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Water Vapor
Water vapor (as distinct from clouds, which are treated separately)
is a strong absorber of infrared radiation. If you have been in both humid
and dry climates and experienced the temperature change from day to night,
you will have observed that in dry climates the temperature decreases
rapidly near sunset, whereas in a humid climate, the drop in temperature is
much less pronounced. The difference is due to radiation from the surface
that is absorbed by water vapor in the humid atmosphere and re-radiated
back to the surface. In the dry climate, this outgoing radiation
penetrates
the atmosphere and escapes from the earth to outer space. So, in fact,
water vapor is the largest absorber of infrared radiation from the earth
and is therefore the most important greenhouse gas in the earth's atmosphere.