Fair-weather cumulus clouds are powered by the heat of their own condensation
The fluffy, flat-bottomed clouds of a summer afternoon run on a feedback loop. Warm air rises and cools until its vapour condenses, and condensing releases heat that pushes the air higher still. The flat base marks the height where that loop switches on, and in deserts it can sit above 6,000 metres.
Cumulus takes its name from the Latin for heap or pile. These are low clouds, usually under 2,000 metres unless they swell into the taller congestus form, and they may float alone, in clusters or in lines. They belong to the family of freely convective heaped clouds that also includes cumulonimbus, while stratocumulus, altocumulus and cirrocumulus represent limited convection at low, middle and high levels.
They form when ground-warmed air rises. As it climbs it cools at a steady rate, so its relative humidity climbs too. Once humidity reaches 100 percent, vapour condenses onto tiny particles in the air, releasing latent heat that warms the parcel and drives more convection. Surrounding air is mixed in along the way, so the rising mass grows. How tall the cloud gets depends on the temperature profile and any inversion layers overhead. Moist air produces lower bases: in temperate regions usually below 550 metres, though up to 2,400 metres is possible.
Rain inside them develops in two overlapping steps, as Irving Langmuir described. Surface tension gives very small droplets slightly higher vapour pressure, so they evaporate and their water recondenses on bigger drops. That stops mattering once drops reach about 20 to 30 micrometres; then falling drops sweep up others by collision. Measurements in one study found no liquid water at the cloud base, a peak of up to 1.25 grams per kilogram of air around mid-height, and a sudden drop to zero at the top.
Most cumulus bring little or no rain, and by reflecting sunlight they generally cool the ground. Under instability, humidity and strong temperature gradients, however, they can grow into rain-bearing congestus or cumulonimbus. Wind shear can also roll them into cloud streets, parallel bands that may run more than 480 kilometres, typically under high pressure after a cold front.
Source: Cumulus cloud
