Rain starts long before there is a cloud. The sun warms lakes, rivers and the sea, and water sneaks into the air as invisible vapour. Warm air rises, and as it climbs it gets colder, and cold air cannot hold as much water — so the vapour turns back into tiny liquid droplets clinging to specks of dust. Billions of those droplets together are what you see as a cloud. The droplets bump into each other and join up, growing bigger and bigger, until some are too heavy for the rising air to hold — and those fall to the ground as rain. Run the cycle in the simulator, slide the temperature, and watch the cloud build and the rain come down.
Most people think clouds are dry and the rain somehow appears from nowhere. In fact a single cloud already holds hundreds of tonnes of water, shared across droplets too tiny to fall. Rain comes when those droplets collide and merge until gravity finally beats the rising air.
What's actually happening
A cloud is a strange thing to drop water on you, because it does not look wet. It looks like soft cotton or grey smoke, something dry and weightless. Yet a single modest cloud can hold hundreds of tonnes of water, and the puzzle of rain is really two puzzles: how the water gets up there invisibly, and what finally makes it come back down in drops. Both answers are about a simple fact — warm air can hold a lot of water, and cold air cannot.
It begins with the sun heating water at the surface — oceans, lakes, puddles, wet soil. Some of that water escapes into the air as water vapour, an invisible gas, and warm air near the ground can carry a surprising amount of it. Then that warm, moist air rises, and here is the turn: as air climbs it expands and cools, and cooler air simply cannot hold as much vapour. The moment it cools past its limit, the dew point, the surplus vapour has to become liquid again. It condenses, but it needs something to condense onto, so it latches onto the billions of tiny specks of dust, salt and soot floating in the air, called condensation nuclei. Each speck gathers a microscopic droplet of water, and billions of those droplets hanging together are exactly what a cloud is. The simulator shows this stage clearly: warm the surface, watch the vapour rise, and see the droplets appear as the air aloft cools.
But cloud droplets are tiny, far too light to fall — they just float. The last step is growth. The droplets drift and collide, merging into bigger drops in a process called collision and coalescence, and in colder clouds ice crystals help by growing fast and then melting on the way down. A drop has to grow roughly a million times in volume from a cloud droplet before it is heavy enough that gravity finally beats the updraught holding it up. When it crosses that threshold it falls, sweeping up smaller droplets as it goes, and lands on you as rain. Temperature runs the whole show: cooler air holds less vapour, so it sets how high the cloud forms and how readily it tips over into rain. Slide the temperature in the simulator and you can watch the cloud thicken and the first heavy drops let go.
Warm air holds water and cold air cannot, so rising air cools, drops its vapour as droplets, and rains once they grow heavy enough to fall.
- 1Press Run cycle and watch water evaporate from the surface, rise, and condense into droplets that gather into a cloud.
- 2Lower the temperature and notice the cloud forms sooner and thicker, because cooler air holds less vapour.
- 3Keep it running and watch the droplets collide and grow until the heavy ones fall as rain back to the surface.
Common questions
A typical fluffy cumulus holds roughly 500 tonnes of water, but it is shared across billions of droplets so tiny they float. The weight is real, just spread out too finely to fall.
Pure vapour struggles to condense on its own, so it latches onto specks of dust, sea salt and soot called condensation nuclei. Cloud seeding exploits this by adding particles to coax droplets to form.
Cloud droplets drift and collide, merging into bigger drops by collision and coalescence. A drop must grow about a million times in volume before gravity can beat the updraught holding it up.