Reader Jane wrote in to note that clouds look so nice and fluffy and lighter-than-air, so they certainly can’t weigh much. Right?
When Peggy LeMone was in junior high, a friend’s dad pondered that same question, and she kept it in the back of her mind for years. Now all grown up, LeMone is a researcher at the National Center for Atmospheric Research, and she’s figured out an answer. Today, she shares her cloud-weighing technique with us.
First, figure out how dense the cloud is. Scientists have measured the water density of a typical cumulus cloud (the white, fluffy ones you see on a nice day) as 1/2 gram per cubic meter—about a small marble’s worth of water in a space you and a friend could comfortably sit in. The density will be greater for different types of clouds.
Next, figure out how big the cloud is. By measuring a cloud’s shadow when the sun is directly above it, you can get an idea of its width. LeMone does this by watching her odometer as she drives under a cloud. A typical cumulus, she says, is about a kilometer across, and usually roughly cubical—so a kilometer long and a kilometer tall, too. This gives you a cloud that’s one billion cubic meters in volume.
Do the math with the density and volume to determine the total water content of the cloud. In this case, it's 500,000,000 grams of water, or 1.1 million pounds. That’s a lot of weight to wrap your head around, so LeMone suggests putting it in more familiar terms, like elephants. That cloud weighs about as much as 100 elephants. If you’re a Democrat and you’re feeling partisan, she says, you could substitute 2500 donkeys. If you care more for dinosaurs than politics, you could also say the cloud weighs about as much as 33 apatosauruses.
If all those elephants or donkeys or dinosaurs were hanging out in the sky, they’d fall. So how does a several-hundred-ton cloud stay afloat? For one thing, the weight isn’t concentrated in a hundred elephant-sized particles or even a billion marble-sized ones. It’s distributed among trillions of really tiny water droplets spread out over a really big space. Some of these droplets are so small that you would need a million of them to make one raindrop, and gravity’s effect on them is pretty negligible.
What’s more, the cloud is less dense than dry air, so it's buoyant. It also helps that all those little droplets get some lift from updrafts of warm air. Those droplets don’t float forever, though. When the cloud’s water density increases and the droplets get bigger and heavier, the cloud eventually does fall, bit by little bit, in the form of rain.