On a sunny afternoon, puffy white clouds float overhead like giant cotton balls. They look soft, light, and almost weightless. But clouds are made of water, and water is heavy. So how much is actually up there?
The short answer is: a lot, but spread very thin. A cloud is a huge collection of microscopic water droplets or ice crystals floating in the air. In one study of puffy cumulus clouds, the wettest part held up to about 1.25 grams of water per kilogram of air. Spread across a cloud that's a kilometer or more wide, those tiny amounts add up to hundreds of tons.
Doing the Math
Here's a rough back-of-the-envelope estimate. A kilogram of air takes up roughly a cubic meter, a bit more than a cubic yard. So the wettest part of that cumulus cloud held about a gram of water per cubic meter, which is only about a quarter teaspoon.
Now imagine a modest cloud about a kilometer, or roughly six-tenths of a mile, on each side. That's a billion cubic meters of air. If every bit were as wet as the wettest part, it would hold around a billion grams of water, or about 1,000 metric tons. The water is thinner near the top and bottom, so the real total would be lower, but still easily in the hundreds of tons.
Where the Water Hides
That same study found that the water isn't spread evenly. At the very base of the cloud, the concentration was zero. It climbed rapidly to a peak near the middle, then slowly dropped off toward the top before falling to zero again at the cloud's upper edge. Clouds can even have holes with no droplets at all, where wind tears in drier air or strong downdrafts evaporate the water.
The droplets themselves are incredibly small, some as tiny as about 5 micrometers across. That's far thinner than a human hair. In the growing cumulus clouds that researchers studied, they counted anywhere from 23 to 1,300 droplets in each cubic centimeter of air. The bases of those clouds sat roughly 1,600 to 4,900 feet above the ground.
So Why Doesn't It Fall?
If a cloud holds that much water, why doesn't it crash to the ground? The secret is in the droplet size. Particles that small fall incredibly slowly, and the rising air that builds cumulus clouds easily keeps them aloft. The water isn't one heavy blob. It's billions of tiny specks, each almost weightless on its own.
Rain begins when droplets grow. First, larger droplets slowly gain water at the expense of smaller ones. Once they reach around 20 to 30 micrometers, they start falling and bumping into other droplets, merging and growing until they're heavy enough to drop out as rain.
Bigger Clouds, Much More Water
Fair-weather cumulus clouds usually produce little or no rain. But under the right conditions, they can grow into towering cumulonimbus clouds, the thunderstorm giants. An average thunderstorm is about 15 miles across and reaches roughly 40,000 feet high, so it can hold vastly more water than a small puffy cloud.
So the next time you lie on the grass watching clouds drift by, give them a little more respect. Those fluffy shapes may look as light as a feather, but together they're carrying an astonishing load of water across the sky.
