The Changing Terrestrial Solar Spectrum
Absorption and scattering levels change as the
constituents of the atmosphere change. Clouds are the most
familiar example of change; clouds can block most of the
direct radiation. Seasonal variations and trends in ozone layer
thickness have an important effect on terrestrial ultraviolet
level.
The ground level spectrum also depends on how far the
sun's radiation must pass through the atmosphere. Elevation
is one factor. Denver has a mile (1.6 km) less atmosphere
above it than does Washington, and the impact of time of
year on solar angle is important, but the most significant
changes are due to the earth's rotation (Fig. 4). At any
location, the length of the path the radiation must take to
reach ground level changes as the day progresses. So not only
are there the obvious intensity changes in ground solar
radiation level during the day, going to zero at night, but the
spectrum of the radiation changes through each day because
of the changing absorption and scattering path length.