When you're flying in an airplane above a blanket of clouds, the pillars of white and gray look soft, fluffy and lighter than air. But don't be fooled — those bouncy-looking clouds are much, much heavier than they appear.
So just how much does a cloud weigh? And how do you weigh a cloud? We asked the experts to find out.
Clouds are composed mainly 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 vapors released by trees, but they are generally very tiny.
There are a couple of ways to measure the weight of a cloud. The first is to weigh the water vapor that composes it — and to do that, "you need to know something about the dimensions of the cloud," Armin Sorooshian, a hydrologist at the University of Arizona, told Live Science. You also have to know how densely packed the droplets are.
Several years ago, Margaret LeMone, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado, wondered about the weight of the water in an average cumulus cloud. 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 not typically cube shaped, but cumulus clouds are frequently about as tall as they are wide, so this assumption helped streamline the volume calculation. Then, based on prior research, she estimated the density of water droplets at around 1/2 gram per cubic meter. "I came up with around 550 tons [499 metric tons] of water," LeMone said.
That's approximately the weight of 100 elephants suspended above your head. "It's really impressive," Soroohsian said.
Of course, different types of clouds have different weights. For example, "cirrus clouds are much lighter, because they have far less water per unit volume," LeMone told Live Science. And cumulonimbus clouds (the dark thunderheads you see just before a storm) tend to be much heavier.
However, "the entire volume of the cloud is not just the droplets; there's air, too," 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 clouds are so heavy, why don't they fall down? For one thing, "the droplets are so small that they don't fall very fast," LeMone said. The average water droplet in a cloud is roughly 1 million times smaller than a raindrop — about the size ratio of Earth to the sun. High-altitude wind currents blow these tiny droplets along, keeping them in the air for much longer than if they were static.
Heat convection also helps keep the drops aloft. "A cloud is actually less dense than the air directly below it," Sorooshian said. As warm air (and warm water) rises, it becomes more buoyant than the cold air (and cold water) beneath 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 cloud droplets cool and condense into one another, they grow, eventually becoming so heavy that they plummet to Earth. Although a raindrop is much bigger than a cloud droplet, each raindrop is still only 0.08 inch (2 millimeters) in diameter, according to the University Center for Atmospheric Research. Those small drops spread out the weight enough that 550 tons of water doesn't crash down on your head all at once.
So, the next time you see a happy little cloud passing overhead, just remember: 100 elephants. And thank your lucky stars for heat convection.
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Joanna Thompson is a science journalist and runner based in New York. She holds a B.S. in Zoology and a B.A. in Creative Writing from North Carolina State University, as well as a Master's in Science Journalism from NYU's Science, Health and Environmental Reporting Program. Find more of her work in Scientific American, The Daily Beast, Atlas Obscura or Audubon Magazine.