Nib asked
How do clouds stay in the sky?
Clouds don't really stay up: their droplets are sinking all the time, but so slowly (about a centimetre a second) that rising air carries them back up, and a puffy cumulus cloud is just the place where air is rising and cooling.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the puzzle
A small fair-weather cumulus cloud about a kilometre across holds roughly half a gram of liquid water in every cubic metre. Across a cubic kilometre that adds up to around 500 tonnes, the weight of about a hundred elephants. Yet it hangs there.
A single puffy cloud holds around 500 tonnes of water, and still it hangs in the sky.
2it does fall
Clouds do not float the way a balloon does. Each droplet is heavier than air and sinks, but it is so tiny that the air's drag almost cancels its weight, so it sinks at only about a centimetre a second. Any rising air, even a gentle breeze lifting off warm ground, easily beats that.
Cloud droplets are heavier than air and do sink, but so slowly that rising air beats them.
a common mix-upA cloud is not a solid blob that floats in place. Its droplets sink all the time, and it only stays because air is rising through it.
3how small
A typical cloud droplet is about 20 micrometres (0.02 millimetres) across. A raindrop is around 2 millimetres, a hundred times wider, and since volume goes with width cubed, one raindrop holds about a million cloud droplets. The drawing is not to scale: at true scale the droplet would be a speck.
A cloud droplet is about 0.02 mm across; a million of them make one raindrop.
4fall speed
Air drag matters more for small things, so tiny drops reach a very low steady fall speed. A 20-micrometre cloud droplet falls about 1.2 centimetres a second. Drizzle drops, a tenth of a millimetre wide, fall about a quarter of a metre a second, and a 2-millimetre raindrop about 6.5 metres a second. The graph is logarithmic on both axes.
Falling speed shoots up with size: about 1 cm/s for a cloud droplet, 6.5 m/s for a raindrop.
Why does a cloud droplet fall so slowly?
Show the answer
It is so tiny that air drag nearly balances its weight at a very low speed, about a centimetre a second.
5rising air
Sun warms the ground, the ground warms the air above it, and that warmer, lighter air rises. As it climbs it cools, and cooler air cannot hold as much vapor, so the vapor condenses into a cloud of tiny droplets.
Sun-warmed ground makes air rise, and the cloud forms where the rising air cools and condenses.
6the race
In a fair-weather cumulus the rising air typically moves around a metre a second, and much faster in big storms. A droplet's sinking speed is about a hundred times smaller, so the air carries it upward. The arrows are not drawn to scale.
Rising air moves about a hundred times faster than a droplet sinks, so droplets get carried up.
7a fountain
Rising air cools as it climbs, and cooler air cannot hold as much vapor, so vapor condenses into fresh droplets at the cloud's top and growing edge. Where droplets sink into warmer, drier air below, they evaporate back into invisible vapor. A cloud is like a standing wave or a fountain: its shape stays while the water moving through it is constantly replaced.
New droplets form on top while old ones evaporate below, so the cloud keeps its shape while its water changes.
Where does a cloud's water go when it sinks out the bottom?
Show the answer
It evaporates back into invisible vapor in the warmer, drier air below the cloud.
8the whole idea
The whole picture: each droplet is tiny, so it sinks at only about a centimetre a second. Rising air is much faster, so droplets are carried up and the cloud stays. A puffy cumulus cloud is really the place where air is rising and cooling. When droplets collide and grow big enough that they fall faster than the air rises, they leave the cloud as rain.
A cloud stays because rising air outpaces its tiny sinking droplets, and rain falls once drops grow big enough to win.
Clouds aren't parked in the sky, they are rising air made visible, a fountain of vapor that never stops moving.