This is electromagnetic radiation (light energy) emitted by the sun. This energy is transmitted through space in the units of electromagnetic energy called photons. The strength of the solar energy that reaches our outer atmosphere is called the solar constant and has a value of approximately 2. 0 calories per minute per square centimeter. Roughly one-half (1. 0 cal min^-1 cm^-2) of all the solar radiation that enters the upper atmosphere is available at the earth's surface. Of the fifty percent loss, approximately 32 percent is reflected back into space by either clouds, dust, or aerosols; the remaining 18 percent is absorbed by clouds, dust, water, and atmospheric gases. The short high-energy wavelengths that are absorbed by the Earth are re-radiated at longer, lower energy wavelengths mostly in the IR wavelength region. These lower-energy wavelengths are, in part, prevented from escaping into space by the greenhouse gases in the atmosphere. Solar radiation is the fuel source that provides a mechanism for all biological processes on earth. Without solar radiation, life on earth would not exist. (This statement, of course, neglects the contribution to the earth's surface temperature from radioactive decay in the earth. )