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Where rain comes from

The short answer

A cloud is made of droplets about 0.02 mm across. A typical raindrop is about 2 mm across: 100 times wider, so 1,000,000 times the volume. Drops grow by sweeping up smaller ones as they fall, and fall speed grows with the square of the radius: a 20 µm radius droplet falls about four times as fast as a 10 µm one. A 2 mm raindrop falls at about 5.6 m/s, so it covers 1 km in about 3 minutes; a 20 µm droplet would take about 23 hours. Updrafts hold the small droplets up until they join into drops heavy enough to fall as rain.

1:40ScienceBeginnerraincloud dropletsraindropsStokes' lawwater cycle

Chapters and transcript

0:00
A cloud is made of tiny droplets.
A cloud is made of countless tiny water droplets, each far smaller than a raindrop. Each droplet is light enough to be held up by the air.
0:06
A typical cloud droplet is about 0.02 mm across.
A typical cloud droplet is about 0.02 mm across. The droplets on screen are not to scale.
0:13
A typical raindrop is about 2 mm: 100 times wider.
A typical raindrop is about 2 mm across, which is about 100 times the width of a cloud droplet.
0:17
Volume grows with the cube: 100 × 100 × 100 = 1,000,000.
Volume grows with the cube of the width. A raindrop 100 times wider holds 100 × 100 × 100 = 1,000,000 times the water, which is why it takes so many droplets to make one drop.
0:23
A bigger drop falls faster, so it sweeps up smaller ones.
A falling drop meets the droplets in its path and takes them in, growing as it goes. Here the sweep is simplified: every droplet the drop touches joins it.
0:33
Fall speed grows with the square of the radius.
Stokes' law gives the fall speed of a small drop as proportional to the square of its radius, so a larger droplet falls much faster.
0:38
10 µm radius: 1.2 cm/s. 20 µm radius: 4.8 cm/s.
Stokes' law gives 1.2 cm/s for a droplet of 10 µm radius, and 4.8 cm/s for one of 20 µm radius.
0:46
A 2 mm raindrop falls at about 5.6 m/s.
A raindrop 2 mm across falls at about 5.6 m/s, which is a few hundred times faster than a cloud droplet.
0:52
Over 1 km: about 3 minutes for the raindrop, 23 hours for the droplet.
At these speeds, a raindrop covers 1 km in about 3 minutes, while a 20 µm radius droplet at 1.2 cm/s would take about 23 hours. The raindrop's fall is shown in 6 seconds of film time, which is not to scale.
1:02
Updrafts hold the small droplets up.
Rising air, an updraft, keeps the small droplets from falling out of the cloud.
1:10
Droplets join until they are too heavy to stay up.
Droplets keep joining as they meet others. Once a drop is heavy enough, the updraft can no longer hold it, and it falls.
1:20
Rain is millions of droplets, joined and falling.
Rain is many droplets joined into drops heavy enough to fall. Rivers then carry that water back to the sea, and the cycle repeats.

Sources

Every figure in the film comes from these. The animation is illustrative.