Solar radiation describes the
visible and near-visible (ultraviolet and near-infrared) radiation emitted from
the sun. The different regions are described by their wavelength range within the
broad band range of 0.20 to 4.0 µm (microns). Terrestrial radiation is a term used
to describe infrared radiation emitted from the atmosphere. The following is a
list of the components of solar and terrestrial radiation and their approximate
Ultraviolet: 0.20 - 0.39 µm
Visible: 0.39 - 0.78 µm
Near-Infrared: 0.78 - 4.00 µm
Infrared: 4.00 - 100.00 µm
Approximately 99% of solar, or
short-wave, radiation at the earth's surface is contained in the region from 0.3
to 3.0 µm while most of terrestrial, or long-wave, radiation is contained in the
region from 3.5 to 50 µm.
Outside the earth's atmosphere,
solar radiation has an intensity of approximately 1370 watts/meter2. This is the
value at mean earth-sun distance at the top of the atmosphere and is referred to
as the Solar Constant. On the surface of the earth on a clear day, at noon, the
direct beam radiation will be approximately 1000 watts/meter2 for many locations.
The availability of energy is
affected by location (including latitude and elevation), season, and time of day.
All of which can be readily determined. However, the biggest factors affecting the
available energy are cloud cover and other meteorological conditions which vary
with location and time.
Historically, solar measurements
have been taken with horizontal instruments over the complete day. In the Northern
US, this results in early summer values 4-6 times greater than early winter
values. In the South, differences would be 2-3 times greater. This is due, in
part, to the weather and, to a larger degree, the sun angle and the length of