The term RADIATION refers to the process by which electromagnetic energy is propagated through space. Radiation moves at the speed of light, which is 186,000 miles per second (297,600 km per second) and travels in straight lines in a vacuum. All of the heat received by Earth is through this process. It is the most important means of heat transfer.

SOLAR RADIATION is defined as the total electromagnetic energy emitted by the Sun. The Suns surface emits gamma rays, x-rays, ultraviolet, visible light, infrared, heat, and electromagnetic waves. Although the Sun radiates in all wavelengths, about half of the radiation is visible light with most of the remainder being infrared. (See fig. 1-2-5.)

Energy radiates from a body by wavelengths which vary inversely with the temperature of that body. Therefore, the Sun, with an extremely hot surface temperature, emits short wave radiation. Earth has a much cooler temperature (15C average) and therefore reradiates the Suns energy or heat with long wave radiation.

Insolation (an acronym for INcoming SOLar radiATION) is the rate at which solar radiation is received by a unit horizontal surface at any point on or above the surface of Earth. In this manual, insolation is used when speaking about incoming solar radiation.

There is a wide variety of differences in the amounts of radiation received over the various portions of Earths surface. These differences in heating are important and must be measured or otherwise calculated to determine their effect on the weather.

The insolation received at the surface of Earth depends upon the solar constant (the rate at which solar radiation is received outside Earths atmosphere), the distance from the Sun, inclina-tion of the Suns rays, and the amount of insolation depleted while passing through the atmosphere. The last two are the important variable factors.

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