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Earth   By John Dobson Published 2004-10-22 12:54:27 From 1995
"Why are the clouds so flat on the bottom?" is the wrong
question to ask. The flat bottom is just the boundary between
the denser dry air below and the less dense wet air above. (The
molecular weight of water is only eighteen. The molecular weight
of nitrogen and oxygen are twenty-eight and thirty-two because
they both form diatomic molecules; so dry air is conspicuously
denser than wet air and stays near the ground while the wet air
floats above it.) But "Why are the clouds so lumpy on top?" is
the right question to ask. And it is because of the phase change
of the water. It takes eighty-one calories per gram to melt ice
without any change in temperature. (A mixture of ice and water
will stay at the freezing point till all the ice has