Clouds appear light and wispy, drifting effortlessly across the sky. This visual perception contrasts sharply with their true mass, as clouds contain an astonishing amount of water. This weight raises a compelling question: how do these massive structures stay suspended in the atmosphere?
The Astonishing Weight
A typical cumulus cloud, a common fluffy white cloud, can hold a surprising amount of water. Spanning about a cubic kilometer, such a cloud can contain approximately 550,000 kilograms of water. This mass is equivalent to the weight of roughly 100 adult elephants. Larger storm clouds can weigh millions of kilograms.
How Cloud Weight is Determined
Scientists determine cloud weight by estimating the amount of water within them. Clouds are not solid masses of water, but vast collections of microscopic water droplets or ice crystals suspended in the air. These tiny particles collectively account for the cloud’s substantial weight. The density of these droplets within a cloud is measured as its liquid water content.
The process involves estimating the cloud’s volume and multiplying it by its average liquid water content. Specialized instruments, such as radiometers or aircraft-mounted probes, measure the concentration of water droplets or ice crystals per unit volume of air. For instance, a typical cumulus cloud might have a liquid water content of around 0.5 grams per cubic meter.
Each individual droplet is incredibly light, perhaps only a millionth of a gram. However, the sheer number of trillions of these droplets within a cloud creates its considerable mass. A cloud’s weight is thus a function of its size and water concentration.
Why Clouds Remain Aloft
Despite their weight, clouds do not fall to the ground due to several atmospheric processes. The individual water droplets or ice crystals that make up a cloud are very small and have a low terminal velocity. A typical cloud droplet might fall at a rate of only a few centimeters per second.
The atmosphere contains constant upward air currents, or updrafts. These rising air currents, often caused by the sun warming the ground and creating thermals, push cloud particles upward. Updrafts are strong enough to counteract the slow downward pull of gravity on individual droplets.
A cloud floats because its overall density, considering the spaces between droplets and the surrounding air, is roughly equivalent to the density of the air around it. This is analogous to a hot air balloon: the heated air inside makes the structure less dense than cooler outside air, allowing it to rise. Clouds achieve buoyancy through their dispersed structure and constant interplay with air currents.