Greenhouse physics

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Abstract
The 'greenhouse effect' has now become an accepted addition to modern vocabulary, describing as it does the increase in the Earth's surface temperature above its effective emitting temperature brought about by the absorption and reemission of long-wavelength radiation by certain gases in the atmosphere. The Earth is heated by solar radiation (with wavelengths up to about 4 μm). About 30% of this incoming radiation is reflected back to space by clouds and the Earth's surface, but the net solar flux is balanced by outgoing long-wavelength (thermal, or infrared) radiation, at an effective emitting temperature of 255 K. This would also correspond to the Earth's surface temperature in the absence of atmospheric absorption. However, the atmosphere, although relatively transparent to solar radiation, absorbs (and re-emits) strongly at long wavelengths. As a result, most of the long-wavelength radiation emitted to space originates from a mean height of about 5.5 km rather than the surface. Remembering that warm air rises and cools adiabatically – temperature in fact decreases with height (at about 6 K km-1) – it is clear that the Earth's surface must be considerably warmer (288 K) than the value deduced from net solar absorption.

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