The NSRDB 1991–2005 update is a serially complete collection of hourly values of the three most common measurements
of solar radiation (i.e., global horizontal, direct normal, and diffuse horizontal) over a period of time adequate to
establish means and extremes and at a sufficient number or locations to represent regional solar radiation climates.
Nearly all of the solar data in the NSRDB are modeled, and only 40 sites have measured solar data—none of them with a 
complete period of record.