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Information on Insolation can be found at Wikipedia. What's this?

By definition, insolation is the incident solar radiation. That is, the amount of solar radiation reading the earth per day. This is usually measured in units of Btu/square ft/d.

This is the sunlight that directly hits a surface and causes energy in the form of heat. This energy that we eventually feel as heat, is due to a nuclear reaction in the sun which is converted into energy. Only about 19% of this is absorbed by the atmosphere, 31% is reflected by the atmosphere, and the remaining 50% gets to earth absorbed almost totally.