18.10 The Solar Constant
Although we get only a tiny part of the sun's total output, it is enough to sustain virtually all life on the earth. Astronomers call the rate of the flow of radiant energy from the sun that reaches the earth through space the solar constant. At the top of the atmosphere, 1.367 kilowatts (kW) of the sun's energy falls on each square meter of the atmospheres outer "surface But not all this energy makes it to the earth's surface.
Although the sun gives off a lot of visible light, the majority of our solar energy is from the invisible infrared band. As light passes through the atmosphere, most of the short wavelength rays and some portions of the infrared are absorbed. Clouds and airborne particles in the atmosphere reflect as much as 32% of the sun's energy. Clouds, dust, and water vapor absorb up to another 18%. This filtering process warms the atmosphere slightly. Under the least favorable conditions, up to half the incoming light does not even reach us! However, on a sunny day with the sun directly overhead, 60-75% of the incoming solar energy can reach the earth's surface.
When the sun's rays are nearly vertical to the earth's surface, the input of solar energy at the earth's surface is greatest. This can happen only within a narrow band of latitudes between 231/2° north and south of the Equator, and then only twice a year at any given lo-cation. At other times of the year, and at other latitudes, the sun's rays hit the ground at some angle, reducing the rate of energy absorbed by the ground. water, or ice. The farther a location is from the Equator, the more slanted the rays of the sun, even at its highest position in the sky. The more slanted the sun's rays, the less energy they deposit at the earth's surface.