How clouds form

A cumulus cloud holds about half a gram of water in each cubic metre, so a cloud 1 km on each side carries about 500 tonnes.

A cloud is liquid water and ice, not vapour

It is easy to think of a cloud as a patch of steam, but water vapour is an invisible gas. You can see a cloud because it is made of billions of tiny liquid droplets or ice crystals that scatter sunlight. Each droplet formed when vapour condensed onto a speck of dust, salt or smoke, a cloud condensation nucleus. Whether a particle activates into a cloud droplet depends on its size and composition and on the supersaturation of the air, not a universal minimum radius.

For vapour to condense, the air has to cool to its dew point, the temperature at which it is saturated. Near the ground that can happen on a clear, calm night, which gives dew and fog. High in the sky it almost always happens because air is lifted. Rising air expands as the pressure around it falls, and expansion cools it. NOAA gives the rate for unsaturated air as 9.8 °C per kilometre.

Four ways to lift air

Sunshine heats the ground unevenly, and bubbles of warm air rise from the warmest patches, like blobs in a lava lamp. This is convection, and it builds cumulus clouds on summer afternoons. Wind blowing against a mountain range is forced up the slope; that gives clouds that cap peaks and ridges, and the rain on the windward side. Along a weather front, warm air slides up over colder, denser air and wide sheets of cloud form. Finally, where winds flow together from different directions near the ground, the air has nowhere to go but up.

Why something so heavy floats

The USGS works out the weight of a cloud. A typical cumulus holds about 0.5 g of water per cubic metre. A cloud 1 km on each side has a billion cubic metres, so it carries about 500,000,000 g, or 500 tonnes, of droplets. It does not fall because the droplets are spread through a huge volume of air, and that moist air is slightly less dense than the dry air around it. Each droplet falls so slowly, about 1.3 cm a second on average according to the NWS, that a gentle updraft keeps it aloft. Only when droplets grow into raindrops do they fall out.

Clouds also keep changing. At the edges, where they mix with drier air, droplets evaporate; inside, where air is still rising, new ones form. NOAA describes a cloud as a balance between condensation and evaporation, which is why a cumulus can grow, shrink and disappear within twenty minutes while you watch.

You can make a small cloud yourself on a cold morning. Breathe out, and the warm, moist air from your lungs mixes with the cold air outside, cools below its dew point and condenses into a little white puff. It vanishes within a second or two as it mixes with more dry air and the droplets evaporate again. A cloud in the sky is the same thing on a far larger scale, kept alive by air that goes on rising.