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Re: What keeps clouds up?



Dear David,

Here is Hewitt's entire answer (apart from my inadvertent errors):

"The presence of of clouds has to do with updrafts and small terminal
speeds of falling droplets. A typical cumulus cloud is in the region of
updraft speeds of at least 1 m/s -- more than the terminal speed of
of typical droplets and more than enough to support droplets. But
even for a cloud in no updraft, the droplets drift so slowly out of the
bottom of the cloud, and evaporate so quickly, that they have little
chance of reaching the earth. They're replaced by new droplets
forming above.

"Raindrops are huge compared with typical cloud droplets. Their
terminal speed is greater than the speed of most updrafts and the
indeed fall from the cloud -- and they evaporate slowly enough to
reach the ground."

There is also a small cartoon ballon at bottom of the piece that alleges:

"Terminal speed of a water droplet (or other sphere) is proportional
to the square root of its radius."

That would be a big surprise to Stokes if one applies it to the water
droplets in a cloud which are surely in the laminar flow regime. I note
your reference to this same behaviour for raindrops, previously
unknown to me. I trust your report somewhat more than I would trust
Hewitt's because I have read other misconceptions from him. Hewitt
applies it to spheres in general, and one would be led to believe that
it applies to the cloud droplets which are the subject of his piece.

The matter of some droplets getting large enough to have terminal
speeds in the binocular-observable region in unusual circumstances I
freely acknowledge, but it is also the case that updrafts approaching
a hundred meters per second also occur in unusual circumstances,
and that these can support falling hailstones of substantial size. One
might also mention v1rga, the frequently observed phenomenon in
which incipient precipitation fails to reach the ground but can be
observed "hanging" beneath a cloud. There is so much that could be
said that would illuminate the matter for the questioner. In my opinion
his answer just doesn't satisfy the student's legitimate need to know.

Leigh