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Name: Art
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Question:
What percent of available moisture in the atmosphere falls as rain?


Replies:
Art,

Only about 0.001 percent of Earth's water is in the atmosphere, and perhaps only a few percent of that is really available at any one time to be used for precipitation. Most of the water vapor in the atmosphere gets cycled through precipitation every few weeks, although some in the upper parts of the atmosphere may remain there for months.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


That is very difficult to say because the "available" moisture, by which I assume the amount of water vapor present, is difficult to determine. It depends upon the temperature of the air, which in turn depends upon altitude, the movement of the air (e.g. up and over mountains), supersaturation of the atmosphere(i.e. clouds, fog, snow, etc.), relative humidity -- the list is long and overlapping. There are many of these variables that need to be specified in order to give a credible answer.

Vince Calder


Art,

Virtually all of the water in the atmosphere ends up being precipitated, although it could take a very long time before water in the stratosphere, for instance, would be precipitated out. The 5% figure that you quote sounds like the amount that is available at any one time (I said "a few percent") for precipitation.

Since most of the water vapor in the atmosphere is in the troposphere, where the active weather systems are, almost all of it gets cycled (precipitated) within several weeks, with the average residence time of a water molecule in the atmosphere being only about 10 days.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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