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Insolation

The only source of energy for the earth’s atmosphere is the sun which has a surface temperature of more than 5982°C (10 800°F). The energy travels through space for a distance of 150 million km (93 million miles) and reaches us as solar energy or radiant energy in the process called insolation.

This radiation from the sun is made up of three parts, the visible ‘white’ light that we see when the sun shines and the less visible ultra-violet and infra-red rays. The visible ‘white’ light is the most intense and has the greatest influence on our climate. The ultra-violet rays affect our skin and cause sunburn when our bare body is exposed to them for too long a period. The infra-red rays can penetrate even dust and fog and are widely used in photography. Only that part of the sun’s radiation which reaches the earth is called insolation.

What matters most is the effect of the atmosphere upon the incoming solar radiation. It is estimated that of the total radiation coming to us, 35 percent reaches the atmosphere and is directly reflected back to space by dust, clouds and air molecules. It plays practically no part in heating the earth and its atmosphere. Another 14 percent is absorbed by the water vapour, carbon dioxide and other gases. Its interception by the air causes it to be ‘scattered’ and “diffused’ so that the visible rays of the spectrum between the ultra-violet and infra-red give rise to the characteristic blue sky that we see above us. The remaining 51 percent reaches the earth and warms the surface.

In turn, the earth warms the layers of air above it by direct contact or conduction, and through the transmission of heat by upward movement of air currents, that is, by convection. This radiation of heat by earth continues during the night, when insolation from the sun cannot replace it. As a result of this, the earth’s surface cools at night.

 

The pattern of insolation-receipt is governed by two basic factors: latitude, earth rotation and revolution. Latitude determines the effects of overhead and oblique sun rays on insolation-receipt, and the earth’s rotation and revolution determines the patterns of day and night, and the seasons.

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