Terrestrial radiation is a term used to describe infrared radiation emitted from the earth.
Outside the earth's atmosphere, solar radiation has an intensity of approximately 1370 watts per meter 2 . This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant. On the surface of the earth on a clear day, at noon, the direct beam radiation will be approximately 1000 watts per meter 2 for many locations. All of which can be readily determined. However, the biggest factors affecting the available energy are cloud cover and other meteorological conditions which vary with location and time.