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The high, thin cirrus clouds in the Earth's atmosphere act in a way similar
to clear air because they are highly transparent to shortwave radiation
(their cloud albedo forcing is small), but they readily absorb the outgoing
longwave radiation. Like clear air, cirrus clouds absorb the Earth's radiation
and then emit longwave, infrared radiation both out to space and back to
the Earth's surface. Because cirrus clouds are high, and therefore cold,
the energy radiated to outer space is lower than it would be without the
cloud (the cloud greenhouse forcing is large). The portion of the radiation
thus trapped and sent back to the Earth's surface adds to the shortwave
energy from the sun and the longwave energy from the air already reaching
the surface. The additional energy causes a warming of the surface and atmosphere.
The overall effect of the high thin cirrus clouds then is to enhance atmospheric
greenhouse warming.
Image from the Space Shuttle Endeavour on July 1, 1993 (STS-57) showing
wispy cirrus clouds.