Some portion of energy, which is provided to the surface by solar irradiance, is emitted back as the so-called “Earth’s heat engine”. Almost all of this energy is consisted in infrared radiation and because of that, it is known as long-wave radiation. Intensity of this process is so high, that if it was not counterpoised by the sun energy supply, the Earth would cool down very quickly.
A large part of the heat radiated by the Earth is consumed by the atmosphere, which after warming up radiates as well. A major portion of the energy from this radiation goes back to the Earth surface (approximately 70%) and is absorbed, constituting an important heat source, especially at night, when there is no sun energy. By the virtue of the fact that this range of waves is absorbed by water, clouds radiation is very strong, therefore even minor cloudiness at night restricts cooling down of the surface.
Because long-wave radiation comes both from the Earth surface and the atmosphere, it is good to know what part of the energy emitted by the Earth comes back to the surface as an atmospheric feedback radiation. This difference is called effective radiation. Devices used to measure its concentration should be directed both up and down, so as to record both radiation streams and these devices are known as pyrgeometers. To measure the radiation balance, i.e. the difference between total solar irradiance and effective radiation, balance is used.
Elaborated by K. Wałaszek