droplets are heavier than air but tiny, so they fall slowly through rising air and appear to hang
A cloud stays aloft because gravity and air resistance reach a stalemate at very low speeds. Gravity pulls a droplet downward with a force proportional to its mass, and mass scales with the cube of the droplet's radius. Air resistance, by contrast, scales roughly with the droplet's cross-sectional area, which scales with the square of the radius. Shrink a droplet and the ratio of drag to weight grows sharply, which is why a 10 micrometre droplet falls about a centimetre per second while a 1 millimetre raindrop falls several metres per second.
Take a modest fair-weather cumulus, roughly a kilometre on each side and a kilometre tall. That gives a volume near one cubic kilometre, or 10⁹ cubic metres. Even with only about half a gram of liquid water per cubic metre of cloud, the total liquid mass comes to roughly 500,000 kilograms. Spread across roughly 10²¹ droplets, each one averages around half a picogram, and at terminal velocity falls only a few centimetres each second. In the time it takes one droplet to drift the kilometre back to the ground, the parcel of air beneath it has typically been replaced by warmer, rising air, so the cloud appears stationary.
Many people assume that because clouds look weightless, the water inside must somehow cancel gravity. It does not. The droplets obey normal physics; they simply have too little mass for gravity to overpower drag. Another misconception is that warm air alone keeps clouds up. Warm air is necessary for an updraft, but the droplets would still descend slowly even in still air; updrafts only slow or reverse that descent. Pilots and glider pilots know that the strongest lift is found near the cloud base, which is the visible evidence of this rising column of air.
This slow-falling model describes non-precipitating clouds of small droplets. Once droplets grow past roughly 100 micrometres, often by colliding and merging, their fall speed becomes large enough that updrafts can no longer support them, and rain begins. Storm clouds can carry updrafts of several metres per second, but even those updrafts lose against hailstones, which fall because they are large enough, and dense enough, that drag cannot match gravity. Fog, by contrast, can be thought of as a cloud whose droplets are forming or sitting in air with little vertical motion at all, allowing them to settle through shallow layers near the ground.
Cram Clouds are heavy. Right? So why do they just float up there?
Rep They are not floating. They are falling, just incredibly slowly.
Cram A whole cloud of water, falling? That cannot be right.
Rep It is. A cumulus cloud can weigh a hundred thousand kilograms. It just does not fall fast.
Cram What stops gravity from winning?
Rep The droplets are tiny. A typical cloud droplet is a hundred times thinner than a human hair.
Rep Tiny things have huge air resistance compared to their weight, so they reach terminal velocity in millimetres per second.
Cram So each drop just drifts down at a snail's pace?
Rep Right. And while they drift, rising air underneath is pushing them back up.
Cram There is an updraft under every cloud?
Rep Yes. Sun heats the ground. Warm air rises. It carries the droplets and keeps the cloud level.
Cram So a cloud is really falling and rising at the same time?
Rep Exactly. The droplets fall through the rising air and never hit the ground.
Cram Then fog is just a cloud that won the race downward?
Rep Close. Fog is a cloud sitting in air too still to lift it. Same droplets, no updraft.
Cram So clouds are not weightless. They are just falling slowly.
Rep That is the mental model. Heavy things in the sky, held aloft by tiny size and rising air.