Green
House Effect
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.
Cited
from: # http://en.wikipedia.org/wiki/Greenhouse_effect
