Weather Data Request Frequently Asked Questions

Here you can find answers to common questions about weather data.

Q. How can I get help?

A. First, download the manual.
After reading the manual if you still have questions you can email Nicholas Long. (Do not use weather@nrel.gov because a person does not read the emails.)

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Q. Why are there multiple files in the email?

A. Since there are many variables needed to make this process operate seamlessly, missing data may occur from time to time. If there is more than 48 consecutive hours of missing data then the data file is stopped and a new file is created when the data starts again.

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Q. What are the available formats for output?

A. There are two different 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 Weather Data Sources.

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

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

ColumnUnits/Notes
1. Local Date and TimeMM/DD/YYYY HH:MM1
2. Greenwich Mean TimeMM/DD/YYYY HH:MM
3. Dry Bulb TemperatureTenths of Degree Celsius
4. Dew Point TemperatureTenths of Degree Celsius
5. Wind DirectionDegrees (0º = North)
6. Wind SpeedTenths of Meter Per Second
7. Wind GustsTenths of Meter Per Second
8. Wind VariabilityRange in Degrees
9. AltimeterHundreds of Pascals
10. Visibility Tenths of Kilometers
11. Weather DescriptionMI, BC, PR, TS, BL, SH, DR, F
12. Weather PrecipitationDZ, RA, SN, SG, IC, PL, GR, GS, UP
13. Weather ObservationBR, FG, FU, VA, SA, HZ, PY, DU
14. Weather OtherSQ, SS, DS, PO, FC
15. Cloud Cover Level 1Identifier + Height in Hundreds of Feet
16. Cloud Cover Level 2Identifier + Height in Hundreds of Feet
17. Cloud Cover Level 3Identifier + Height in Hundreds of Feet
18. Sea Level PressureHundreds of Pascals

1If the program does not find a valid GMT offset for the station then this column will be the same as GMT.

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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. For a quick understanding of the values you can also download this site's manual.
The units of the data are as follows:

Entry

Units/Notes

1. Local Date and TimeYYYYMMDDHH
2. Dry Bulb TemperatureTenths of Degree Celsius
3. Dew Point TemperatureTenths of Degree Celsius
4. Wind DirectionDegrees (0º = North)
5. Wind SpeedTenths of Meter Per Second
6. AltimeterHundreds of Pascals
7. VisibilityTenths 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.

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Q. What time zone is the data reported in?

A. All the data is 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.

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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 simple breaks the file and leaves the missing data empty. The only data that are 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. 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.

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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.

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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.

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Q. Can I retrieve data without having to use the web interface?

A. Yes, to receive data without using the web interface please refer to the manual. The data retrieval is still email based and will require an email client.

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Q. What is the asterisk and can I remove it for the filled data in the 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)

Where XX is the cell that contains the first data point with or without the asterisk. Next, fill this formula down for the remainder of the column.

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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 was 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.

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

A. If you requested data that was not available in the database, then the dates are automatically adjusted to include any dates that fall within the requested range.

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Q. How do I interpret the Stations Listing File?

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

Column

Units/Notes

1. Irrelevant 
2. Irrelevant 
3. Station IDICAO Code
4. Station Name or Airport Name 
5. City 
6. State or ProvidenceIf Available
7. Country 
8. Continent Index 
9. LatitudeDegrees, Minutes
10. LongitudeDegrees, Minutes
11. ElevationMeters
12. WMO IdentifierIWEC 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. LatitudeRadians
19. LongitudeRadians
20. County 

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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).

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