kPa)
es - ea = saturation vapor pressure deficit (kPa)
Time steps used in ETo calculations:
The above equation yields an ETo value in mm/day. It uses values of
temperature, relative humidity, wind speed, and solar radiation averaged over a
day. It is, therefore, said to be using a daily time step for ETo calculations.
Similarly, hourly averages of temperature, relative humidity (RH), wind speed,
and solar radiation can be used in an hourly time step. Choice of the time step
to be used in ETo calculations depends on availability of required data (e.g.,
hourly averaged temperature data may or may not be available), accuracy of
ETo values desired by users, and the situation in which ETo results will be used.
Given that the main climatic factors which determine ETo values (solar
radiation, temperature, relative humidity, and wind speed) vary throughout the
day, it is obvious that using hourly data values and an hourly time step, rather
than their daily averages and a daily time step, will yield more accurate results.
Visual Weather uses an hourly time step for better accuracy in ETo
The FAO56 Penman-Montieth equation for the hourly time step is:
ETo =
0.408 ∆ (Rn - G) + γ (37 / (Thr + 273)) u 2 (e° (Thr ) - e a )
∆ + γ (1 + 0.34 u 2 )
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
ETo = reference evapotranspiration (mm/hour)
Rn = net radiation at the crop surface (MJ/m²/hour)
G = soil heat flux density (MJ/m²/hour)
Thr = mean hourly air temperature (°C)
e°(Thr) = saturation vapor pressure at Thr (kPa)
ea = average hourly actual vapor pressure (kPa)
ea = e°(Thr)*RHhr/100 ; RHhr is average hourly relative humidity (%)
u2 = average hourly wind speed (m/s), measured at 2 m height.
Other variables and their units are the same as in equation (1).
Equations used to calculate the value of each variable in equation (2), related
quantities, their units, etc., are presented below.
Please refer to Table 1 at the end of this Appendix for listings of
all quantities and their symbols and units used in the ETo
Calculations for ∆[2, 3] (slope of the vapor pressure curve).
4098[0.6108 e (17.27T)/(T+237.3) ]
(T + 237.3) 2
T is temperature in °C.
Calculations for Rn (net radiation at the crop surface).
The net radiation, Rn, is the algebraic sum of radiation at the crop's
surface, with the sign convention that the incoming radiation is positive
and the outgoing radiation is negative. The incoming radiation is higher in
energy (i.e., shorter in wavelength) than the outgoing radiation (which is
longer in wavelength). The net radiation at the crop's reference surface is:
Rn = Σ (Radiation) =
(Radiation received at reference crop surface) + (- Radiation lost)
Rn = (Solar radiation received at reference crop) + ( - Solar Radiation
reflected from the reference crop's surface) + (- Long wave radiation
outgoing from the crop/earth's surface)
Rn = Rs + (-α RS) + (Rnl) = RS (1-α) + (Rnl) =
RS (1-0.23) + Rnl = 0.77Rs + Rnl
By definition the value of α (albedo or canopy reflection coefficient) is
0.23 for the reference crop, then the net solar or short wave radiation
received at the crop, Rns = RS (1-α) = RS (1-0.23) = 0.77 RS
In the above equation, Rnl represents the outgoing, lower energy (longer
wavelength) radiation.
In the daytime incoming net radiation > outgoing net radiation, Rns > Rnl,
therefore Rn is positive.
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
In the nighttime incoming net radiation < outgoing net radiation, Rns < Rnl,
therefore Rn is negative.
The overall value of Rn is positive over a 24-hour period.
Having determined Rns = 0.77Rs, next Rnl must be calculated. Calculation
of Rnl is based on equations used in explaining black body radiation.
2a. Calculations for Rnl (long wave radiation at the crop surface).
The amount of energy radiated back into the atmosphere is proportional to
the fourth power of the absolute temperature of a radiating surface (Rnl
αT ). This is known as Stefan-Boltzmann's law, in context with black
body radiation. The proportionality constant for this relation is known as
Stefan-Boltzmann constant, σ.
The value of the Stefan-Boltzmann constant is:
4.903x 10 -9 MJ m-2 K -4 day -1 = 4.903x 10-9 x 10 6
(1 day x 24 hours / day x 3600 sec / hour)
= 5.675 x 10 W m K
However, the radiation emitted into the atmosphere gets absorbed by
clouds, vapors, dust and gases like CO2. Thus the magnitude of Rnl would
be less than that predicted by the Stefan-Boltzmann law. Therefore, in
calculating Rnl, the Stefan-Boltzmann law has to be corrected by
considering factors such as humidity and clouds and assuming that
concentrations of other absorbers remain constant. The following
equation is used to calculate the long wave Rnl for the hourly time steps.
Rnl = σ Thr (0.34 - 0.14 (e a) ) (1.35 (RS / RSO) - 0.35)
where, Rnl = net outgoing longwave radiation (MJ m hour )
σ = 2.043 x 10 (MJ m K hour )
ea = actual vapor pressure (kPa)
RS= measured solar radiation (MJ m hour )
RSO = calculated value of clear-sky solar radiation (MJ m hour )
RS/RSO = relative value of short wave radiation, 0.2 <RS/RSO <= 0.8
(no units)
Thr = hourly average temperature in Kelvin
During the daytime the ratio RS/RSO is calculated using the measured
values of Rs.
For a complete cloud cover, RS/RSO = relative value of short wave
radiation, 0.3.
The hourly values of the ratios, RS/RSO, are calculated as follows.
2b. Calculation of the ratio Rs/Rso.
Rs = measured value of solar radiation (MJ/m /hour)
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Rso = clear sky solar radiation; i.e., solar radiation with no cloud cover
(MJ/m /hour)
The general equation used to calculate the solar radiation is:
Rs = (as + bs (n/N))Ra
as = constant = 0.25
bs= constant= 0.50
n = actual sunshine hours on a given day
N = expected sunshine hours on a given day
Ra= extraterrestrial radiation
If the day is entirely clear then n = N and
Rs = (as + bs )Ra = (0.25 + 0.50)Ra ˜ 0.75Ra = Rso
If the day is entirely cloudy, then n = 0 and
Rs = (as )Ra ˜ 0.25 Ra
Therefore the ratio, Rs/Rso on a completely cloudy day would be:
Rs/Rso ˜ 0.25 Ra/0.75 Ra ˜ 0.33 about 0.3. Therefore, 0.3 < Rs/Rso < 1.0
define limits of the ratio.
In the nighttime
Rs = 0. But Rs/Rso should not be taken to be 0 or 0.3, since the sky may
be cloud free. For the nighttime
Rs/Rso = Rs/Rso calculated for a time 2 to 3 hours before sunset.
To identify the hour that is 2 to 3 hours before sunset apply the following
definition to the hour.
It is the hour for which the solar time angle ω, satisfies the following
(ωs - 0.79) <= ω < (ωs - 0.52)
where ωs is the sunset hour angle (in radians), which is calculated as
ωs = π/2 - arctan[ tan(φ)tan(δ) / X ]
where X = 1 -[ tan(φ)] [tan(δ)] and X = 0.00001 if X <= 0
Thus, to find out the value of Rs/Rso for the nighttime,
identify the value for a few hours before sunset, ω, by applying the
condition above
b) calculate the value of Rs/Rso for the near sunset hour
c) use this value of Rs/Rso for the nighttime (when Rs ˜ 0)
The lower limit on Rs/Rso should be 0.32
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
In the beginning of a dataset or datalogger initiation, calculations for the
value of ω at near sunset may be missing. In such cases for the first night
Rs/Rso = 0.7 is used as an average approximation. On subsequent nights,
the user can utilize the preceding approach based on ω near sunset and
compute Rs/Rso.
The actual vapor pressure, ea, can be calculated as shown later in items 5
and 6 below.
2c. Calculations for RSO (clear sky radiation).
Measured values of solar radiation, RS , are actually extraterrestrial
radiation (Ra) reaching the earth corrected by daily conditions; i.e.,
whether a particular day is clear or overcast. The following equation
shows this relation:
RS= (aS + bS (n/N)) Ra
Where n = number of hours of actual sunshine (hours)
N =maximum possible number of hours of expected sunshine
For a completely overcast day n = 0; therefore,
RS= aS Ra, the constant aS = fraction of extraterrestrial radiation reaching
the earth on a overcast day.
For an entirely clear day n = N
RS= (aS + bS )Ra = clear sky radiation , aS + bS = fraction of extraterrestrial
radiation reaching the earth on a clear day. Thus,
RS = (aS + bS )Ra = RSO
When calibrated values of aS and bS are not available,
RSO = (0.75 + 2 x 10 z )Ra z= station elevation in meters
2d. Calculations for Ra (extraterrestrial radiation).
The extraterrestrial radiation, Ra, for hourly steps can be calculated as
Ra =
12 (60)
G Sc dr[(ω 2 - ω1)sin(φ )sin(δ ) + cos( φ )cos(δ )(sin(ω 2) - sin(ω1))]
GSc = Solar constant = 0.0820 MJ m Min =1.36 x 10 W/m = 1.36 k
dr = inverse relative distance Earth-Sun =
1 + 0.033 cos ( (2 π /365) J ) , J is day of the year
δ = solar declination (radians) =
0.409 sin ((2 π/365) J - 1.39)
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
φ = latitude of the location (radians ) = ( π/180) x latitude in degrees
ω1 = solar time angle at the beginning of period (radians) =
ω - π t1/24 = ω - π /24 for hourly step
ω2 = solar time angle at the end of period (radians) = ω + π t1/24 = ω + π
/24 for hourly step, since t1 = 1
ω = solar time angle at midpoint of the period =
( π/12) [ (time + 0.06667( Lz - Lm ) + S c) -12]
time = standard (military) clock time at the midpoint of the period
(hours); e.g., t =14.5 hours between 1400 and 1500 hours.
Lz = Longitude of the center of the station's time zone (west of the
Greenwich line as 0 ) around the earth in clockwise direction (degrees
west of Greenwich)
Lm = Longitude of the measurement site (west of the Greenwich line as
Sc = seasonal correction for the solar time (hour) =
0.1645 sin (2b) -0.1255 cos(b) -0.025 sin(b)
b = 2π (J-81)/364 where J is day of the year between
1 (January 1) and 365 or 366 (December 31)
Calculation of G (soil heat flux).
During the day, Rs > 0 for a reference crop, G = 0.1* Rn
In the night time, Rs= 0, and G = 0.5* Rn
Calculation of γ (psychrometric constant).
γ = cP P/ελ
where, cP = specific heat capacity at constant pressure = 1.013 x 10-3
ε = ratio of molecular weight of water to molecular weight of dry
air = 0.622
λ = latent heat of vaporization of water = 2.45 MJ/kg
P = atmospheric pressure (kPa)
Substituting values of constants, γ becomes
γ = 0.000665 P
where the atmospheric pressure is obtained from the following equation:
 293 - 0.0065Z 
P = 101.3 
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
where, Z = Elevation in meters. In the above calculation the temperature
is assumed to be 20°C; hence,
T (Kelvin) = T (°C) + 273 =293. Note that by taking the value of
temperature to be 20 °C at all altitudes the temperature dependence of γ
has been ignored.
At sea level Z= 0; therefore, P = 101.3 kPa. At a higher altitude the
pressure P drops below this value, since the air is less dense. As the
altitude increases, P decreases and so does γ.
Visual Weather computes the atmospheric pressure, P, based on the userentered elevation.
Note that the latent heat of vaporization of water, λ is also a function of
temperature. However, its variation with temperature is small enough to
be neglected. For the FAO 56 PM equation the value of λ is assumed to
be constant, λ = 2.45 MJ/kg.
Calculation of e°(Thr) (saturation vapor pressure at air temperature
The saturation vapor pressure is related to air temperature, T
e°(Thr) = 0.6108 e
(17.27T) / (T + 237.3)
Note that the right-hand side of this equation forms a part of the
numerator in equation (3) above.
Calculation of ea (average hourly actual vapor pressure).
ea = e°(Thr)*RHhr/100
RHhr is the average hourly relative humidity (%)
Finally these individual results are combined to calculate hourly values of
ETo by using equation (2).
Denominator = ∆ + γ (1 + 0.34 u2 )
First term = 0.408 ∆ (Rn-G) / (∆ + γ (1 + 0.34 u2 ))
Second term = γ (37/(Thr + 273)) u2 (e°(Thr)-ea)) / (∆ + γ (1 + 0.34 u2 ))
ETo (mm/hour) = First term + Second term
The 24 hourly values of ETo calculated using equation (2) are added to
derive an ETo (mm/day) value for a day. This process is repeated 7 times
for weekly reports and 30 times for a monthly report.
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Additional Notes:
Visual Weather accesses hourly average values of temperature (T,
°C), relative humidity (RH, fraction), wind speed (u, m/s), and total
hourly solar radiation (Rs, W/m2) from the database.
Solar radiation values are stored in W/m but these are multiplied by a
factor of 0.0036 to convert them into MJ/m /hour, in order to
maintain the same units as those used in the FAO 56 publication.
Wind speed varies with altitude. Anemometers are either 2 or 3
meters for agronomy, and 10 m above the ground level if the
application is in meteorology. However, FAO 56PM equation
requires that wind speeds be specified at a standard height of 2
meters. The following equation is applied to determine wind speed at
2 m height, regardless of the anemometer position above the ground
u2 = u*
Where, u2= wind speed at standard 2 meter height
D = ln (67.8*Ht -5.42)
Ht= Height is the height of anemometer above the ground level
If Ht = 2 meters then u2 = u
A.2 Vapor Pressure Deficit of the Air
The vapor pressure deficit (kPa) of the air is the difference between the value
of saturated vapor pressure (maximum vapor it can accommodate at a given
temperature without condensation) and the actual vapor pressure (the amount
of vapor it holds at the time).
This difference is calculated by calculating the saturated vapor pressure of the
VP deficit = (e°(Thr)-ea))
e°(T) saturated vapor pressure (kPa) of the air at temperature T
(Ref. Equation (21) above)
actual vapor pressure (kPa) of the air
(Ref. Equation (22) above)
This calculation is performed within the hourly ETo calculation and displayed
in ETo reports.
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
A.3 Crop Water Needs, Crop Coefficients
A crop's life cycle consists of four stages, initial period, development stage,
mid-season stage, and the late-season stage.
The initial stage is the length of time (days) between the planting date and
the date at which approximately 10% of the ground surface is covered
with green vegetation.
The development stage is the number of days between the 10% ground
cover to nearly full ground cover. The full ground cover often coincides
with flowering.
The mid-season starts at full ground cover and ends when a crop shows
signs of maturity, e.g. yellowing of leaves.
The late season lasts between crop's maturity to its harvest.
During each stage a crop's water need will vary. This requires adjusting the
amount of water supply to the crop.
The amount of water (in mm/day or inches per day) needed by a crop is
calculated by multiplying the reference crop evaporation (ETo) described
earlier by the crop coefficient (Kc) applicable to that crop in a given stage of
its growth.
ETc = ETo x Kc
These topics, along with useful data applicable to several crops can be obtained
by accessing the following web site:
Information available at this web site should serve only as a guide. It is the
responsibility of users to provide the correct value of Kc applicable to their
situation. Contacting local agricultural agencies may provide information
about crops grown in that region.
Table 1. Names, symbols and units of all quantities used in calculations of hourly ETo values.
Mean hourly temperature
Mean hourly RH
Mean hourly wind speed at 2 meter height
Total hourly solar radiation
Meteorological Data
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Data Related to Station's Location
Weather station's longitude
Always west of Greenwich line
(with Greenwich line as 0 degrees)
Weather station's latitude
North or south of the Equator
N or S
No Units
Day of the year
J or DOY
Begin time hour (military time, hour; e.g., 1400)
End time hour (military time, hour; e.g., 1500)
Standard (military) time at step's midpoint (e.g.; 14.5)
Slope of the vapor pressure curve
Specific heat capacity at constant pressure
Ratio of mol weights of water to dry air
no units
Latent heat of vaporization of water
Atmospheric pressure
Psychrometric constant
Saturation vapor pressure at T
Actual vapor pressure
Vapor pressure deficit of the air at T
Solar constant
GSc= 0.082
Standard meridian (midpoint of the time zone)
Anemometer position above the ground level
Data Related to Time
Calculated Values
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Constant used in seasonal correction
sin (b)
Seasonal correction
Inverse relative sun-earth distance
Solar declination
Solar time angle at the beginning of period
Solar time angle at the ending of period
Solar time angle at midpoint of period
Length of the step (applies to hourly step only)
t1 = 1
Extraterrestrial radiation
Clear sky radiation
Ratio of radiation
no units
Ratio of radiation with total cloud cover
Rs/RSO= 0.3
no units
Ratio of radiation at nighttime in humid, subhumid climates
Rs/RSO= 0.4 to 0.6
no units
Ratio of radiation at nighttime in arid, semi-arid climates
Rs/RSO= 0.7 to 0.8
no units
Constant used in calculating Ra
as= 0.75
Constant used in calculating Ra,
bs*Z = 2 x 10-5*Z
(Z = elevation in meters)
Short wave, high energy radiation
Stefan-Boltzmann constant
SIGMA = 2.043 x 10 -10
SIGMA*(absolute temp)4
Long wave, low energy radiation
Net radiation (short wave-long wave radiation)
Soil heat flux (if Rs>0 then 0.1*Rn, else 0.5*Rn)
Net radiation - soil heat flux
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Denominator (Denom) in the equation for calculating ETo
Denom = ∆ + γ (1+0.34 u2)
First term in the equation for calculating ETo
First term = 0.408 * ∆ * (Rn-G) / Denom
Second term in the equation for calculating ETo
Second term =(γ * (37/(T+273)) * u2* (eo-ea)) /
Hourly ETo value
ETo (mm/hour) = first term + second term
Crop Evapotranspiration, Guidelines for Computing Crop Water
Requirements, FAO Irrigation and Drainage Paper 56, Richard G. Allen,
Luis S. Pereira, Dirk Raes, and Martin Smith, Food and Agriculture
Organization of the United Nations, Rome, 1998.
'Uber einige meteorologische Begriffe' , Tetens,O. , Z. GeoPhys., 6:297309, 1930.
'On the computation of saturation vapor pressure', Murray, F.W., J. Appl.
Meteor., 6:203-204, 1967.
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
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Appendix B. Growing Degree Days (GDD)
B.1 Growing Degree Days
This section explains the concept of growing degree-days (GDD) and methods
used in Visual Weather to calculate its hourly values.
Growing degree-days (GDD) is a measure of temperature condition that is
favorable to plant growth. A range of temperatures is defined by entering lower
and upper temperature limits. Temperatures lying within these limits are
assumed to be conducive for growth of a given plant. The lower and upper
limits of temperatures favorable to plant growth vary from plant-to-plant and
are provided by the user. Generally, temperatures above 50°F (10°C) are
considered favorable for plant growth.
B.1.1 Method
An hourly average temperature value, Ta, is compared to the upper and
lower temperature limit.
If Ta > upper temperature limit, then Ta is set equal to upper temperature
limit, and
If Ta < lower temperature limit, then Ta is set equal to lower temperature
If upper temperature limit >Ta > lower temperature limit, then
GDD = Ta- lower temperature limit is calculated for every hour.
The 24 hourly GDD values are summed.
The GDD value calculated per day is added to the previous day's GDD value.
This process is continued and a cumulative GDD value is reported. When the
cumulative GDD value reaches a threshold for a given plant value, as judged
by a user, then it is assumed that it has grown to an extent where it must be
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Appendix C. Dew Point and Wet Bulb
C.1 Dew Point
Dew point is the temperature at which air saturates, upon cooling, without
change in its water content.
C.1.1 Method
The value of saturation vapor pressure, es (kPa), is calculated using the
polynomial in Ta, the air temperature
es=(a0 + Ta*(a1+Ta*(a2+Ta*(a3+Ta*(a4+Ta*(a5+a6*Ta))))))*0.1;(1)
where Ta is the mean hourly temperature and the polynomial coefficients
The quantity es is in mbars. Since 1000 mbars = 100 kPA, 1 mbar = 0.1
kPa. Thus, multiplying the polynomial result by 0.1 converts es from
mbar into kPa.
The vapor pressure, ea (kPa), is calculated from ea = RH * eo/100, where
RH is a measured value of relative humidity (%).
The dewpoint, Td (Celsius), is calculated using the equation
Td = C3 *ln(ea/ C1) / (C2-ln(ea/ C1))
where C1 = 0.61078, C2 = 17.558, and C3 = 241.88
C.2 Wet Bulb Temperature
Evaporation requires energy. The required energy is supplied by the
surrounding. The surrounding cools due to loss of energy. For this reason,
lower temperature can be noticed if a wet cloth is wrapped around mercury
bulb and water is allowed to evaporate by blowing air on the cloth. This 'wet
bulb temperature' is compared to the 'dry bulb temperature' (mercury bulb
without wet cloth) in the same surrounding. The wet bulb temperature (Tw) is
always lower than the dry bulb temperature (Ta).
Wet bulb temperature is the lowest temperature that can be obtained by
evaporating water into surrounding air at constant pressure.
Appendix C. Dew Point and Wet Bulb Temperatures
C.2.1 Method
Calculate the dewpoint temperature, Td. Express Td in Kelvin.
Td (Kelvin) = Td (Celsius) + 273.15
Calculate the atmospheric pressure, P (mBars), using equation
P = Po* EXP(-Elevation/8500.0)
where Po = 1013 mBar = atmospheric pressure at the sea level.
Calculate the saturation vapor pressure, es (mBar), at air temperature, Ta
es = EXP(C15-C1*Ta-C2/Ta)
where C1 = 0.0091379024, C2 = 6106.396, and C15 = 26.66082.
Calculate the vapor pressure, ed (mBar), at dewpoint, Td (Kelvin)
ed = EXP(C15-C1*Td-C2/Td)
where C1 = 0.0091379024, C2 = 6106.396, and C15 = 26.66082.
Estimate the first value of the wet bulb temperature, Tw (Kelvin )
First Guess Tw = (f*P*Ta + Td*S) / (f*P + S)
where f = Cp/L*ε = 0.006355 (Kelvin ) , Cp = heat capacity of water at
constant pressure = 1004 (J/kg*K) , L = Latent heat of vaporization of
water = 2.54 x 10 (J/kg) ε =mass of water vapor/mass of dry air = 0.622,
P = atmospheric pressure (mBar) calculated above, Ta= air temperature
(Kelvin), Td= dewpoint (Kelvin), and S = (es-ed) /(Ta-Td).
Difference between quantities, Diff, as shown below is calculated
Diff = f*P*(Ta-Tw) - (ew-ed)
where ew = is vapor pressure at Tw, calculated using equation similar to
equation (6).
If the quantity Diff < 0.0001 then looping calculations are stopped and
wet bulb temperature is reported. Otherwise, new Tw is guessed (see
equations (9a) and (9b), the loop index is increased and calculation is
To find a next guess value of Tw, find derivative, DiffDer, of the Diff
DiffDer = ew* (C1 - C2/Tw ) - f*P
Next Guess of Tw = Tw - Diff / DiffDer
Appendix C. Dew Point and Wet Bulb Temperatures
New value is again tested as before. This calculation are repeated until the
condition Diff < 0.0001 is met.
Loop can be iterated at least 10 times, if not more. The final value, Tw, is
Calculating dewpoint from RH and air temperature, Technical Note 16,
December 22, 20001 Campbell Scientific Limited, England.
Iribane, J.V., Godson, W.L., Atmospheric Thermodynamics, 3 rd Edition,
pp. 259, D. Reidel, 1981.
Appendix C. Dew Point and Wet Bulb Temperatures
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Appendix D. Wind Chill
D.1 Wind Chill
Wind chill is a degree of ‘coldness’ experienced by exposed human skin due to
increasing wind speeds at a given ambient temperature. Wind chill
temperature drops with increasing wind speeds at a constant air temperature or
with decreasing air temperatures at a constant wind speed. The wind chill
effect is more pronounced when the wind speed increases and air temperature
drops. Since the degree of coldness varies from person-to-person, the wind
chill may have different meaning to different individuals in the same
Wind chill temperatures are meaningful at wind speeds exceeding 3 mph
(~1.3 m/sec) and temperatures plummeting below 50°F (10°C). For wind
speeds below 3 mph the reported wind chill temperature is equal to the ambient
air temperature.
D.1.1 Method
The wind chill equivalent temperature (°F) can be obtained by using the
following equation :
Wind Chill (°F) = 35.74 + 0.6215T – 35.75 (V ) + 0.4275T (V )
where, T = Air Temperature in °F and V = Wind Speed in mph.
Visual Weather calculates wind chill from the raw data of average values of
temperatures and wind speeds. Users need not enter any parameters to
calculate the wind chill.
D.2 References
Bulletin of the American Meteorological Society, pp. 1743-1744, Vol. 74,
No. 9, September 1993.
National Weather Service Announcement, August 2001,
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Appendix E. Chill Hours
E.1 Chill Hours
Chill hours represent the number of hours the temperature stayed at or below a
certain base (or reference) temperature provided by the user.
E.1.1 Method
The hourly average temperature value, Ta, is compared to the base
If Ta < = BaseTemp, then the number of chill hours = 1, and
If Ta > BaseTemp, then the number of chill hours = 0
A value of either 1 (if the hourly average temperature stayed at or below
the base temperature) or 0 (if the hourly average temperature stayed
above the base temperature) is assigned to each hour of the day.
These values are summed at the end of each 24-hour period. The total
value represents the number of chill hours; i.e., the number of hours the
temperature remained at or below the base temperature.
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