On p119 in the Feb. '97 issue of The Physics Teacher, Paul Hewitt treats
the question "..why is it that all the water droplets in a cloud don't
fall to the ground?" He points out that individual droplets have small
terminal velocity.
Yet this can't be the whole answer, since the total mass of a cloud is
quite large. Yes, individual droplets fall very slowly. But why don't
the individual droplets fall as a group, dragging the air between along
with them? This behavior is seen when Ultrasonic-Humidifier fog is poured
into clear air. The mist-laden air is heavy and sinks rapidly, even
though the individual droplets would fall very slowly. The mist
constitutes a denser fluid than the surrounding air, so it falls.
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?
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