Why doesn't the sky fall?

[randy10926 at comcast.net]

Air molecules are never at rest. They undergo constant random thermal motions of a variety of types. The simplest type is that of uniform translation. The mean speed is about 500 m/s which is greater than the speed of sound (340 m/s). 

Randy

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I thought it was time to stir the pot while we wait for the results from the Worlds.
Can anyone explain why gravity doesn't pull all the air molecules down to earth?  Are they lighter than space?  What is their mean speed?
I don't think this will help answer the weathervaning question and won't help us fly any better but I thought it might be fun.
Jim O
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