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What keeps clouds from pouring downwards like humidifier mist? For some
reason the density of humid air must not increase as cloud droplets form.
A thought-experiment. Take some air with non-zero absolute humidity. Now
move all the water molecules together in groups to form a population of
droplets. The droplets are heavy, but the air left behind is lighter, so
the average density doesn't change and the cloud will not fall. But I
suspect that this reasoning is incomplete. Is humid air lighter or
heavier than dry air? And if the humidity is suddenly removed from air,
does the partial pressure fall, requiring that the parcel of air shrink
and become denser?
I'm confused. Why do clouds stay up?