A cloud weighs about as much as a few whales
We picture a cloud as something vaporous, light, almost weightless. That is a trick of scale. A modest summer cumulus (the kind that drifts past in dozens on a blue afternoon) is roughly one kilometre across and one kilometre deep. Inside it, the water held as tiny droplets weighs on the order of 500 tonnes. Roughly seven or eight blue whales. Hanging in the air. Above your head. Not falling.
The big storm clouds carry far more: tens of thousands of tonnes of water, sometimes more. A vast suspended reservoir.
Each droplet falls so slowly it floats
The trick is size. The water in a cloud is nothing like the fat raindrops we imagine: each droplet is about 0.01 mm across. A thousand times smaller than a raindrop. At that scale the air pushes back enormously compared to the droplet’s weight, the same physics that keeps a speck of dust drifting almost stationary in a sunbeam. A cloud droplet falls at about one centimetre per second. In perfectly still air, it would take 28 hours to descend a single kilometre.
And the air is never perfectly still. Clouds form precisely where moist air is rising, at tens of centimetres per second, often much faster. That gentle upward breath is enough to keep the droplets aloft almost indefinitely. They hover like fog pressed against an invisible ceiling: the level where the air cools enough for vapour to condense.
When rain is born
As long as the droplets stay tiny, the cloud holds. But inside, they bump into one another, stick, merge. Some grow faster than others. Once they reach about half a millimetre across, their weight finally wins out over the rising air. They begin to fall in earnest, picking up speed, up to 9 metres per second for a large raindrop.
A cloud that rains is a cloud that has let its droplets grow too big. In the most violent storms, the vertical currents are so powerful that they catch the falling drops, hurl them back up, freeze them, release them, freeze them again — and that is how hail is made.