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►Climate effect of solar radiation:
The average energy density of solar radiation
just above the Earth's atmosphere, in a plane perpendicular to the rays,
is about 1367 W/mē, a value called the solar constant (although it
fluctuates by a few parts per thousand from day to day). The Earth
receives a total amount of radiation determined by its cross section (
 R2),
but as the planet rotates this energy is distributed
across the entire surface area (4
R2). Hence, the average incoming solar radiation (known as "insolation")
is 1/4th the solar constant or ~342 W/mē. At any given location and
time, the amount received at the surface depends on the state of the
atmosphere and the latitude.
On Earth, solar radiation is obvious
as daylight when the sun is above the horizon. This is during daytime, and
also in summer near the poles at night, but not at all in winter near the
poles. When the direct radiation is not blocked by clouds, it is experienced
as sunshine, a combination of bright yellow light (sunlight in the strict
sense) and heat. The heat on the body, on objects, etc.,
that is directly produced by the radiation should be distinguished from the
increase in air temperature.
The amount of radiation intercepted by a planetary body varies as the square
of the distance between the star and the planet. The Earth's orbit and
obliquity change with time, sometimes achieving a nearly perfect circle, and
at other times stretching out to an eccentricity of 5%. The total insolation
remains almost constant but the seasonal and latitudinal distribution and
intensity of solar radiation received at the Earth's surface also varies
(for example see a graph). For example, at latitudes of 65 degrees the
change in solar energy in summer & winter can vary by more than 25% as a
result of the Earth's orbital variation. Because changes in winter and
summer tend to offset, the change in the annual average insolation at any
given location is near zero, but the redistribution of energy between summer
and winter does strongly affect the intensity of seasonal cycles.
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Solar Energy -
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