The solar radiation and meteorological elements contained in the data base are listed in Table 1-1 . Atmospheric pressure is reported in millibars because these units are commonly used in computer models to estimate solar radiation and are consistent with standard NWS reporting practices. Appendix A contains factors to convert all elements to other commonly used units. All data are referenced to local standard time. The solar radiation elements are the radiant energy integrated over the hour preceding the designated time.
Meteorological elements are the values observed at the designated time.
When a station contains only modeled solar radiation data, it is referred to as a Secondary station. Primary stations contain measured solar radiation data for at least a portion of the 30-year record. Their locations are shown in Figure 1-1 . Appendix B provides more comprehensive information about each of the stations incorporated into the data base. Because of the importance of these and other measured data, a brief history of solar radiation measurements in the United States is presented in Part 2 .