Peggy LeMone was in junior high, when she started pondering a simple (but not so much) question: “How much does a clould weigh?”. Now all grown up, LeMone is a researcher at the National Center for Atmospheric Research, and she’s figured out an answer.
First, find out how dense the cloud is. Scientists have measured the water density of a typical cumulus cloud as 1/2 gram per cubic meter.
Next, figure out how big the cloud is. A typical cumulus 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. Putting it in more familiar terms: that cloud weighs about as much as 100 elephants.
If all those elephants 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. So gravity’s effect on them is pretty negligible.
What’s more, 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.
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