Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how do clouds stay in the sky
Here is a shock: one puffy cloud holds about five hundred tonnes of water, like a hundred elephants. So why doesn't it come crashing down?
The truth is, the cloud isn't floating. Its droplets are falling, all the time, just so slowly that the slightest rising air beats them.
Why so slowly? Size. A cloud droplet is about twenty micrometres across, thinner than a hair. A million of them fit in one raindrop.
Small things fall slowly because air drag grows as they shrink. A raindrop drops about six metres a second. A cloud droplet creeps down one centimetre a second.
Sun-warmed ground heats the air above it, and warm air rises, carrying vapor up. Where it cools, the vapor condenses into droplets: that is the cloud.
How fast? Rising air moves about a metre a second, a hundred times faster than a droplet sinks, so the droplets get carried up.
At the cloud's top, cooling air keeps making fresh droplets. At its base and edges, drier air makes old ones evaporate. The shape stays while the water changes.
So a puffy cloud is a place where air rises and cools. And when droplets grow big enough to outrun the updraft, they fall as rain.
Clouds aren't parked in the sky, they are rising air made visible, a fountain of vapor that never stops moving.