Why do clouds float in the sky even though they have tons of water in them?

Clouds are the visible part of a continuous cycle in which water evaporates into the air, condenses and returns to Earth mostly as rain. The cycle starts when the Sun warms the Earth, which in turn, warms pockets of moist air. When an air pocket heats up, its molecules bounce around more actively and begin to take up more space. As a result, the air expands and becomes less dense. The surrounding heavier air then pushes its way underneath the warm pocket, which rises like a hot-sir balloon.

Only when the water condenses does it become visible, and when enough droplets condense, they form a cloud. Clouds can grow to vast proportions if enough air is heated. A thunderhead can tower up to 12 kilometers high, and half a million tons of water can be released in a single downpour. Clouds stay aloft because the droplets are so tiny that they remain suspended on air currents. Each mist particle is minuscule – only one millionth the mass of a final raindrop. They do not have any appreciable fall velocity and most of them continue to float with the surrounding air. Sometimes the larger particles drop out of the cloud, only to evaporate and be swept up again. At other times they leave the sides of the cloud to be replaced by water vapor from below, which condenses as tiny droplets. So there is no perceptible change in the cloud’s shape as well as size. The mass of water the cloud holds also remains more or less the same.

What makes this great mass float in the air is its relative lightness compared to the mass of air in which it resides. Take an example of a small cloud at an altitude of some 3,000 meters. Assume that it occupies a volume of 1 cubic kilometer and there is a 1 gram droplet per cubic meter. The total mass of these water particles would be about 1,000 tons, whereas the mass of the air in that same cubic kilometer would not be less than 1,00,000 tons. The heavier air keeps the cloud buoyant.

Additional reading:
Cloud (Wikipedia)

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