The earth intercepts only a minute fraction of the energy radiated by the sun – less than one two-billionth. Even this seemingly insignificant amount, to our surprise, represents several hundred thousand times the electric generating capacity of the United States.

Solar radiation provides more then 99.9 percent of the energy that heats the earth. Undoubtedly, the radiant energy from the sun is the most important control of our weather and climate.

According to Trewartha, the per-minute energy flow from the sun to the earth is nearly equal to the total amount of energy consumed by man on earth. The most astonishing fact about the incoming solar radiation (insolation) that strikes the earth’s surface is that it is equal to about 23 billion horse power.

Actually it is this amount of energy received from the sun that acts as the driving force for all the atmospheric as well as biologic processes on the earth. Besides, all other sources of energy found on earth such as coal, oil, and wood etc. are nothing but converted form of solar energy.


The word ‘insolation’ is a contraction of “incoming solar radiation”. It consists of a bundle of rays of radiant energy of different wavelengths. The sun emits radiant energy in the form of electromagnetic waves.

The wavelength and frequency of these waves which travel with the speed of light (2, 97,600 km per second) are really staggering. The longer waves, largely absorbed in the atmosphere, are called infra-red rays.

The shorter waves are called the ultra-violet rays representing the other end of the solar spectrum. It is the visible portion of the spectrum lying between the invisible ultraviolet and infra-red rays that most effectively heat the earth.

In fact, the rays of the solar spectrum reaching the earth’s surface are converted, after absorption by the surface, from short-wave to long-wave radiant energy. This is what is known as heat.


However, there are various factors which cause temporal and spatial variations in the amount of this heat. Various processes of weather and climate are fuelled by the energy gained by the earth’s surface and its atmosphere.