A national laboratory of the U.S. Department of Energy 

Office of Energy Efficiency & Renewable Energy 

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Real-Time Weather Data User’s Guide 

NREL/BR-550-34303 Access Guide March 2006 

Updated February 2006 

N. Long

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Real-Time Weather Data 

Access Guide 

Updated February 2006 

N. Long

Prepared under Task No. BEC3.4005, BEC6.1002 

User’s Guide 

NREL/BR-550-34303 

March 2006 

National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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Acknowledgments 

This work was made possible under the Department of Energy’s (DOE’s) Office of 

Energy Efficiency and Renewable Energy’s High Performance Building’s Initiative. We 

appreciate the support and guidance of Dru Crawley, Program Manager for High 

Performance Buildings as well as the following people who reviewed this document prior 

to publication: Ron Judkoff, Paul Torcellini, Mike Deru, and Sheila Hayter (National 

Renewable Energy Laboratory) and Linda Lawrie (U.S. Army Engineer Research and 

Development Center).

iii
Table of Contents 

Introduction .......................................................................................................................1

Parsing Data .....................................................................................................................1

Filling Data ........................................................................................................................1

Verification of Filling Routines................................................................................................... 3

Accessing the Database ...................................................................................................3

Requesting Data .......................................................................................................................... 3

Station Listings............................................................................................................................ 4

Weather Output Format............................................................................................................... 5

CSV ......................................................................................................................................... 6

IWEC....................................................................................................................................... 6

Contact......................................................................................................................................... 6

FAQ............................................................................................................................................. 7

References......................................................................................................................11

Tables 

Table 1 Format for Manual E-mail.................................................................................................. 4

Table 2 Station File Format............................................................................................................. 5

Table 3. Column and Units of CSV Files....................................................................................... 6

Table 4. Column and Units of CSV Files....................................................................................... 7

Table 5. Column and Units of IWEC Files .................................................................................... 8

Table 6. Station File Format........................................................................................................... 9

Figures 

Figure 1. Filling routine verification .............................................................................................. 3

iv
Introduction 

The National Weather Service (NWS) collects weather information from stations around the 

world. There are approximately 4,000 stations in the world that are currently monitored. This 

weather information is collected, parsed, and stored into a local database at the National 

Renewable Energy Laboratory (NREL). The data are available to anyone via the Internet and email. 

This database is based on an earlier database developed by the Gas Technology Institute (GTI, 

formerly the Gas Research Institute) beginning in 1998 for 234 locations in the United States. 

GTI and its subcontractors (Mason-Grant and Rocket Science) graciously donated that data and 

allowed DOE to incorporate it into this new database—providing data for some locations from 

late 1998 through today (although there may be significant gaps in the data). 

The accuracy and continuity of the data is dependent on many procedures operating seamlessly. 

The NWS ftp site must be active, the local data collection computer must be on and stable, and 

local Internet services must be available. If any of these variables are unavailable then missing 

data will occur. Also, the data received is only as good as the data reported. The parser and 

database does not determine if the data was off by a certain percent because of a failing sensor or 

unusual conditions during the time of the reading. The majority of the data that is collected is the 

dry bulb/dew point temperatures, pressure, and wind conditions. In some cases more information 

was available and is included in the database. This paper will document how the data are parsed, 

how the data are filled, and how to access the database. 

Parsing Data 

The weather-parsing program allows for gigabytes of data to be processed into an accessible 

indexed database format. The current format of the weather data received by the NWS is 

extremely inconvenient to read especially for trending historical data; therefore, a parsing 

program was created to simplify the data. Even though there are many consistent errors with the 

format of the weather data files, the data parser checks for many of the errors and compensates 

for them. 

Many stations have multiple readings around the hour. The program finds the closest entry to the 

hour and uses those values. However, if newer information (such as cloud cover) is received in a 

reading and that data point is not already stored, then the new data point is stored as if it occurred 

for that hour. The overwriting of old data will not occur unless the new time stamp is closer to 

the hour of interest or the value was not previously stored. 

Filling Data 

The weather database does not have a contiguous collection of data. There are basically two 

different failure areas; the first is on the NREL side where the connection to the Internet was 

interrupted either by a network failure or a computer failure preventing the weather data to be 

downloaded. The other failure area is on the NWS side, where the station was not accessed to 

collect the data and store it in a file. Also, heavy traffic to the NWS ftp site has prevented several 

data sets from being retrieved. 

Upon request of the data, the missing data can be exported with interpolated values for the holes. 

The filled values are not stored in the database; they are calculated every time a data request is 

received. If the user requests filled data there are two different routines that operate to smooth the 

data while a third routine simply breaks the file and skips the missing data. The only data that are 

1
filled are the dew point and dry bulb temperatures. If the missing period is less then 6 hours then 

the data are simply filled linearly or as seen in Equation (1). 

 n

tt

tftf tftf n ⋅ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

− +=

12

2 1

1

)()( )()( (1) 

Where: f(tn) is the time step to fill 

 f(t1) and f(t2) are the values around the missing time step 

If there are more than 6 hours and less then 48 hours of missing data, then the data are filled by 

taking the trend of the first previous day that is valid as seen in Equation (2). The trend is offset 

by the values that surround the missing data to smooth the filled data. 

( ) ( ) ( ) n

tt

tftftftf tftftftf d d

n dn d ⋅ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+−

−−− +−+= − −

− − 1

)()()()( )()()()(

12

2 2 1 1

1 1 (2) 

Where: f(tn) is the time step to fill 

 f(t1) and f(t2) are the values around the missing time step 

 d is the offset back to the previous valid day 

If the missing data covers more than 48 hours, then the program creates a new file where the data 

starts again. The naming convention for the new filename is an underscore and the valid file 

dates, for example: KDEN_20020824-20031015. 

The filling data routine does not scan for values outside of a given range, for instance if the 

temperature suddenly dropped 50oC, then there must have been an error with the reading of the 

station or parsing of the data file. In this case the user must manually identify these outliers and 

correct them. 

Note: When filling, if the dew point temperature is larger than the dry bulb then the program will 

force the dew point to equal the dry bulb. 

2
Verification of Filling Routines 

Figure 1 shows the dew point and dry bulb temperatures for the real data and filled data. This is 

the worst-case scenario because an abundant amount of data is missing over several days. For the 

large pieces of missing data, the trend is continued from the previous days, while for the smaller 

pieces of missing data the trend is linear. 

-200

-150

-100

-50

0

50

100

150

200

250

300

Temperature (Tenth Deg C)

03/09/2003 03/10/2003 03/11/2003 03/12/2003 03/13/2003 03/14/2003 03/15/2003 03/16/2003 03/17/20

Filled Dry Bulb

Non-Filled Dry Bulb

Fillled W et Bulb

Non-Filled W et Bulb

Figure 1. Filling routine verification 

Accessing the Database 

Requesting Data 

There are two different methods to retrieve data from the database. The preferred way to request 

data is to access the weather request web page currently located at 

http://www.eere.energy.gov/buildings/energyplus/cfm/weatherdata/weather_request.cfm. The 

process is simple and outlined on the web page. It is possible to enter the 4-character ICAO 

station ID directly or to search for a valid ID by selecting the “search” option on the home page. 

Upon completion of the request, data should be sent within an hour. 

The second way of retrieving data is to manually send an email to weather@nrel.gov with a 

specific format. It is suggested to use the web interface explained in the previous paragraph 

because a mistype in the e-mail may prevent the data from being sent. However, if it is chosen to 

use a manual e-mail, the format must be as follows: 

3
Table 1 Format for Manual E-mail 

Format Example

To: weather@nrel.gov

Subject: Weather Data 

Body: 

StationID=<Enter 4-character ICAO Station ID> 

StartDate=<Enter Start Date in MM/DD/YYYY> 

EndDate=<Enter End Date in MM/DD/YYYY> 

OutputType=<1 for CSV, 2 for IWEC> 

FillData=<0 for No, 1 for Yes>

To: weather@nrel.gov 

Subject: Weather Data 

Body: 

StationID=KDEN 

StartDate=08/24/2002 

EndDate=10/15/2002 

OutputType=1 

FillData=1

The station ID must be on the first line of the body and appear exactly as above. Also, it is not 

necessary to enter the greater than or less than signs in the text. The format of the start date and 

end date are the same. If the dates entered in the StartDate and EndDate fields are out of range of 

the actual data, then the program will converge on the valid dates. If the e-mail is in the correct 

format and the return address is valid, then the user should receive an e-mail with the data within 

an hour. 

If no data are received within eight hours, the process should be repeated. Certain e-mail clients 

do not allow for large attachments to be sent during peak hours. In this case the data may take 

longer to receive. Also, some e-mail clients do not allow for large attachments at all. In this case 

the requester should break up the dates to receive smaller files, then piece the files together 

manually. 

It is possible that no information is to be found for the station entered and in this case an empty 

email will be returned to the user. If the data request occurs during the night or during a weekend 

(MDT) then the process may take longer because maintenance operations are running during 

these hours. 

Station Listings 

It is possible to retrieve a listing of all the valid stations and the data range by selecting “Retrieve 

a list of stations and data availability” on the web page or by sending an e-mail to 

weather@nrel.gov with “Station Listings” in the subject (the body can be blank). 

The format of the station file is below. It is semicolon delimited with the following information 

in each column, starting at column 1. If the entry is null, then the column will contain a period. 

4
Table 2 Station File Format 

Column Units/Notes 

1. Irrelevant 

2. Irrelevant 

3. Station ID ICAO Code 

4. Station Name or Airport Name 

5. City 

6. State or Providence If Available 

7. Country 

8. Continent Index 

9. Latitude Degrees, Minutes 

10. Longitude Degrees, Minutes 

11. Elevation in Meters Meters 

12. WMO Identifier – IWEC Code 

13. Time Zone Offset From GMT 

14. First Valid Timestamp 

15. Last Valid Timestamp 

16. Number of Valid Records 

17. Percentage of Valid Records 

18. Latitude in Radians 

19. Longitude in Radians 

20. County 

21. Alternate 3-character Station Code 

This file is automatically created every weekend to update the valid station dates. Also, the 

database is continually adjusted to reflect station information because exhaustive information 

(such as GMT to LST offsets) for the stations could not be found and are continually adjusted 

when new information is found. 

Weather Output Format 

After a request has been received via the e-mail with appropriate fields, the local program will 

attempt to output the data. If there was an error with the e-mail format (this will typically occur if 

the user manually sends an email instead of using the web interface) then the user will receive an 

email with the correct format outlined in the body. If no data are available for the site, then the 

user will receive an email with an attachment stating “Station or Data For Station Not Found.” 

Other errors may occur if the subject has the wrong title. If no email is received within 8 hours, 

try requesting the data again. 

When requesting data, the program searches for a station listing that details the parameters of the 

station such as longitude, latitude, and time offset for Greenwich Mean Time (GMT). All data 

requests are in GMT. If the user requests data that is outside of the available time stored in the 

database, then the program will only report the data that falls within the bounds. If no station 

information was found or the time offset was not present, then a line in the output file will warn 

the user that the time stamps are not in local standard time. All IWEC files need to be in local 

standard time before using them in an energy simulator. Conversely, if the time offset values are 

found, then the time stamps will be in local standard time. 

The output of the weather data is available in two formats: Comma Separated Values (CSV) and 

International Weather for Energy Calculations (IWEC). 

5
CSV 

The CSV files have the following columns and respective units: 

Table 3. Column and Units of CSV Files 

Column Units/Notes 

1. Local Date and Time MM/DD/YYYY HH:MM1

2. Greenwich Mean Time1 MM/DD/YYYY HH:MM 

3. Dry Bulb Temperature Tenths of Degree Celsius 

4. Dew Point Temperature Tenths of Degree Celsius 

5. Wind Direction Degrees (0o = North) 

6. Wind Speed Tenths of Meter Per Second 

7. Wind Gusts Tenths of Meter Per Second 

8. Wind Variability Range in Degrees 

9. Altimeter Hundreds of Pascals 

10. Visibility Tenths of Kilometers 

11. Weather Description MI, BC, PR, TS, BL, SH, DR, F 

12. Weather Precipitation DZ, RA, SN, SG, IC, PL, GR, GS, UP 

13. Weather Observation BR, FG, FU, VA, SA, HZ, PY, DU 

14. Weather Other SQ, SS, DS, PO, FC 

15. Cloud Cover Level 1 Identifier + Height in Hundreds of Feet 

16. Cloud Cover Level 2 Identifier + Height in Hundreds of Feet 

17. Cloud Cover Level 3 Identifier + Height in Hundreds of Feet 

18. Sea Level Pressure Hundreds of Pascals 

More information on the cloud cover and weather descriptors can be found in the Surface 

Weather Observations and Reports Handbook, http://www.ofcm.gov/fmh-1/pdf/fmh1.pdf. [1]. If 

filled data is requested then there will be an asterisk next to the filled value. An easy way to 

remove the asterisk is to load the data into Microsoft Excel and type the following formula into 

the cell adjacent to the asterisk. The “XX” is the cell that contains the data. Simply fill this 

formula, (3), down to the end of the column. 

= XXTrimRightIF = XXTrimLenXXTrimLeftValue − XX ))),1))((),(((,"*")1),((( (3) 

IWEC 

The IWEC format is based on the field position. The information in the IWEC file is available in 

the IWEC manual published by ASHRAE. [2]. 

Contact 

For more information please contact Nicholas Long, nicholas_long@nrel.gov. Do not e-mail 

questions to weather@nrel.gov because a person does not read these e-mails. 

 

1

 If the program does not find a valid GMT offset for the station, then this column will be the same as 

GMT. 

6
FAQ 

Q. How can I get help?

A. First, download the manual, 

http://www.eere.energy.gov/buildings/energyplus/pdfs/weatherdata_guide_34303.pdf. After 

reading the manual, if you still have questions you can e-mail Nicholas Long, 

nicholas_long@nrel.gov. Do not use weather@nrel.gov because a person does not read the emails. 

Q. Why are there multiple files in the e-mail? 

A. Since there are many variables needed to make this process operate seamlessly, missing data 

may occur from time to time. If there are more than 48 consecutive hours of missing data, then 

the data file is closed and a new file is created where the data starts again. 

Q. What are the available formats for output? 

A. There are two formats available for output: Comma Separate Values (CSV) and International 

Weather for Energy Calculations (IWEC). CSV is the raw data separated by commas for easy 

import into several different spreadsheet programs. IWEC files contain a specific format used by 

programs to evaluate energy performance of buildings. For more information on IWEC files visit 

http://www.eere.energy.gov/buildings/energyplus/weatherdata_sources.html. 

Q. What are the units when the data are in CSV format?

A. The columns and units in the CSV format are as follows: 

Table 4. Column and Units of CSV Files 

Column Units/Notes 

1. Local Date and Time MM/DD/YYYY HH:MM1

2. Greenwich Mean Time MM/DD/YYYY HH:MM 

3. Dry Bulb Temperature Tenths of Degree Celsius 

4. Dew Point Temperature Tenths of Degree Celsius 

5. Wind Direction Degrees (0o = North) 

6. Wind Speed Tenths of Meter Per Second 

7. Wind Gusts Tenths of Meter Per Second 

8. Wind Variability Range in Degrees 

9. Altimeter Hundreds of Pascals 

10. Visibility Tenths of Kilometers 

11. Weather Description MI, BC, PR, TS, BL, SH, DR, F 

12. Weather Precipitation DZ, RA, SN, SG, IC, PL, GR, GS, UP 

13. Weather Observation BR, FG, FU, VA, SA, HZ, PY, DU 

14. Weather Other SQ, SS, DS, PO, FC 

15. Cloud Cover Level 1 Identifier + Height in Hundreds of Feet 

16. Cloud Cover Level 2 Identifier + Height in Hundreds of Feet 

17. Cloud Cover Level 3 Identifier + Height in Hundreds of Feet 

18. Sea Level Pressure Hundreds of Pascals 

1

If the program does not find a valid GMT offset for the station then this column will be the same as GMT. 

7
Q. What are the units when the data are in IWEC format?

A. The format for the IWEC files are very cryptic, to get a full understanding of IWEC files 

download the IWEC Manual published by ASHRAE. The units of the data are as follows: 

Table 5. Column and Units of IWEC Files 

Entry Units/Notes 

Local Date and Time YYYYMMDDHH 

Dry Bulb Temperature Tenths of Degree Celsius 

Dew Point Temperature Tenths of Degree Celsius 

Wind Direction Degrees (0o = North) 

Wind Speed Tenths of Meter Per Second 

Altimeter Hundreds of Pascals 

Visibility Tenths of Kilometers 

If the program does not find a valid GMT offset for the station, then the data will state that an 

error occurred while trying to offset the data. In this instance, all the data is in GMT and should 

be converted by the user. Also, the hours are from 1 to 24 not 0 to 23. 

Q. What time zone is the data reported in?

A. All the data are stored in Greenwich Mean Time (GMT) but upon retrieval, the program 

attempts to convert the time steps to local standard time (LST). If the time steps are not 

converted, it is most likely because the station database did not contain an offset variable for the 

time zone. Also, when the time steps are not converted, a warning line is inserted into the data 

file to warn that the steps are incorrect and that they need to be corrected manually. 

Q. How are missing data filled?

A. When there are missing data, the requestor has the option of filling the hole or leaving it 

blank. If the user requests filled data there are two different routines that operate to smooth the 

data while a third routine simply breaks the file and leaves the missing data empty. The only data 

that are filled is the dew point and dry bulb temperatures. If the missing period is less then 6 

hours, then the data are simply filled linearly. If there are more than 6 hours and less then 48 

hours of missing data, then the data are filled by taking the trend of the first valid previous day. 

The trend is offset by the values that surround the missing data to smooth the filled data. 

If the missing data covers more than 2 days or 48 hours, then the program creates a new file 

where the data starts again. The naming convention for the new filename is an underscore and 

the valid file dates, for example KDEN_20020824-20031015. 

Note: When filling data, if the dew point temperature is larger than the dry bulb then the program 

will force the dew point to equal the dry bulb. 

Q. Does the parser check the data to make sure the values seem reasonable?

A. No, the parser does not check the data for reasonability. There are cases when the 

temperature increases or decreases more than a reasonable amount. Manual plotting or 

subsequent processing on the data sets should be used to determine these values. 

Q. Can I receive data without having the data filled?

A. Yes, when you request the data from the web page make sure the “Fill Missing Time Steps” 

under “Step 2: Choose Dates and Formats” checkbox is not selected. The data you receive will 

only contain the data that is stored in the local database. 

8
Q. What is the asterisk and can I remove it for the data in CSV Format?

A. The asterisk indicates that the data point was filled. The best way to remove the asterisks for 

the filled data is to import all the data into Microsoft Excel, make sure to put the data into 

individual columns by selecting “Text to columns” under the “Data” menu. Create new columns 

next to the columns of data that contain the asterisks. In the new columns enter the following 

formula: 

=IF(Right(Trim(XX,1)=”*”,Value(Left(Trim(XX),Len(Trim(XX))-1)),XX) (4) 

Where XX is the cell that contains the first data point with or without the asterisk. Then simply 

fill this formula down to the end of the column. 

Q. What is the file naming convention? 

A. The typical naming convention is the station ID with the start date and end date appended. 

For example KDEN_20020824-20021015.csv contains data for Denver from August 24, 2002 

through October 15, 2002. If you chose to have IWEC information, then if the WMO code is 

found in the station database, then the station ID is replaced with the WMO code. For example 

724760_20020824-20021015.iwc contains the same information as above but in IWEC format. 

Q. Why are the dates in the file different than the dates I requested?

A. If you requested data that were not available in the database, then the dates are automatically 

adjusted to include any dates that fall within the requested range. 

Q. How do I interpret the Stations Listing File?

A. The station file is comma delimited and contains the following information in each column: 

Table 6. Station File Format 

Column Units/Notes 

1. Irrelevant 

2. Irrelevant 

3. Station ID ICAO Code 

4. Station Name or Airport Name 

5. City 

6. State or Providence If Available 

7. Country 

8. Continent Index 

9. Latitude Degrees, Minutes 

10. Longitude Degrees, Minutes 

11. Elevation in Meters Meters 

12. WMO Identifier – IWEC Code 

13. Time Zone Offset From GMT 

14. First Valid Timestamp 

15. Last Valid Timestamp 

16. Number of Valid Records 

17. Percentage of Valid Records 

18. Latitude in Radians 

19. Longitude in Radians 

20. County 

9
Q. What is the source of the data?

A. The National Weather Service (NWS) collects weather information from stations across the 

world in METAR format. There are over 4,000 stations in the world that are currently monitored. 

This weather information is collected, parsed, and stored into a local database at the National 

Renewable Energy Laboratory (NREL). 

Q. What happens if my mail server does not allow large attachments?

A. Certain e-mail clients do not allow for large attachments to be sent during peak hours. In this 

case the data may take longer to receive. Also, some e-mail clients do not allow for large 

attachments at all. In this case the requester should break up the dates to receive smaller files, 

then piece the files together manually. If you believe your mail client does not allow large 

attachments, then please wait 24 hours before requesting the data again. 

10
References 

[1] Surface Weather Observations and Reports. FCM-H1-1995. Washington, D.C.: U.S. Department of 

Commerce/National Oceanic and Atmospheric Administration. December 1995. 

[2] International Weather for Energy Calculations (IWEC Weather Files) User’s Manual. ASHRAE. 

March 31, 2001. 

11