Why Clouds Don’t Fall

Hundreds of Tonnes of Fluff

In a cumulus cloud a kilometre on each side, every cubic metre of air holds about half a gram of liquid water. Altogether that is about 500 tonnes — as much as a hundred elephants.

That water is split into countless droplets, each a few hundredths of a millimetre across, far smaller than the width of a hair.

A cumulus cloud, schematic · droplets enlarged thousands of times

A Very Slow Fall

The smaller a droplet, the more the air holds it back for its size. A cloud droplet 0.02 millimetres across falls through still air at only about a centimetre a second.

The rising air beneath a cumulus cloud moves tens of centimetres, even metres, a second. The droplets can’t fall out of it; they are carried upward. Even in perfectly still air, a droplet would need nearly a day to fall a kilometre, and any that drift out of the cloud’s base evaporate. So the cloud stays aloft.

A Million Into One

For rain, the droplets have to grow. Tossed up and down inside the cloud, they bump into each other and merge, getting bigger and bigger. A raindrop 2 millimetres across takes about a million cloud droplets.

Once a drop is too big for the rising air to hold, it falls: a 2-millimetre raindrop falls at about 6.5 metres a second.

Raindrop fall speeds: Gunn & Kinzer (1949)

Raindrops Aren’t Teardrop-Shaped

Pictures often show raindrops pointed at the top and round at the bottom, but real ones aren’t. Small raindrops are round; big ones are squashed by the air rushing past, flat underneath, like a little hamburger bun.

Bigger still, a drop is blown into a bag shape and breaks into smaller ones. Between flattening and breaking apart, raindrops never fall much faster than about 9 metres a second, however hard it rains.