A cloud is full of water — so why doesn't it fall on your head?

A puffy cloud can hold as much water as a hundred elephants. Yet it just hangs there. What's holding all that water up? Let's find out… then make it rain.

1The two things you need

Rising air, and how size changes falling

Two simple ideas do all the work. Watch each one move:

Air keeps puffing up

Air isn't empty. Warm air near the ground rises, like a slow invisible fan blowing straight up. It pushes on anything light.

Size changes falling

Tiny things drift, big things drop. A speck of dust floats forever. A fat drop plummets. Same stuff — the size is what changes.

2The two kinds of water drop

A floater and a faller, side by side

A cloud is not "light water." It's water split into two very different sizes:

The floater

A tiny droplet

Smaller than a speck of flour. The gentle puff of rising air holds it up easily.

The faller

A fat raindrop

Hundreds of droplets clumped into one. Too heavy for the puff — it drops.

3Your turn — set the size

Drop one droplet into the rising air

The puff of air blows up at the same gentle speed the whole time. You only change one thing: how big the droplet is. Watch where it goes.

floating
Droplet sizea tiny speck
TINY SPECKFAT DROP

4Now grow the whole cloud

What if every droplet got bigger? 🌧️

Here's a whole cloud of tiny droplets, all floating. The rising air stays just as gentle. You're going to grow every droplet at once. But first — a guess.

Guess before you find out

You grow the droplets bigger and bigger while the puff of air stays the same gentle speed. What happens to the floating cloud?

5So which size wins?

Neither! Each size trades something

Tiny droplets float for hours

So light the rising air carries them, letting a cloud drift across the whole sky and stay up all day.

The catch: they're far too small to reach the ground — a cloud of them alone can't water the garden.
Fat drops actually fall

Heavy enough to beat the puff and drop as real rain you can feel.

The catch: each one had to bump into thousands of tiny droplets to grow that big — so not every cloud gets to rain.

A cloud floats because its water is split into specks too tiny for the rising air to drop. It only rains when those specks clump big enough that the air finally lets go.

Psst, grown-ups: cloud droplets are roughly 10–20 µm across, with terminal fall speeds of only a few millimetres per second — easily offset by the gentle updrafts inside a cloud, so they stay suspended (they are not buoyant; the water is heavier than air, just slow to fall). Fall speed climbs steeply with radius (Stokes' law: speed ∝ radius² for small droplets), so as droplets grow by colliding and coalescing, one eventually crosses the updraft speed and falls out as a raindrop (~1–2 mm, falling several m/s). A cloud is visible liquid water or ice, not water vapour.