How hard the air pushed
Why Is Some Rain Gentle and Some Rain Heavy?
Drizzle falls from a lazy cloud; a downpour falls from a cloud that fought gravity for hours.
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Step 1 of 5
Drops only fall when they beat the rising air.
Inside a cloud, air is moving upward. A droplet falls only when it is heavy enough that gravity wins, so the strength of that updraught sets the drop size.
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Step 2 of 5
A weak cloud lets tiny drops go.
In a shallow cloud with almost no lift, a droplet only has to grow a little before it sinks out. It arrives as drizzle you barely feel.
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Step 3 of 5
A strong updraught keeps drops in play.
Fast rising air holds a drop up for a long time. All the while it collides with others and grows, sometimes riding up and down several times.
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Step 4 of 5
Big drops fall fast and hit hard.
A drizzle drop drifts down at walking pace. A downpour drop is many times wider and arrives at around thirty kilometres an hour.
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Step 5 of 5
Which is why a downpour ends quickly.
The tall cloud dumps everything it lifted in a few minutes and collapses. Drizzle drifts on for hours because nothing dramatic is happening at all.
Drops bigger than about five millimetres cannot survive — air resistance flattens and tears them apart as they fall.
The short version
Gentle rain falls from shallow clouds with no lift; heavy rain falls from tall clouds whose updraughts grew the drops huge.
Try it yourself
Hold a sheet of dark paper out in rain for two seconds. Drizzle leaves pinpricks; a shower leaves coin-sized splats.
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