Distribution of Solar Radiation
The unequal distribution of solar radiation over the Earth is the primary cause of weather and climate. The rate of receipt of solar energy varies with latitudes, seasons, and time of the day, i.e., the angle of the sun’s rays with the surface of the Earth.
A) Distribution of Solar Radiation Without Atmosphere
The annual march of the solar radiation at the top of the atmosphere is shown. In the tropics, the intensity of solar radiation remains quite high throughout the year with little seasonal variation. The noon rays are vertical twice a year at all places situated between the two tropics. As a result, the solar radiation curve shows two maxima and two minima for the low latitudes.
The mid-latitude curve (40º) is broadly representative of the belts lying between 23½º and 66½º in each hemisphere. It shows a single strong maximum and a single minimum, both of which coincide with the solstice. The curves show a large seasonal variation.
The high latitude (80º), which represents belts poleward of the Arctic and Antarctic circles, resembles that of the middle latitudes. The only difference in this curve is that it reaches zero during winter when there is no solar radiation.
B) Distribution of Solar Radiation With Atmosphere (Without Clouds)
The distribution of solar radiation with the atmosphere but without clouds is shown. A solar beam, while passing through the atmosphere (without clouds), is depleted by scattering, reflection, and absorption. Thus, the amount of radiation reaching the surface of the Earth is less than that received at the outer limits of the atmosphere. The depletion is maximum at high latitudes due to the higher obliquity of the solar beam, resulting in the beam passing through much greater thickness of the atmosphere than in the lower latitudes.
At the time of the equinoxes, the latitudinal distribution of solar radiation is symmetrical about the equator, with the amount decreasing to zero at each pole.
During the summer solstice, the more nearly vertical solar beam and longer days combined together produce a broad maximum in the middle latitudes of the northern hemisphere. The same feature is observed in the middle latitudes of the southern hemisphere during the winter solstice.
Latitudinal variation of radiation is smaller in the summer hemisphere compared to the winter hemisphere.
C) Distribution of Solar Radiation With Atmosphere and Clouds
The clouds reflect a large amount of solar radiation, reducing the amount reaching the surface of the Earth. The maximum total annual radiation is not found at the equator but rather at about latitude 20ºN and 20ºS. The lesser amount near the equator in the southern hemisphere is due to greater cloudiness and more ocean surface in the southern hemisphere.
In the equatorial belt, the area of least solar radiation coincides with the warm continents, where convective clouds are abundant. Maximum solar radiation is received in the sub-tropics, which are relatively less cloudy regions.
In the high latitudes, the lowest annual radiation values are over oceans because of abundant cloudiness.
