How Can a Cloud Weigh Millions of Pounds and Still Float?

A cloud looks almost weightless, yet even a modest one can contain hundreds of tons of liquid water.

Consider a typical fair weather cumulus cloud with a volume of about one cubic kilometre. If each cubic metre contains roughly half a gram of liquid water, the cloud contains about 500,000 kilograms of water, approximately the mass of one hundred elephants. Larger storm systems can contain vastly more.

The cloud does not fall as a single object because its water is divided among an immense number of microscopic droplets. Each droplet is so small that air resistance nearly balances its downward motion. Gentle rising currents can keep the droplets suspended or carry them upward.

The surrounding air also matters. A cloud is not a container filled only with water. It is a region of moist air containing droplets or ice crystals. Warm, humid air can be less dense than the cooler, drier air around it, helping the entire cloud forming region rise.

Droplets collide and combine as conditions change. Once they become large enough, gravity overcomes the support provided by moving air and resistance. They begin falling as rain. In strong thunderstorms, powerful rising currents can carry growing drops and ice particles upward repeatedly before they finally escape.

A cloud therefore floats in much the same sense that dust floats in a sunbeam. Its total mass may be enormous, but that mass is distributed among particles individually small enough to remain suspended.

The surprising part is not that clouds eventually fall. It is that the atmosphere can hold so much water above us before they do.