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Green House Effect
















Green House Effect

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The distinction between the greenhouse effect and real greenhouses.
A modern Greenhouse in RHS Wisley. The term "greenhouse effect" is an analogy to greenhouses, and as all such things is not exact, and can and has been abused.
There is considerable confusion on the matter and a more thorough discussion is useful for understanding how greenhouses and the greenhouse effect work. Many sources make the "heat trapping" analogy of how a greenhouse limits convection to how the atmosphere performs a similar function through a different mechanism involving absorption and emission of infrared absorbing gases.

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 heats up an enclosure similarly to one with a glass cover. Thus greenhouses work by a different mechanism, primarily by preventing convective cooling.

On the other hand, if one limits infrared radiation from the greenhouse, especially at night, one can substantially increase the temperature of the greenhouse, or limit the amount of heating that is needed. Aluminized screens which reflect the infrared thermal radiation so that it cannot heat the greenhouse windows are used for this purpose . As energy prices rise, similar screens are coming into increasing use in greenhouses.

This is an important hint for understanding the Woods experiment. In that case when a glass window was used at the top of the tube, it absorbed thermal energy transferred by radiation and radiatively emitted that energy to the outside from the top of the tube. When a rock salt window which transmits infrared was used, essentially the same amount of thermal infrared radiation was passed to the outside by radiation from the bottom and walls of the cylinder.

In the atmosphere, as in the greenhouse, sunlight heats the surface. A radiative balance is achieved when the thermal energy radiated to space matches the energy absorbed from the sun. Without greenhouse gases all of the energy radiated from the surface would reach space at the speed of light. As a practical matter none of the energy radiated from the surface at wavelengths that can be absorbed by the principal greenhouse gases reaches space directly. Rather due to cycles of absorption and emission, radiation at those frequencies is trapped within the atmosphere and can only be emitted by greenhouse gases high in the troposphere where the lapse rate ensures that it is significantly colder. Since the rate of emission by the colder molecules is much slower, the entire earth system must heat up in order to restore a radiative balance. In this way, the greenhouse effect limits the emission from the earth by radiation.

Both the greenhouse effect and the greenhouse limit the rate of thermal energy flowing out of the system. In that way they are similar. On the other hand in the case of a greenhouse, cutting off convection is the principal limit on the flow of energy. In the case of the greenhouse effect the rate of radiation from the Earth to space is limited by the greenhouse gases.

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