Because of the small size of the earth compared to the sun and the great distance between the two (149 million kilometres), only a minute percentage of solar radiation—one in two billion parts—is intercepted by the earth. But even this small amount of solar radiation reaching the earth is of great significance. All biological and many of the physical phenomena of the earth are controlled by this tiny proportion of solar radiation.
The incoming solar radiation is known as insolation. It is received in the form of short waves which are of electromagnetic nature. These short-wave or ultraviolet rays are absorbed partly by the atmosphere and, on reaching the earth, are converted into long-wave radiation by reflection. The earth’s surface receives this radiant energy at the rate of two calories per square centimetre per minute.
The angle of incidence or the angle which the sun’s rays make with the earth’s surface, determines the amount of solar radiation which a particular place on the earth will receive. A smaller angle means the same amount of radiation will have to serve a larger area on the earth and the intensity will be less concentrated. A larger angle means the sun’s rays will be nearly vertical over the place and the given amount will have to serve a smaller area. As a result, the radiation received will be more concentrated and the intensity will be greater. See Fig. 2.2 (i) and
(ii) For instance, at 45° latitude, the amount of radiation received is only 75% of what is received at the equator. At Arctic/Antarctic Circles and at Poles, this figure is 50% and 40% respectively.
2. Duration of Sunshine:
The amount of solar radiation received obviously depends on the length of time that the sun shines over a particular place. At the equator, where the duration of sunshine is 12 hours daily throughout the year, the amount of radiation received is more compared to the other places on the earth. At winter solstice (22 December), the southern hemisphere receives more sunshine as it is summer there, while at summer solstice (21 June), the northern hemisphere receives more sunshine as it is summer time there. [Fig. 2.2 (iii)].
3. Transparency of Atmosphere:
The amount of cloud cover and its thickness, dust and water vapour which determine the transparency of the atmosphere affect the reflection, absorption and transmission of solar radiation. If the wavelength (X) of the radiation is more than the radius of the obstructing particle (such as a gas), then scattering of radiation takes place. If the wavelength is less than the obstructing particle (such as a dust particle), then total reflection takes place. Absorption of solar radiation takes place if the obstructing particles happen to be water vapour, ozone molecules, carbon dioxide molecules or clouds.