greenhouse effect
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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.
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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.
Related category
• ECOLOGY
AND THE ENVIRONMENT |
