Real greenhouses The greenhouse effect of the atmosphere is named by analogy to greenhouses which get warmer in sunlight, but the mechanism by which the atmosphere retains heat is different.[35] A greenhouse works primarily by allowing sunlight to warm surfaces inside the structure, but then preventing absorbed heat from leaving the structure through convection, i.e. sensible heat transport. The greenhouse effect heats the Earth because greenhouse gases absorb outgoing radiative energy, heating the atmosphere which then emits radiative energy with some of it going back towards the Earth. A greenhouse is built of any material that passes sunlight, usually glass, or plastic. It mainly heats up because the Sun warms the ground inside, which then warms the air in the greenhouse. The air continues to heat because it is confined within the greenhouse, unlike the environment outside the greenhouse where warm air near the surface rises and mixes with cooler air aloft. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (R. W. Wood, 1909) that a greenhouse with a cover of rock salt (which is transparent to infra red) heats up an enclosure similarly to one with a glass cover.[3] Thus greenhouses work primarily by preventing convective cooling.[4][36] In contrast, the greenhouse effect heats the Earth because rather than retaining (sensible) heat by physically preventing movement of the air, greenhouse gases act to warm the Earth by re-radiating some of the energy back towards the surface. This process may exist in real greenhouses, but is comparatively unimportant there.
Posted on: Sat, 24 Jan 2015 12:00:24 +0000
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