by Jim Beam
Wednesday, October 29, 2003
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From experience just carrying a container of water, we also know that water is much heavier than air. Since pressure is related to weight, water pressure must be far greater than air pressure, and of course it is.
Gravity keeps the atmosphere wedded to the earth. Without gravity, earth's atmosphere would float away to outer space. Since gravity diminishes with distance from the center of the earth, air weighs less at altitude than at sea level. Literally, air at altitude is "thinner" (meaning less dense) compared to sea level, and air becomes progressively less dense with increasing altitude. A cubic foot of air on the summit of Mt. Everest contains only about a third as many molecules as a cubic foot at sea level, and hence weighs only about a third as much. (Everything weighs less at altitude, including people, but we don't notice the difference except in outer space. You would not feel lighter on the top of a high mountain. In outer space, miles from earth's center of gravity, weightlessness is experienced.)