How much does a cloud weigh?
When you fly in an airplane above the cloud cover, the white and gray columns appear soft, fluffy and lighter than air. But don’t be fooled – those clouds that look like they’re bouncing are much, much heavier than they seem.
So how heavy is a cloud? And how to weigh the cloud? We asked the experts to find out.
Clouds are mostly composed of air and millions of tiny water droplets that form when water condenses around a “seed” particle. Seed particles can be anything from nitric acid to steam given off by trees, but they are generally very small.
There are several ways to measure cloud weight. The first is to weigh the water vapor that makes it up—and to do that, “you have to know something about the dimensions of the cloud,” Armin Sorooshian (opens in a new tab), a hydrologist at the University of Arizona, told Live Science. You also need to know how tightly packed the droplets are.
Related: Why do nuclear bombs create mushroom clouds?
Few years ago, Margaret LeMona (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 the average cumulus cloud. So she did mathematics. First, she measured the size of the cloud’s shadow and estimated its height, assuming a roughly cubic shape. Clouds are usually not cube-shaped, but cumulus clouds are often as tall as they are wide, so this assumption helped simplify the volume calculation. Then, based on previous research, she estimated the density of water droplets to be about 1/2 gram per cubic meter. “I reached about 550 tons [499 metric tons] water,” LeMone said.
That’s about the weight of 100 elephants hanging above your head. “It’s really impressive,” Soroohsian said.
Of course, different types of clouds have different weights. For example, “cirrus are much lighter, because they have far less water per unit volume,” LeMone told Live Science. And cumulonimbus clouds (the dark thunderstorms you see right before a storm) tend to be much heavier.
However, “the entire volume of the cloud is not just droplets; there is also air,” Sorooshian said. If one wanted to take LeMone’s calculations a step further, one could take into account 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 — roughly the size ratio Earth to the sun. Wind currents at high altitudes blow these tiny droplets away, keeping them in the air much longer than if they were static.
Convection of heat also helps keep droplets in the air. “The cloud is actually less dense than the air immediately below it,” Sorooshian said. As warm air (and warm water) rises, it becomes buoyant against the cold air (and cold water) below it, like a layer of foam on top of a latte.
Of course, clouds can be said to “fall” in the form of rain. When the cloud droplets cool and condense into each other, they grow, eventually becoming heavy enough to fall to Earth. Although a raindrop is much larger than a cloud drop, each raindrop is still only 0.08 inches (2 millimeters) in diameter, according to University Center for Atmospheric Research (opens in a new tab). Those little drops spread the weight enough to keep 550 tons of water from crashing down on your head at once.
So the next time you see a happy little cloud pass overhead, remember: 100 elephants. And thank your lucky stars for convection heat.
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