Solar Radiation

Solar radiation is measured atop the meteorological mast at the shore laboratory using an Eppley Model PSP (Precision Spectral Pyranometer). Approximately 99% of solar, or short-wave, radiation at the earth's surface is contained in the region from 0.3 to 3.0 Ám, which corresponds to wavelength between the ultraviolet and near infrared. Above the earth's atmosphere, solar radiation has an intensity of approximately 1380 watts per square meter (W/m2). This value is known as the Solar Constant. At our latitude, the value at the surface is approximately 1000 W/m2 on a clear day at solar noon in the summer months. The difference between this value and the Solar Constant is due to transmission loss to the atmosphere. The clear sky value is considerable less in the winter. Clouds can dramatically reduce this value by reflecting the solar radiation back out to space.

The PSP senses this radiation with a thermopile that produces a millivolt signal that is directly proportional to the downwelling solar irradiance. The PSP uses a glass dome that uniformly transits radiation between 0.285 and 2.8 Ám. A calibration constant is applied to provide the total solar irradiance in watts per square meter.