Why do clouds float if they are full of water?
Explored it? Here's the recap
Why do clouds float if they are full of water?
The short answer
A cloud floats because its water is split into billions of microscopic droplets, each so small that the gentle upward push of rising air easily holds it up. The cloud isn't lighter than air — its water is just broken into pieces too tiny to fall quickly.
How it works
Inside and below a cloud, warm air rises like a slow invisible fan. How fast a water drop falls depends mostly on its size: a tiny droplet falls only a few millimetres per second, slow enough that the rising air keeps it suspended. As droplets bump into each other and grow, they fall faster and faster. When a droplet finally grows big enough that its falling speed beats the upward push of the air, the air can no longer hold it and it drops out of the cloud as rain.
What people get wrong
People often think clouds float because water is lighter than air, or because a cloud is made of fluffy steam. Neither is true. A cloud is made of ordinary liquid water, which is much heavier than air. It floats only because that water is divided into specks too tiny to fall fast, and because rising air pushes up on them. Rain is not new, heavier water arriving from somewhere else; it is the same droplets that have clumped together until they are too big for the air to carry.
The catch
Tiny droplets float for hours, so a cloud can drift across the sky and stay up all day, but they are far too small to reach the ground, so a cloud of them alone cannot water anything. Big drops are heavy enough to fall as real rain you can feel, but each one has to collide with thousands of tiny droplets to grow that large, which takes time and the right conditions, so not every cloud gets to rain.
Questions kids ask
If a cloud is so heavy, why doesn't gravity just pull it down?
Gravity does pull on every droplet, but each droplet is so tiny that it falls only a few millimetres per second. The gentle upward push of rising air is enough to balance that slow fall, so the droplets stay suspended instead of dropping.
So what is rain, really?
Rain is the cloud's own droplets after they have clumped together into drops big enough to fall fast. Once a drop grows large enough that its falling speed beats the rising air, the air can't hold it anymore and it falls to the ground.
Is a cloud lighter than the air around it?
No. The water in a cloud is much heavier than air. The cloud floats only because that water is broken into microscopic droplets that fall very slowly, and because rising air pushes up on them.
Why do some clouds rain and others just drift by?
A cloud only rains if its droplets get the chance to collide and grow large enough to fall. If conditions keep the droplets small, the cloud keeps floating and drifts away without raining.
For grown-ups
Cloud droplets are roughly 10–20 micrometres across, with terminal fall speeds of only a few millimetres per second, easily offset by the modest updrafts inside a cloud, so they stay suspended. They are not buoyant; the water is denser than air but slow to fall. Fall speed rises steeply with radius (Stokes' law: speed is proportional to radius squared for small droplets), so as droplets grow by collision and coalescence, one eventually exceeds the updraft speed and falls out as a raindrop (about 1–2 mm across, falling several metres per second). A cloud is visible liquid water or ice, not water vapour.