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.