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Clouds are made of extremely small droplets of water ranging from 1 to 100 microns. Their high surface area to volume ratio means that air resistance would greatly slow their fall.

However, regardless of a water droplets total size, water's density is still much greater than air, and over time the earth's gravitational pull should cause the water droplet to fall with respect to its surrounding gas.

Why then, do water droplets in clouds appear to float at great altitudes rather than fall to the earth? If drawing a Newton force diagram of a water droplet in a cloud, what forces pulling upwards equalize the force of gravity pulling it down?

Qmechanic
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Cory Klein
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1 Answers1

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This link does the calculations, including viscosity. They do fall:

A water droplet with a 10 nm radius falls at 12 nm/s in air. It would take 2.6 years for this droplet to fall one meter. It is only when the small droplets begin to coalesce into larger droplets that they fall with significant speed.

Italics mine.

From the link provided by Rob:

Cloud dropplets are about a mm apart and on average of micron size, and most water is mainly as water vapor in all but thunder clouds. They fall so slowly that even small updrafts keep them aloft.

anna v
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