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Re: What keeps clouds up?



On Sat, 22 Mar 1997, Fred Bucheit wrote:

When water evaporates from a water droplet it does not add mass to the
atmosphere, it displaces air thereby making the air lighter. Diatomic
nitrogen has an atomic mass of 28 and diatomic oxygen has an atomic mass of
32 but a water molecule has a mass of 18.
Humid air is less dense than dry air and therefore tends to rise in the
atmosphere.

Ah, that helps. I didn't know how to visualize evaporated water as
compared to nitrogen. So, in simple language, water vapor acts more like
He than like CO2.

This implies that some clouds should indeed fall downwards, since
conversion of the water from gas to liquid-mist should make the net
density increase significantly. But if the cloud resulted from plume of
high-absolute-humidity air which was initally rising, the appearance of
droplets might just slow its upward motion, rather than reversing it.

I'm led to suspect that condensation energy P/V/T effects might dominate
here. If the tiny effects of droplet terminal velocity can be totally
ignored, maybe the effects of water vapor density are small and can be
mostly ignored, and this is why textbook explanations usually center on
heating and pressure rather than on H2O-gas density or on droplet mass.
What I was missing was a textbook explanation which includes even the
negligable effects.

Overall, it sounds to me like the original question, "why do clouds stay
up", is an improper question in the sense that the cloud is not a physical
object, instead it is a complicated system. Its behavior doesn't follow
one or two simple physical laws, so the answer to the question cannot be
brief or simple. Asking about clouds is more akin to asking why animal
populations vary, or about "oscillons" in vibrating bead beds.

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