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Solar Energy

The driving force behind all meteorological changes taking place on the earth is solar energy. Each minute, the outer portions of the earth's atmosphere receive an average of 2 calories/sq cm. This value is known as the solar constant. Although the solar constant changes over very long periods of time, it does not vary enough to affect the general nature of the earth's weather over short periods of time.

The solar energy reaching the outer atmosphere may experience a variety of fates. Thirty percent of all solar energy is lost to space by means of scattering and by reflection off clouds and the earth's surface. Another 19% is absorbed by gases in the atmosphere and by clouds. About a quarter of it (25%) reaches the earth's surface directly; another quarter (26%) eventually reaches the surface after being scattered by gases in the atmosphere.

An important factor in determining the fate of solar radiation is its wavelength. Shorter wavelengths tend to be absorbed by gases in the atmosphere (especially oxygen and ozone) while radiation of longer wavelengths tends to be transmitted to the earth's surface.

Solar radiation that reaches the earth's surface is absorbed to varying degrees, depending on the kind of material on which it falls. Since darker colors and rougher surfaces absorb radiation better than lighter colors and smoother surfaces, soil tends to absorb more solar radiation than water.

Solar energy that reaches the earth's surface is re-radiated back to the atmosphere as heat, also referred to as infrared radiation. Infrared radiation consists of much longer wavelengths. This re-radiated energy is likely to be absorbed by certain gases in the atmosphere such as carbon dioxide and nitrous oxide. This absorption process, the greenhouse effect, is responsible for maintaining the planet's annual average temperature.

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