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greenhouse effect





greenhouse effect
The greenhouse effect can be explained with the aid of Wien's displacement law, which states that the wavelength at which a blackbody radiates most intensely varies inversely with its absolute temperature. Thus the radiation originating in the hot Sun is of much shorter wavelength than that radiated from the cool Earth or its atmosphere. Since the atmosphere, particularly when laden with water vapor, is far more opaque to the long-wave radiation characteristic of the Earth than it is to incoming solar radiation, it tends to absorb the former radiation and reradiate it, largely back toward the surface, ensuring that the Earth's surface is maintained at a somewhat higher temperature than would be the case were all the energy radiated from the surface lost directly into space. Actually, less than half the short-wave solar radiation arriving at the top of the atmosphere is absorbed at the Earth's surface. Much is scattered into space by minute particles in the atmosphere or absorbed by atmospheric dust, ozone, carbon dioxide, or water vapor. This last, absorbed energy becomes involved in the long-wavelength radiation processes. Some energy is transferred from the surface to the atmosphere by convection and as latent heat of vaporization of water.
A popular term used to describe the heating effect due to the trapping of long wavelength radiation by greenhouse gases produced from natural and human sources. The greenhouse effect allows shorter wavelength solar infrared to penetrate the Earth's atmosphere but absorbs the longer wavelength infrared returning to space. Because of it the temperature at the Earth's surface is some 18°C warmer than would otherwise be the case.

Sunlight radiated at visible and near-ultraviolet wavelengths provides most of the Earth's energy income. After absorption it is reradiated, but at longer, infrared wavelengths, the Earth being much cooler than the Sun. Although the atmosphere is transparent to the incoming solar radiation, that reradiated from the Earth's surface is strongly absorbed by atmospheric water vapor and carbon dioxide. That absorbed is again reradiated, the majority back toward the surface.


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