When flying in an airplane above a blanket of clouds, the pillars of white and gray appear soft, fluffy and lighter than air. But make no mistake, these bouncing clouds are much, much heavier than they look.
So how much does a cloud weigh? And how do you weigh a cloud? We asked the experts to find out.
Clouds are composed mostly of air and millions of tiny water droplets, which form when water condenses around a “seed” particle. Seed particles can be anything from nitric acid to fumes released by trees, but they are usually very small.
There are several ways to measure the weight of a cloud. The first consists in weighing the water vapor that composes it – and for that, “you have to know something about the dimensions of the cloud”, Armin Sorooshian (opens in a new tab), a University of Arizona hydrologist, told Live Science. You also need to know the droplet density.
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A few years ago, Margaret LeMone (opens in a new tab), an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado, wondered about the weight of water in an average cumulus. So she did the math. First, she measured the size of a cloud’s shadow and estimated its height, assuming a roughly cubic shape. Clouds are generally not cube-shaped, but cumulus clouds are often about as tall as they are wide, so this assumption helped streamline the volume calculation. Then, based on previous research, she estimated the density of water droplets to be around 1/2 gram per cubic meter. “I came with about 550 tons [499 metric tons] of water,” LeMone said.
This is approximately the weight of 100 elephants hanging over your head. “It’s really impressive,” Soroohsian said.
Of course, different cloud types have different weights. For example, “cirrus clouds are much lighter because they contain much less water per unit volume,” LeMone told Live Science. And cumulonimbus clouds (the dark thunderstorms you see just before a thunderstorm) tend to be much heavier.
However, “the entire volume of the cloud is not just droplets, there is also air,” Sorooshian said. If someone wanted to take LeMone’s calculations a step further, they could factor in the weight of the air between each droplet.
But if the clouds are so heavy, why don’t they fall? For one thing, “the droplets are so small that they don’t fall very quickly,” LeMone said. The average water droplet in a cloud is about 1 million times smaller than a raindrop – about the size ratio of Earth under the sun. High-altitude wind currents blow these tiny droplets, keeping them in the air much longer than if they were static.
The thermal convection also helps to keep the drops in the air. “A cloud is actually less dense than the air directly below it,” Sorooshian said. As the hot air (and hot water) rises, it becomes more buoyant than the cold air (and cold water) below, like a layer of foam on a latte.
Of course, the clouds can be said to “fall” as rain. As the cloud droplets cool and condense into each other, they grow in size, eventually becoming so heavy that they collapse to Earth. Although a raindrop is much larger than a cloud droplet, each raindrop is still only 0.08 inches (2 millimeters) in diameter, according to the University Center for Atmospheric Research (opens in a new tab). Those little drops distribute the weight enough that 550 tons of water won’t crash on your head all at once.
So the next time you see a happy little cloud pass over your head, just remember: 100 elephants. And thank your lucky stars for the convection heat.