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!

Conclusion

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 less atmosphere).