by Andrew Worley
What is Insolation?
The single significant source of heat for the atmosphere
is from our Sun (the radiation from other stars is very
small). The radiant energy from the sun which strikes the
earth is termed insolation (from incoming solar radiation).
It consists of electromagnetic waves mainly between infra
red and ultra violet.
Insolation is mainly absorbed by the earth. Some of the energy is absorbed by
the atmosphere, but the surface of the earth receives 2/3 of the energy.
Energy is also reflected by the earth but at a longer wavelength. Clouds will
reflect the low wavelength radiation from the earth.
Reception of Insolation is conditioned by:
a) The solar constant or energy output from the sun which
depends on the distance from the sun (ironically this is
not in fact constant as the sun goes through
cycles in which the solar constant changes by several percent).
b) The transparency of the atmosphere (may be altered by volcanoes or rogue
objects from space).
c) The length of the sunlight period.
d) Angle of incidence of the sun’s rays (at a large angle
the insolation will be more spread out).
The least important factor is (a).
Calculations have shown that the solar constant vary between 1.88 and 2.01
gram calories per cm per minute. This is a lot of energy (4.5 million horsepower
per mile squared – the equivalent of 4500 mini metros in the City of London).
Transparency of the atmosphere has an important bearing on the amount of
insolation which reaches the earths surface. Dust, cloud, water vapour, volcanic
ash, biblical plagues all have an important baring upon the amount of insolation
which reaches the earths surface as they absorb and reflect insolation.
Transparency also relates to latitude. At high latitudes the suns rays must
pass through a thicker layer of atmosphere, which in the northern hemisphere is
worst in winter when the sun is ‘lowest’ in the sky. Duration of
sunlight is of course very important.
What about Aspect?
In the Northern hemisphere southern facing slopes receive more insolation
where as north facing ones are often in the shade. The maximum number of
sunlight hours may be reduced by so much that some valleys in Switzerland only receive
direct sunlight at noon!
Therefore, the total amount of insolation an area receives will be greatest
at the equator and decreases as the poles are reached, although other factors
such as aspect and transparency of the atmosphere are important – thus the
worlds distribution of insolation is closely linked to latitude. In fact the
equator receives 2.5 times more incoming solar radiation than at the poles. As
the seasons shift so does the zones of minimum and maximum insolation.
Actual observation on the earth show a slight variation of distribution from
the latitudal problem because of the humidity of air varies. Dry and high areas receive
most (the air is clearer at higher altitudes and the radiation passes through