Preface

This user's manual describes typical meteorological year (TMY) data sets derived from the 1961-1990 National Solar Radiation Data Base (NSRDB). Because they are based on more recent and accurate data and will make possible more accurate performance and economic analyses of energy systems, these data sets are recommended for use in place of earlier TMY data sets derived from the 1952-1975 SOLMET/ERSATZ data base.

To distinguish between the old and new TMY data sets, the new TMY data sets are referred to as TMY2s. TMY and TMY2 data sets cannot be used interchangeably because of differences in time (solar versus local), formats, elements, and units. Unless they are revised, computer programs designed for TMY data will not work with TMY2 data.

The TMY2s are data sets of hourly values of solar radiation and meteorological elements for a 1-year period. Their intended use is for computer simulations of solar energy conversion systems and building systems to facilitate performance comparisons of different system types, configurations, and locations in the United States and its territories.

Because they represent typical rather than extreme conditions, they are not suited for designing systems to meet the worst-case conditions occurring at a location.

The TMY2 data sets and this manual were produced by the National Renewable Energy Laboratory's (NREL's) Analytic Studies Division under the Resource Assessment Program, which is funded and monitored by the U.S. Department of Energy's Office of Solar Energy Conversion.

Approved for the NATIONAL RENEWABLE ENERGY LABORATORY
Thomas D. Bath, Director Dave Rennè , Manager
Analytic Studies Division Resource Assessment Program

SECTION 1

Overview

This user's manual describes typical meteorological year (TMY) data sets derived from the 1961-1990 National Solar Radiation Data Base (NSRDB). Based on more recent and accurate data, these data sets are recommended for use in place of earlier TMY data sets (NCDC 1981) that were derived from the 1952-1975 SOLMET/ERSATZ data base (SOLMET - Vol. 1 1978 and SOLMET - Vol. 2 1979). To distinguish between the two TMY data sets, the new TMY data sets are referred to as TMY2s.

TMY and TMY2 data sets cannot be used interchangeably because of differences in time (solar versus local), formats, elements, and units. Unless they are revised, programs designed for TMY data will not work with TMY2 data.
Section 1 of the manual provides general information about the TMY2s and how they were developed; Section 2 lists the stations and provides station identifying information and classification; Section 3 details the contents of the TMY2 files and provides the location in the hourly records of data values and their source and uncertainty flags; Section 4 compares the TMY2s with 30-year data sets; Appendix A provides a description of the procedures used to develop the TMY2s; Appendix B provides a key for present weather elements; and Appendix C contains a table of unit conversion factors for converting SI data to other units.

Typical Meteorological Year - A Description

A TMY is a data set of hourly values of solar radiation and meteorological elements for a 1-year period. It consists of months selected from individual years and concatenated to form a complete year. The intended use is for computer simulations of solar energy conversion systems and building systems. Because of the selection criteria, TMYs are not appropriate for simulations of wind energy conversion systems.

A TMY provides a standard for hourly data for solar radiation and other meteorological elements that permit performance comparisons of system types and configurations for one or more locations. A TMY in not necessarily a good indicator of conditions over the next year, or even the next 5 years. Rather, it represents conditions judged to be typical over a long period of time, such as 30 years. Because they represent typical rather than extreme conditions, they are not suited for designing systems and their components to meet the worst-case conditions occurring at a location.

NSRDB - Source of Data for the TMY2s

The TMY2s were derived from the NSRDB, Version 1.1, which was completed in March 1994 by the National Renewable Energy Laboratory (NREL). The NSRDB contains hourly values of measured or modeled solar radiation and meteorological data for 239 stations for the 30-year period from 1961-1990. A complete description of the NSRDB and how it was produced is presented in its user's manual (NSRDB - Vol. 1 1992) and the final technical report (NSRDB - Vol. 2 1995). The original version of the NSRDB, Version 1.0, was completed in August 1992. Version 1.1 corrects two types of minor errors in Version 1.0 that affected about 10% of the stations (Rymes 1994).

Succeeding the older 1952-1975 SOLMET/ERSATZ data base, the NSRDB accounts for any recent climate changes and provides more accurate values of solar radiation for several reasons:

• Better model for estimating values (More than 90% of the solar radiation data in both data bases are modeled.)
• More measured data, some of which is direct normal radiation
• Improved instrument calibration methods
• Rigorous procedures for assessing quality of data.

A comparison of the old and new data bases provided an incentive for developing the TMY2s. On an annual basis, 40% of the NSRDB and SOLMET/ERSATZ stations are in disagreement for global horizontal radiation by more than 5%, with some stations showing disagreement of up to 18% (Marion and Myers 1992). For direct normal radiation, 60% of the NSRDB and SOLMET/ERSATZ stations are in disagreement by more than 5%, with some stations showing disagreement of up to 33%. Disagreement between the two data bases is even greater when compared on a monthly basis.

An analysis of cloud cover data indicated little or no change for the two periods; consequently, most of the disagreement for NSRDB and SOLMET/ERSATZ data is attributed to differences in reconstructing the instrument calibrations and differences in the solar radiation models (NSRDB - Vol. 2 1995).

Because of differences in the data bases from which they were derived, the old TMYs and the new TMY2s will differ. For some stations, the differences may be minor, but other stations will have large differences.

Methodology

Except for a few changes to the weighting criteria, which accounts for the relative importance of the solar radiation and meteorological elements, the TMY2s were created using similar procedures that were developed by Sandia National Laboratories (Hall et al. 1978) to create the original TMYs from the 1952-1975 SOLMET/ERSATZ data. Studies by Freeman (1979), Siurna, D'Andrea, and Hollands (1984), and Menicucci and Fernandez (1988) have shown that this procedure gives reasonable results. Sandia's procedure has also been adopted by Siurna, D'Andrea, and Hollands (1984) for developing TMYs for Canada.

The Sandia method is an empirical approach that selects individual months from different years from the period of record. For example, in the case of the NSRDB that contains 30 years of data, all 30 Januarys are examined, and the one judged most typical is selected to be included in the TMY. The other months of the year are treated in a like manner, and then the 12 selected typical months are concatenated to form a complete year.

The 12 selected typical months for each station were chosen from statistics determined by using five elements: global horizontal radiation, direct normal radiation, dry bulb temperature, dew point temperature, and wind speed. These elements are considered the most important for simulation of solar energy conversion systems and building systems.
For other elements in the TMY2s, the selected months may or may not be typical. Cloud cover, which correlates well with solar radiation, is probably reasonably typical. Other elements, such as snow depth, are not related to the elements used for selection; consequently, their values may not be typical. Even though wind speed was used in the selection of the typical months, its relatively low weighting with respect to the other weighted elements prevents it from being sufficiently typical for simulation of wind energy conversion systems.

Appendix A contains a more detailed description of the procedures used to develop the TMY2s.

TMY2 Station Classification

The TMY2 station classification pertains to the amount of measured meteorological data available for a station to select typical months to form the typical meteorological year. Of a possible 30 candidate months, Class A stations had a minimum of 15 candidate months, without more than 2 consecutive hours of missing data, from which a typical month was selected. For Class B stations to achieve a minimum of 15 candidate months, data filling for periods of up to 47 hours were required. For some elements not required for the selection of the typical meteorological months, the data are unfilled in the TMY2 data files. The elements horizontal visibility, ceiling height, and present weather may be missing for up to 2 consecutive hours for Class A stations and for up to 47 hours for Class B stations. No data are missing for more than 47 hours, except for snow depth and days since last snowfall for Colorado Springs, Colorado.

Data Elements

Table 1-1 shows the data elements in the TMY2 data files. These are the same elements as for the 30-year NSRDB, except that illuminance and luminance elements were added to support building energy analysis. The table includes information by element and station classification to alert the user to the possibility of missing data. Definitions of the elements and their units are provided in Table 3-2 of Section 3.

Where to Order

TMY2 data sets are available over Internet from NREL's Renewable Resource Data Center (RReDC). The Universal Resource Locator (URL) address of the RReDC is http://rredc.nrel.gov. Users should have World Wide Web (WWW) browsing software, such as Mosaic or Netscape, to access the RReDC.
TMY2 data sets for all 239 stations may also be obtained on a CD-ROM. A "Readme" file, which describes the contents, is included on the CD-ROM.
The TMY2 CD-ROM may be ordered by

• fax using the TMY2 CD-ROM fax order form
• electronically by filling out the RReDC's TMY2 Data CD-ROM online order form.

Cost for the CD-ROM is $15.00 (US).

References

Freeman, T. L. (1979). Evaluation of the "Typical Meteorological Years" for Solar Heating and Cooling System Studies. SERI/TR-8150-1. Golden, CO: Solar Energy Research Institute.
Hall, I.; Prairie, R.; Anderson, H.; Boes, E. (1978). Generation of Typical Meteorological Years for 26 SOLMET Stations. SAND78-1601. Albuquerque, NM: Sandia National Laboratories.
Marion, W.; Myers, D. (1992). A Comparison of Data from SOLMET/ERSATZ and the National Solar Radiation Data Base. NREL/TP-463-5118. Golden, CO: National Renewable Energy Laboratory.
Menicucci, D.; Fernandez, J. (1988). A Comparison of Typical Year Solar Radiation Information with the SOLMET Data Base. SAND87-2379. Albuquerque, NM: Sandia National Laboratories.
NCDC (1981). Typical Meteorological Year User's Manual. Asheville, NC: National Climatic Data Center.
NSRDB - Vol. 1 (1992). User's Manual - National Solar Radiation Data Base (1961-1990).
Version 1.0. Golden, CO: National Renewable Energy Laboratory and Asheville, NC: National Climatic Data Center.
NSRDB - Vol. 2 (1995). Final Technical Report: National Solar Radiation Data Base (1961-1990). NREL/TP-463-5784. Golden, CO: National Renewable Energy Laboratory.
Rymes, M. (1994). "Beyond the NSRDB Version 1.0." Proceedings of the 1994 Annual American Solar Energy Society Conference; June 25-30, 1994, San Jose, CA; pp. 445-450.
Siurna, D.L.; D'Andrea, L.J.; Hollands, K.G.T. (1984). "A Canadian Representative Meteorological Year for Solar System Simulation." Proceedings of the 10th Annual National Conference of the Solar Energy Society of Canada; August 2-6, 1984, Calgary, Alberta; pp. 85-88.
SOLMET, Vol. 1 (1978). User's Manual - Hourly Solar Radiation-Surface Meteorological Observations. TD-9724. Asheville, NC: National Climatic Data Center.
SOLMET, Vol. 2 (1979). Final Report - Hourly Solar Radiation-Surface Meteorological Observations. TD-9724. Asheville, NC: National Climatic Data Center.

SECTION 2

Stations

There are 239 TMY2 stations for the United States and its territories. These are the same stations as for the NSRDB, from which the TMY2 data sets were derived. The stations are National Weather Service stations that collected meteorological data for the period of 1961-1990. Table 2-1 lists the stations by state or territory and provides information describing the station location and the NSRDB and TMY2 classifications.

Compared to the SOLMET/ERSATZ TMYs, there is a net gain of five stations, and some of the station locations have changed. The TMY2 data sets include 37 new stations, but 32 previous SOLMET/ERSATZ TMY stations were not included because these stations were not included in the NSRDB.

Locations

The station locations are described in Table 2-1 by the city and state name, the station Weather Bureau Army Navy (WBAN) identification number, the latitude and longitude in degrees and minutes, and the elevation in meters.

NSRDB Classification

Stations are classified with respect to being NSRDB primary (P) or secondary (S) stations. The 56 primary stations measured solar radiation for a part (from 1 to 27 years) of the 30-year period of 1961-1990. The remaining 183 secondary stations made no solar radiation measurements and therefore use modeled solar radiation data that are derived from meteorological data such as cloud cover.

TMY2 Classification

This classification pertains to the amount of measured meteorological data available for a station to select typical months to form the typical meteorological year. Class A stations, of which there are 216, had a minimum of 15 candidate months without more than 2 consecutive hours of missing data. For the 23 Class B stations to achieve a minimum of 15 candidate months, data filling for periods of up to 47 hours were required. For some elements not required for the selection of the typical meteorological months, the data are unfilled in the TMY2 data files. The elements horizontal visibility, ceiling height, and present weather may be missing for up to 2 consecutive hours for Class A stations and for up to 47 hours for Class B stations. No data are missing for more than 47 hours, except for snow depth and days since last snowfall for Colorado Springs, Colorado.

SECTION 3

Data and Format

For each station, a TMY2 file contains 1 year of hourly solar radiation, illuminance, and meteorological data. The files consist of data for the typical calendar months during 1961-1990 that are concatenated to form the typical meteorological year for each station.

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 Convention

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.

File Header

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.

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.

Missing Data

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.

SECTION 4

Comparison with Long-Term Data Sets

The TMY2 data were compared with 30-year data sets to show differences between TMY2 data and long-term data for the same stations. Comparisons were made on a monthly and annual basis for global horizontal, direct normal, and south-facing latitude tilt radiation; and for heating and cooling degree days. These comparisons give general insight into how well, with respect to long-term conditions, the TMY2s portray the solar resource and the dry bulb temperature environment for simulations of solar energy conversion systems and building systems. On an annual basis, the TMY2s compare closely to the 30-year data sets. The monthly comparisons are less favorable than the annual comparisons.

Solar Radiation Comparisons

Monthly and annual solar radiation for the TMY2 data sets were compared with previously determined (Marion and Wilcox 1994) monthly and annual averages for the 1961-1990 NSRDB, from which the TMY2 data sets were derived. These comparisons were made for global horizontal, direct normal, and a fixed surface facing south with a tilt angle from horizontal equal to the station's latitude.

Results of these comparisons are shown in Figures 4-1 through 4-6. TMY2 values for all stations are plotted against their respective 30-year average from the 1961 - 1990 NSRDB. As indicated by the scatter of the data and the statistical information at the top of the figures, agreement is better on an annual basis than on a monthly basis. This is a consequence of cancellation of some of the monthly differences when the monthly values are summed for the annual value. The statistical information presented is the mean difference between the TMY2 value and the 1961-1990 average and the standard deviation of the differences.

Table 4-1 provides 95% confidence intervals, determined as twice the standard deviation of the differences between TMY2 and NSRDB values, for TMY2 monthly and annual solar radiation. The confidence intervals are given in units of kWh/m2/day. Differences between TMY2 and NSRDB 30-year values should be within the confidence interval 95% of the time.

Heating and Cooling Degree Day Comparisons

Degree days are the difference between the average temperature for the day and a base temperature. If the average for the day (calculated by averaging the maximum and minimum temperature for the day) is less than the base value, then the difference is designated as heating degree days. If the average for the day is greater than the base value, the difference is designated as cooling degree days.

Monthly and annual heating and cooling degree days (base 18.3^oC) calculated from the TMY2 data sets were compared with those for the same stations from NCDC's data tape, "1961-1990 Monthly Station Normals All Elements." This data tape includes temperature and degree day normals for about 4775 stations in the United States and its territories. The normals are averages computed by NCDC for the period 1961-1990.

Results of these comparisons are shown in Figures 4-7 through 4-10. TMY2 values for all stations are plotted against their respective 30-year average from NCDC's data tape. As seen for solar radiation, agreement is better on an annual basis than on a monthly basis.

Table 4-2 provides 95% confidence intervals, determined as twice the standard deviation of the differences between TMY2 and NCDC values, for TMY2 monthly and annual heating and cooling degree days. The confidence intervals are given in units of degree days. Differences between TMY2 and NCDC 30-year values should be within the confidence interval 95% of the time.

References

Marion, W.; Wilcox, S. (1994). Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors. NREL/TP-463-5607. Golden, CO: National Renewable Energy Laboratory

APPENDIX A

Procedures for Developing TMY2s

The TMY2s were created based on the procedures that were developed by Sandia National Laboratories (Hall et al. 1978) to create the original TMYs from the 1952-1975 SOLMET/ERSATZ data. Modifications to the Sandia method were made to better optimize the weighting of the indices, to provide preferential selection for months with measured solar radiation data, and to account for missing data. This appendix begins by summarizing the Sandia method, and then it discusses departures from the Sandia method that were used to create the TMY2 data sets.

Sandia Method

The Sandia method is an empirical approach that selects individual months from different years of the period of record. For example, in the case of the NSRDB that contains 30 years of data, all 30 Januarys are examined and the one judged most typical is selected to be included in the TMY. The other months of the year are treated in a like manner, and then the 12 selected typical months are concatenated to form a complete year. Because adjacent months in the TMY may be selected from different years, discontinuities at the month interfaces are smoothed for 6 hours on each side.

The Sandia method selects a typical month based on nine daily indices consisting of the maximum, minimum, and mean dry bulb and dew point temperatures; the maximum and mean wind velocity; and the total global horizontal solar radiation. Final selection of a month includes consideration of the monthly mean and median and the persistence of weather patterns. The process may be considered a series of steps.

Step 1 - For each month of the calendar year, five candidate months with cumulative distribution functions (CDFs) for the daily indices that are closest to the long-term (30 years for the NSRDB) CDFs are selected. The CDF gives the proportion of values that are less than or equal to a specified value of an index.

Candidate monthly CDFs are compared to the long-term CDFs by using the following Finkelstein- Schafer (FS) statistics (Finkelstein and Schafer 1971) for each index.

where

= absolute difference between the long-term CDF and the candidate month CDF at xi n = the number of daily readings in a month.

Four CDFs for global horizontal solar radiation for the month of June are shown in Figure A-1. Compared to the long-term CDF by using FS statistics, the CDF for June of 1981 compared the best and the CDF for June of 1989 compared the worst. Even though it was not the best month with respect to the long-term CDF, June of 1962 was selected for the TMY2. This was a consequence of additional selection steps described in the following paragraphs. Because some of the indices are judged more important than others, a weighted sum (WS) of the FS statistics is used to select the 5 candidate months that have the lowest weighted sums.

where

wi = weighting for index
FSi = FS statistic for index.

Step 2 - The 5 candidate months are ranked with respect to closeness of the month to the long-term mean and median.
Step 3 - The persistence of mean dry bulb temperature and daily global horizontal radiation are evaluated by determining the frequency and run length above and below fixed long-term percentiles. For mean daily dry bulb temperature, the frequency and run length above the 67th percentile (consecutive warm days) and below the 33rd percentile (consecutive cool days) were determined. For global horizontal radiation, the frequency and run length below the 33rd percentile (consecutive low radiation days) were determined.

The persistence data are used to select from the five candidate months the month to be used in the TMY. The highest ranked candidate month from step 2 that meets the persistence criteria is used in the TMY. The persistence criteria excludes the month with the longest run, the month with the most runs, and the month with zero runs.

Step 4 - The 12 selected months were concatenated to make a complete year and smooth discontinuities at the month interfaces for 6 hours each side using curvefitting techniques.

Weighting and Indice Modifications

The weighting for each index plays a role in the selection of the typical months. Ideally, one would select a month that had FS statistics for each index that were better than all the other months. In practice, this is unlikely because the months might be typical with respect to some of the indices, but not others. By weighting the FS statistics, the relative importance and sensitivity of the indices may be taken into account. The Sandia weighting values and the weighting values used for the TMY2s are compared in Table A-1.

For the TMY2s, an index for direct normal radiation was added. This improves the comparison between annual direct normal radiation for the TMY2s and the 30-year annual average by about a factor of 2 (based on 20 geographically representative NSRDB stations). When only global horizontal radiation is used for the solar index, the TMY annual direct radiation values for the 20 stations were within 4% (95% confidence level) of the 30-year annual average. Using both global horizontal and direct radiation indices reduced the differences to 2%, with no adverse effect on global horizontal radiation comparisons.

Weightings for dry bulb and dew point temperature were changed slightly to give more emphasis to dry bulb and dew point temperatures and less to wind velocity, which is of less importance for solar energy conversion systems and buildings. Neither of the TMY weightings is appropriate for wind energy conversion systems.

The relative weights between solar and the other elements were not found to be particularly sensitive. As an indicator, annual heating and cooling degree days (base 18.3^oC) were compared for the TMY2s and the 30-year period for the 20 stations. With the selected solar weighting of 50% (global and direct), annual heating degree days for the TMY2s were within 5% (95% confidence level) of the 30-year annual average. As an extreme, reducing the solar weighting to zero only reduced the differences to within 2.5%. Differences between the TMY2 annual averages and the 30-year averages for cooling degree days were within 9%, for both 0% and 50% solar weightings.

As a consequence of adding the index for direct normal radiation, the persistence check in Step 3 was modified to determine the frequency and run length below the 33rd percentile (consecutive low radiation days) for daily values of direct normal radiation. This information, along with that for the other persistence indices, was then used to select the month satisfying the persistence criteria.

El Chichon Years

The volcanic eruption of El Chichon in Mexico in March 1982 spewed large amounts of aerosols into the stratosphere. The aerosols spread northward and circulated around the earth. This phenomenon noticeably decreased the amount of solar radiation reaching the United States during May 1982 until December 1984, when the effects of the aerosols had diminished. Consequently, these months were not used in any of the TMY2 procedures because they were considered not typical.

Leap Years

TMY2 files do not include data for February 29. Consequently, data for February 29 were not used in leap year Februarys to determine their candidate month CDFs. However, to maximize the use of available data, data for February 29 were included for determining the long-term CDFs.

Preference for Months with Measured Solar Radiation Data

For a station, the NSRDB may contain both measured and modeled solar radiation data. Because of additional uncertainties associated with modeled data, preference in the selection of candidate months were given to months that contained either measured global horizontal or direct normal solar radiation data. This was accomplished between Steps 2 and 3 by switching the ranking of the first and second ranked candidate months if the second ranked month contained measured solar radiation data, but the first ranked month did not.

Month Interface Smoothing

Curve-fitting techniques were used to remove discontinuities created by concatenating months from different years to form the TMY2s. These techniques were applied for 6 hours each side of the month interfaces for dry bulb temperature, dew point temperature, wind speed, wind direction, atmospheric pressure, and precipitable water. Relative humidities for 6 hours on each side of the month interfaces were calculated using psychometric relationships (ASHRAE 1993) and curve-fitted values of dry bulb temperature and dew point temperature.

Allowance for Missing Data

The NSRDB has no missing solar radiation data, but meteorological data are missing for some stations and months. Consequently, when creating the TMY2s, procedures were adopted to account for missing meteorological data. From these procedures, two classes of TMY2 stations evolved:
Class A and B.
Class A stations are those stations whose 30-year meteorological data records were the most complete and that had an adequate number (15) of candidate months after eliminating any months with data missing for more than 2 consecutive hours. The minimum of 15 candidate months permitted completion of 90% of the stations without extensive data filling. As indicated in Figure A-2, as few as 15 candidate months yielded typical months that were within the range of differences established by 25 or more candidate months when comparing monthly values of direct normal for TMY2 months with monthly averages of direct normal for the 1961-1990 period. This relationship was also found to be true for global horizontal radiation and heating and cooling degree days.

Class B stations had more missing data than Class A stations, and the data were filled for the index elements used to select the TMY2s. Other elements in Class B TMY2s were not filled and may be missing. (Table 1-1 shows elements that may have missing data values in TMY2 files for Class A and B stations.

Class A Stations. There are 216 Class A stations. Missing data for these stations were accounted for in the following fashion:

1. Long-term CDFs in Step 1, based on the 30-year period (excluding the El Chichon period), were determined using only measured data or data modeled (such as solar radiation) from measured or observed data.
2. Months were eligible to be candidate months if they had no missing or filled data for periods greater than 2 hours. This accommodated data from 1965 to 1981 that was digitized by NOAA only every third hour. For the elements used for the indices, the missing data for the 2-hour sequences were replaced with interpolated or modeled values.

Class B Stations. The NSRDB data from which the 23 Class B stations were derived have substantially more missing data than the NSRDB data from which the Class A stations were derived. This situation required filling missing data to have sufficient candidate months from which to select typical months. The additional missing data for the Class B stations resulted from such things as equipment problems and the fact that some stations did not operate at night for some or all of the 30-year period. Criteria were relaxed for Class B stations to permit filled data for periods of up to 47 hours to be used in determining the long-term CDFs, and months were eligible to be candidate months if they had no missing or filled data for periods greater than 47 hours. For Colorado Springs, Colorado, the criteria were further relaxed to permit missing data for snow depth and days since last snowfall.

Data-Filling Methods

The TMY2 data sets required filling some missing data that were not filled during the development of the NSRDB. The NSRDB was made complete with respect to solar radiation elements (NSRDB - Vol. 1 1992). This required NSRDB filling of missing data, at least for daylight hours, for elements used to model solar radiation, such as total and opaque sky cover, dry bulb temperature, relative humidity, and atmospheric pressure.

For other meteorological elements, data were not filled in the NSRDB. Consequently, to develop the TMY2s, missing data for dry bulb temperature (nighttime), dew point temperature, and wind speed required data filling to complete the selection of typical months. These elements, along with global horizontal and direct normal radiation, were used to generate statistics to determine the appropriate selection of typical months.

To maximize the usefulness of the TMY2s, other missing meteorological data were also filled, with the exception of horizontal visibility, ceiling height, and present weather. The discontinuous nature of these three elements did not readily lend itself to interpolation or other data-filling methods.

Data filling for TMY2 Class B stations was more extensive than for the Class A stations. TMY2s for Class A stations were restricted to the selection of typical months that had no more than 2 consecutive hours of data missing, whereas Class B stations could have up to 47 consecutive hours of data missing.

Two-hour gaps in data records for Class A and Class B stations were filled by linear interpolation, except for relative humidity, which was calculated based on psychometric relationships (ASHRAE 1993) using measured or filled dry bulb temperature and dew point temperature. For Class B stations, longer gaps from 3 to 47 hours were filled using filled data from the NSRDB if available; otherwise TMY2 data filling-methods were used.

The NSRDB contains filled data for total and opaque sky cover, dry bulb temperature, relative humidity, and atmospheric pressure. NSRDB data gaps up to 5 hours were filled by linear interpolation. Gaps from 6 to 47 hours were filled for the above elements by using data from adjacent days for identical hours and then by adjusting the data so that there were no abrupt changes in data values between the filled and measured data. Many Class B stations did not operate for parts of the night and/or early morning and late afternoon. For these stations, NSRDB data were filled from sunrise to sunset to allow model estimates of solar radiation. However, nighttime data were not necessarily filled.

The TMY2 data sets used procedures to fill nighttime data and other data not filled in the NSRDB. These procedures were used for total and opaque sky cover, atmospheric pressure, dry bulb temperatures, dew point temperatures, relative humidity, wind speed, precipitable water, broadband aerosol optical depth, snow depth, and days since last snowfall. Data elements not filled are horizontal visibility, ceiling height, and present weather.

The TMY2 data-filling procedures are described in the following paragraphs.

Total and opaque sky cover, and atmospheric pressure were linearly interpolated over any missing nighttime periods
Nighttime dry bulb temperatures were linearly interpolated, and then the filled values were adjusted to preserve nonlinearities, such as more rapid changes in temperature near sunrise and sunset. These adjustments were based on average diurnal profiles determined for each calendar month and appropriately scaled to match the endpoints of the interpolation interval.

Missing daytime dew point temperatures were filled using psychometric relationships (ASHRAE 1993) and measured or NSRDB filled values of dry bulb temperature and relative humidity. The same procedure was also used to fill missing nighttime dew point temperatures if measured or NSRDB filled values of dry bulb temperature and relative humidity were available. Otherwise, missing nighttime dew point temperatures were filled by the procedure used to fill nighttime missing dry bulb temperatures--linear interpolation and then adjustment of filled values based on average diurnal profiles determined for each calendar month.

Missing nighttime relative humidity values were filled using psychometric relationships and dry bulb and dew point temperatures. Dry bulb temperatures used were measured or NSRDB filled or TMY2 filled, and dew point temperatures used were measured or TMY2 filled.

Missing wind speed data, for up to 47 hour gaps, were filled by the procedure used to fill nighttime missing dry bulb temperatures--linear interpolation and then adjustment of filled values based on average diurnal profiles determined for each calendar month.

Missing wind direction and precipitable water, for up to 47 hour gaps, were linearly interpolated. For calm winds, wind direction was set to zero (north).

Broadband aerosol optical depth values in the TMY2s are daily values provided by seasonal functions derived during the development of the NSRDB. The seasonal functions are sinusoidal with respect to the day of the year and have peak values occurring in the summer.

Snow depth and days since last snowfall data were available from the NSRDB for all but Colorado Springs and a few stations at southern latitudes, such as Guam and Puerto Rico. So much data were missing for Colorado Springs that no attempt was made to fill the data, and missing data for the elements snow depth and days since last snowfall were flagged as missing. For the southern latitude sites that do not receive snow, snow depth was set to zero and days since last snowfall was set to 88, meaning 88 or more days.

Quality Control

Data were checked before and after processing to ensure that data were reasonable. NCDC provided information identifying some erroneous dew point temperature data in Version 1.1 of the NSRDB, where dew point temperatures exceeded dry bulb temperatures. During processing of the NSRDB data to generate the TMY2s, dew point temperatures were checked to make sure they did not exceed dry bulb temperatures. If they did, the dew point temperature was calculated using relative humidity and dry bulb temperature, if available; otherwise, the data were considered missing.
NCDC also identified three stations (Chattanooga, Tennessee; Huntsville, Alabama; and Louisville, Kentucky) that had erroneous total sky cover data for the period 1970-1974. The cloud cover data had been set to 10 for non-3-hourly values (correct values were present every 3 hours).
Consequently, modeled solar radiation for these stations and times would be erroneous. For the TMY2s, data for these stations and time periods were excluded.

Post-processing checks revealed that some of the selected TMY2 months had solar radiation values with obvious errors (diffuse radiation values were zero even though global horizontal and direct normal radiation were a few hundred watt hours). Consequently, these stations were reprocessed with the affected data being excluded. The stations with months excluded during the reprocessing because of erroneous solar data are: Boulder, Colorado (2/88, 3/85, 5/85, and 10/85); Lake Charles, Louisiana (2/80); Caribou, Maine (4/78, 7/85, and 7/72); Great Falls, Montana (10/89); Omaha, Nebraska (5/85, 5/89, and 11/81); Ely, Nevada (6/89 and 9/88); Guam, Pacific Islands (1/88, 9/79, and 9/88); El Paso, Texas (12/88); Midland, Texas (5/80 and 12/79); Salt Lake City, Utah (5/88, 8/80, and 10/89); Lander, Wyoming (3/88 and 8/80).

Calculation of Illuminance Data

To facilitate lighting and energy analysis of buildings, hourly values for global horizontal illuminance, direct normal illuminance, diffuse horizontal illuminance, and zenith luminance were added to the TMY2 data sets. These elements were calculated using luminous efficacy models developed by Perez et al. (1990). Inputs to the models are global horizontal radiation, direct normal radiation, diffuse horizontal radiation, and dew point temperature. The luminous efficacy in terms of lumens per watt is determined as a function of sky clearness, sky brightness, and zenith angle.

Assignment of Source and Uncertainty Flags

With the exception of extraterrestrial horizontal and extraterrestrial direct radiation, each data value was assigned a source and uncertainty flags. The source flag indicates whether the data were measured, modeled, or missing, and the uncertainty flag provides 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.

Usually, the source and uncertainty flags in the TMY2 data files are the same as the ones in the NSRDB, from which the TMY2 files were derived. However, differences do exist for data that were flagged missing in the NSRDB, but then filled while developing the TMY2 data sets. Differences are also present for illuminance and luminance data values that were not included in the NSRDB. Uncertainty values apply to the data with respect to the time stamp of the data, and not as to how "typical" a particular hour is for a future month and day. The uncertainty values represent the plus or minus interval about the data value that contains the true value 95% of the time.

The uncertainty assigned to modeled solar radiation data includes only the bias error in the model and not the random error component, which could be several times larger for partly cloudy skies. For partly cloudy skies, an hour can be composed of large or small amounts of sunshine, depending on whether the sun is mostly free of the clouds or occluded by the clouds. Consequently, modeled hourly values may depart significantly from true values for partly cloudy skies. The uncertainty assigned to modeled solar radiation data represents the average uncertainty for a large number of model estimates (such as for a month). When averaging large data sets, random errors tend to cancel, leaving only the bias error.

Uncertainties for values of illuminance and luminance were determined by taking the root-sum-square of the two main sources of error: (1) uncertainty of the solar radiation element (global horizontal, direct normal, or diffuse horizontal radiation) from which the illuminance or luminance element is derived, and (2) uncertainty of the model estimate.
The uncertainty of the model estimates are based on the evaluation presented by Perez et al. (1990) for six test stations. To be conservative, the following model mean bias errors for the stations with the largest errors were used:

• 1.2% for global horizontal illuminance
• 1.6% for direct normal illuminance
• 2.3% for diffuse horizontal illuminance
• 1.2% for zenith luminance.

The uncertainty of the illuminance data value was then determined as the root-sum-square of the model uncertainty and solar radiation element uncertainty.

The use of the bias error, instead of bias and random error, is consistent with the approach in the above paragraph concerning the assignment of uncertainty values to modeled solar radiation elements. Consequently, it also has the same implications. The assigned uncertainty is representative of the average uncertainty for a large number of model estimates (such as for a month), but the actual uncertainty of the individual modeled illuminance and luminance values is greater than indicated.

For meteorological elements, relative uncertainties from the NSRDB were used. These uncertainties do not portray a quantitative evaluation of the uncertainty of the meteorological elements, but rather give relative uncertainties based on the data and the manner in which they were derived (NSRDB-Vol. 1 1992).

The source and uncertainty flags for the solar radiation, illuminance, and meteorological elements are presented in Tables 3-3 through 3-6.

References

ASHRAE (1993). 1993 ASHRAE Handbook: Fundamentals. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
Finkelstein, J.M.; Schafer, R.E. (1971). "Improved Goodness-of-Fit Tests."Biometrika, 58(3), pp. 641-645.
Hall, I.; Prairie, R.; Anderson, H.; Boes, E. (1978). Generation of Typical Meteorological Years for 26 SOLMET Stations. SAND78-1601. Albuquerque, NM: Sandia National Laboratories.
NSRDB - Vol. 1 (1992). User's Manual - National Solar Radiation Data Base (1961-1990). Version 1.0. Golden, CO: National Renewable Energy Laboratory and Asheville, NC: National Climatic Data Center.
Perez, R.; Ineichen, P.; Seals, R.; Michalsky, J.; Stewart, R. (1990). "Modeling Daylight Availability and Irradiance Components from Direct and Global Irradiance." Solar Energy, 44(5), pp. 271-289.

14944 SIOUX_FALLS SD -6 N 43 34 W 96 44 435
85010101000000000000?00000?00000?00000?00000?00000?00000?010A710A7-150A7
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85010102000000000000?00000?00000?00000?00000?00000?00000?0l0A710A7-144A7
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85010103000000000000?00000?00000?00000?00000?00000?00000?0l0A710A7-144A7
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85010106000000000000?00000?00000?00000?00000?00000?00000?0l0A710A7-167A7
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85010107000000000000?00000?00000?00000?00000?00000?00000?004A704A7-183A7
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85010108000000000000?00000?00000?00000?00000?00000?00000?002A702A7-194A7
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85010112053014150357G40782G40064G50374I40735I40098I5013lI500A700A7-167A7
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85010113056214150387G40806G40067G50407I40767I40101I50139I500A700A7-156A7
-244A7047A70978A7320A7067A70193A777777A70999999999003E7050F8000A700E7
85010114053014150359G40788G40064G50377I40742I40098I5013lI500A700A7-144A7
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239A7043A70978A7330A7052A70193A777777A70999999999003E7050F8000A700E7
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85010119000000000000?00000?00000?00000?00000?00000?00000?000A700A7-172A7
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85010120000000000000?00000?00000?00000?00000?00000?00000?000A700A7-178A7
-233A7062A70978A7000A7000A70241A777777A70999999999003E7050F8000A700E7
85010121000000000000?00000?00000?00000?00000?00000?00000?000A700A7-183A7
-239A7062A70978A7260A7015A70241A777777A70999999999003E7050F8000A700E7
85010122000000000000?00000?00000?00000?00000?00000?00000?000A700A7-183A7
-239A7062A70977A7220A702lA70241A777777A70999999999003E7050F8000A700E7
85010123000000000000?00000?00000?00000?00000?00000?00000?000A700A7-178A7
-239A7059A70977A7220A7015A70241A777777A70999999999003E7050F8000A700E7
85010124000000000000?00000?00000?00000?00000?00000?00000?000A700A7-178A7
-239A7059A70977A7240A7010A70241A777777A70999999999003E7050F8000A700E7

1 1111
1 2 3 4 5 6 7 8 9 0 1 2 3 4
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456
7890123456789012345678901234567890123456789012
(for field position identification only)

Figure 3-1. Sample file header and data in the TMY2 format for January 1

Table 3-1. Header Elements in the TMY2 Format
(For First Record of Each File)

Field Position	Element	Definition
002 - 006	WBAN Number	Station's Weather Bureau Army Navy number (see Table 2-1)
008 - 029	City	City where the station is located (maximum of 22 characters)
031 - 032	State	State where the station is located (abbreviated to two letters)
034 - 036	Time Zone	Time zone is the number of hours by which the local standard time is ahead of or behind Universal Time. For example, Mountain Standard Time is designated -7 because it is 7 hours behind Universal Time.
038 - 044 038 040 - 041 043 - 044	Latitude	Latitude of the station N = North of equator Degrees Minutes
046 - 053 046 048 - 050 052 - 053	Longitude	Longitude of the station W = West,E = East Degrees Minutes
056 - 059	Elevation	Elevation of station in meters above sea level
FORTRAN Sample Format: ( 1X,A5,1X,A22,1X,A2,1X I3,1X,Al,lX,I2,1X,I2,1X,Al,lX,I3,1X,I2,2X,I4 ) C Sample Format: ( %s %s %s %d %s %d %d %s %d %d %d )

Table 3-2. Data Elements in the TMY2 Format
(For All Except the First Record)

Field Position	Element	Values	Definition
002 - 009 002 - 003 004 - 005 006 - 007 008 - 009	Local Standard Time Year Month Day Hour	61 - 90 1 - 12 1 - 31 1 - 24	Year, 1961-1990 Month Day of month Hour of day in local standard time
010 - 013	Extraterrestrial Horizontal Radiation	0 - 1415	Amount of solar radiation in Wh/m2 received on a horizontal surface at the top of the atmosphere during the 60 minutes preceding the hour indicated
014 - 017	Extraterrestrial Direct Normal Radiation	0 - 1415	Amount of solar radiation in Wh/m2 received on a surface normal to the sun at the top of the atmosphere during the 60 minutes preceding the hour indicated
018 - 023 018 - 021 022 023	Global Horizontal Radiation Data Value Flag for Data Source Flag for Data Uncertainty	0 - 1200 A - H, ? 0 - 9	Total amount of direct and diffuse solar radiation in Wh/m2 received on a horizontal surface during the 60 minutes preceding the hour indicated
024 - 029 024 - 027 028 029	Direct Normal Radiation Data Value Flag for Data Source Flag for Data Uncertainty	0 - 1100 A - H, ? 0 - 9	Amount of solar radiation in Wh/m2 received within a 5.7° field of view centered on the sun during the 60 minutes preceding the hour indicated
030 - 035 030 - 033 034 035	Diffuse Horizontal Radiation Data Value Flag for Data Source Flag for Data Uncertainty	0 - 700 A - H, ? 0 - 9	Amount of solar radiation in Wh/m2 received from the sky (excluding the solar disk) on a horizontal surface during the 60 minutes preceding the hour indicated
036 - 041 036 - 039 040 041	Global Horiz. Illuminance Data Value Flag for Data Source Flag for Data Uncertainty	0 - 1300 I, ? 0 - 9	Average total amount of direct and diffuse illuminance in hundreds of lux received on a horizontal surface during the 60 minutes preceding the hour indicated 0 to 1300 = 0 to 130,000 lux
042 - 047 042 - 045 046 047	Direct Normal Illuminance Data Value Flag for Data Source Flag for Data Uncertainty	0 - 1100 I, ? 0 - 9	Average amount of direct normal illuminance in hundreds of lux received within a 5.7° field of view centered on the sun during the 60 minutes preceding the hour indicated. 0 to 1100 = 0 to 110,000 lux
048 - 053 048 - 051 052 053	Diffuse Horiz. Illuminance Data Value Flag for Data Source Flag for Data Uncertainty	0 - 800 I, ? 0 - 9	Average amount of illuminance in hundreds of lux received from the sky (excluding the solar disk) on a horizontal surface during the 60 minutes preceding the hour indicated. 0 to 800= 0 to 80,000 lux
054 - 059 054 - 057 058 059	Zenith Luminance Data Value Flag for Data Source Flag for Data Uncertainty	0 - 7000 I, ? 0 - 9	Average amount of luminance at the sky's zenith in tens of Cd/m2 during the 60 minutes preceding the hour indicated. 0 to 7000 = 0 to 70,000 Cd/m2
060 - 063 060 - 061 062 063	Total Sky Cover Data Value Flag for Data Source Flag for Data Uncertainty	0 - 10 A - F 0 - 9	Amount of sky dome in tenths covered by clouds or obscuring phenomena at the hour indicated
064 - 067 064 - 065 066 067	Opaque Sky Cover Data Value Flag for Data Source Flag for Data Uncertainty	0 - 10 A - F 0 - 9	Amount of sky dome in tenths covered by clouds or obscuring phenomena that prevent observing the sky or higher cloud layers at the hour indicated
068 - 073 068 - 071 072 073	Dry Bulb Temperature Data Value Flag for Data Source Flag for Data Uncertainty	-500 to 500 A - F 0 - 9	Dry bulb temperature in tenths of oC at the hour indicated -500 to 500 = -50.0 to 50.0oC
074 - 079 074 - 077 078 079	Dew Point Temperature Data Value Flag for Data Source Flag for Data Uncertainty	-600 to 300 A - F 0 - 9	Dew point temperature in tenths of oC at the hour indicated -600 to 300 = -60.0 to 30.0oC
080 - 084 080 - 082 083 084	Relative Humidity Data Value Flag for Data Source Flag for Data Uncertainty	0 - 100 A - F 0 - 9	Relative humidity in percent at the hour indicated
085 - 090 085 - 088 089 090	Atmospheric Pressure Data Value Flag for Data Source Flag for Data Uncertainty	700 - 1100 A - F 0 - 9	Atmospheric pressure at station in millibars at the hour indicated
091 - 095 091 - 093 094 095	Wind Direction Data Value Flag for Data Source Flag for Data Uncertainty	0 - 360 A - F 0 - 9	Wind direction in degrees at the hour indicated. ( N = 0 or 360, E = 90, S = 180,W = 270 ). For calm winds, wind direction equals zero.
096 - 100 096 - 098 099 100	Wind Speed Data Value Flag for Data Source Flag for Data Uncertainty	0 - 400 A - F 0 - 9	Wind speed in tenths of meters per second at the hour indicated. 0 to 400 = 0 to 40.0 m/s
101 - 106 101 - 104 105 106	Visibility Data Value Flag for Data Source Flag for Data Uncertainty	0 - 1609 A - F, ? 0 - 9	Horizontal visibility in tenths of kilometers at the hour indicated. 7777 = unlimited visibility 0 to 1609 = 0.0 to 160.9 km 9999 = missing data
107 - 113 107 - 111 112 113	Ceiling Height Data Value Flag for Data Source Flag for Data Uncertainty	0 - 30450 A - F, ? 0 - 9	Ceiling height in meters at the hour indicated. 77777 = unlimited ceiling height 88888 = cirroform 99999 = missing data
114 - 123	Present Weather	See Appendix B	Present weather conditions denoted by a 10-digit number. See Appendix B for key to present weather elements.
124 - 128 124 - 126 127 128	Precipitable Water Data Value Flag for Data Source Flag for Data Uncertainty	0 - 100 A - F 0 - 9	Precipitable water in millimeters at the hour indicated
129 - 133 129 - 131 132 133	Aerosol Optical Depth Data Value Flag for Data Source Flag for Data Uncertainty	0 - 240 A - F 0 - 9	Broadband aerosol optical depth (broad-band turbidity) in thousandths on the day indicated. 0 to 240 = 0.0 to 0.240
134 - 138 134 - 136 137 138	Snow Depth Data Value Flag for Data Source Flag for Data Uncertainty	0- 150 A - F, ? 0 - 9	Snow depth in centimeters on the day indicated. 999 = missing data
139 - 142 139 - 140 141 142	Days Since Last Snowfall Data Value Flag for Data Source Flag for Data Uncertainty	0 - 88 A - F, ? 0 - 9	Number of days since last snowfall. 88 = 88 or greater days 99 = missing data
FORTRAN Sample Format: (lX, 4I2, 2I4, 7 (I4, Al, I1), 2( I2, Al, Il), 2 (I4, Al, Il), l(I3, Al, Il), l ( I4, Al, Il), 2 (I3, Al, I1), l (I4, Al, Il), l (I5, Al, Il), l0Il, 3 (I3, Al, Il), l (I2, Al, Il)) C Sample Format: (%2d%2d%2d%2d%4d%4d%4d%1s%1d%4d%1s%1d%4d%1s%1d%4d%1s%1d%4d%1s%1d%4d%1s %1d%4d%1s%1d%2d%1s%1d2d%1s%1d%4d%1s%1d%4d%1s%1d%3d%1s%1d%4d%1s%1d%3d %1s%1d%3d%1s%1d%4d%1s%ld%51d%1s%1d%1d%1d%1d%1d%1d%1d%1d%1d%1d%1d%3d%1s %1d%3d%1s%1d%3d%1s%1d%2d%1s%1d) Note: For ceiling height data, integer variable should accept data values as large as 99999.

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