Section 2 Stations
Each hourly record in the file contains values for solar radiation, illuminance, and meteorological elements. A two-character source and uncertainty flag is attached to each data value to indicate whether the data value was measured, modeled, or missing, and to provide an estimate of the uncertainty of the data value.
Users should be aware that the format of the TMY2 data files is different from the format used for the NSRDB and the original TMY data files.
File naming convention uses the WBAN number as the file prefix, with the characters
TM2 as the file extension. For example, 13876.TM2 is the TMY2 file name for
Birmingham, Alabama. The TMY2 files contain computer readable ASCII characters
and have a file size of 1.26 MB.
The first record of each file is the file header that describes the station. The
file header contains the WBAN number, city, state, time zone, latitude, longitude,
and elevation. The field positions and definitions of these header elements are
given in Table 3-1, along with sample FORTRAN and C
formats for reading the header. A sample of a file header and data for January 1
is shown in Figure 3-1.
Following the file header, 8760 hourly data records provide 1 year of solar
radiation, illuminance, and meteorological data, along with their source and
uncertainty flags. Table 3-2 provides field positions,
element definitions, and sample FORTRAN and C formats for reading the hourly records.
Each hourly record begins with the year (field positions 2-3) from which the typical month was chosen, followed by the month, day, and hour information in field positions 4-9. The times are in local standard time (previous TMYs based on SOLMET/ERSATZ data are in solar time).
For solar radiation and illuminance elements, the data values represent the energy received during the 60 minutes preceding the hour indicated. For meteorological elements (with a few exceptions), observations or measurements were made at the hour indicated. A few of the meteorological elements had observations, measurements, or estimates made at daily, instead of hourly, intervals. Consequently, the data values for broadband aerosol optical depth, snow depth, and days since last snowfall represent the values available for the day indicated.
Data for some stations, times, and elements are missing. The causes for missing
data include such things as equipment problems, some stations not operating at
night, and a NOAA cost-saving effort from 1965 to 1981 that digitized data for
only every third hour.
Although both the NSRDB and the TMY2 data sets used methods to fill data where possible, some elements, because of their discontinuous nature, did not lend themselves to interpolation or other data-filling methods. Consequently, data in the TMY2 data files may be missing for horizontal visibility, ceiling height, and present weather for up to 2 consecutive hours for Class A stations and for up to 47 hours for Class B stations. For Colorado Springs, Colorado, snow depth and days since last snowfall may also be missing. No data are missing for more than 47 hours, except for snow depth and days since last snowfall for Colorado Springs, Colorado. As indicated in Table 3-2, missing data values are represented by 9's and the appropriate source and uncertainty flags.
Source and Uncertainty Flags
With the exception of extraterrestrial horizontal and extraterrestrial direct
radiation, the two field positions immediately following the data value provide
source and uncertainty flags both to indicate whether the data were measured,
modeled, or missing, and to provide an estimate of the uncertainty of the data.
Source and uncertainty flags for extraterrestrial horizontal and extraterrestrial
direct radiation are not provided because these elements were calculated using
equations considered to give exact values.
For the most part, the source and uncertainty flags in the TMY2 data files are the same as the ones in NSRDB, from which the TMY2 files were derived. However, differences do exist for data that were missing in the NSRDB, but then filled while developing the TMY2 data sets. Uncertainty values apply to the data with respect to when the data were measured, and not as to how "typical" a particular hour is for a future month and day. More information on data filling and the assignment of source and uncertainty flags is found in Appendix A.
Tables 3-3, 3-4, 3-5, and 3-6 define the source and uncertainty flags for the solar radiation, illuminance, and meteorological elements.
Table of Contents