Buzz
Reader Jane shared her thoughts, mentioning that clouds appear soft and light, almost like they can't possibly weigh much. Is that really the case?
Back when Peggy LeMone was in junior high, a friend's dad wondered about the very same question, which she kept in her thoughts over the years. Now an adult, LeMone is a researcher at the National Center for Atmospheric Research and has figured out the answer. She now reveals her method of weighing clouds with us.
Start by calculating the cloud's density. Scientists have determined the water density of an average cumulus cloud (the fluffy, white ones on clear days) to be 1/2 gram per cubic meter—equivalent to the amount of water in a small marble, a space that could comfortably fit you and a friend. Other cloud types have different densities.
Next, determine the size of the cloud. You can estimate its width by observing its shadow when the sun is directly above it. LeMone does this by tracking her odometer while driving beneath a cloud. A typical cumulus cloud, she explains, is about a kilometer wide and usually takes on a roughly cubic shape—so it’s about a kilometer long and tall. This results in a cloud with a volume of one billion cubic meters.
Now, apply the density and volume to calculate the total water content in the cloud. In this case, it’s 500,000,000 grams of water, or roughly 1.1 million pounds. To put this massive amount into perspective, LeMone suggests comparing it to elephants. The cloud weighs as much as 100 elephants. If you're into politics, she adds, that's equivalent to about 2500 donkeys. Or, if you prefer dinosaurs, it’s the weight of 33 apatosauruses.
If all those elephants, donkeys, or dinosaurs were hanging out in the sky, they’d fall. So how does such a heavy cloud stay up? Well, the weight isn’t concentrated in giant particles like elephant-sized chunks or even in a billion marble-sized ones. Instead, it’s spread across trillions of incredibly tiny water droplets dispersed over a vast space. Some of these droplets are so small that you would need a million of them to make a single raindrop, and gravity has an almost negligible effect on them.
Furthermore, the cloud is less dense than dry air, which gives it buoyancy. Add to that the fact that the tiny droplets receive some lift from updrafts of warm air. However, these droplets don’t float forever. When the cloud's water density increases and the droplets grow heavier, eventually, the cloud will fall, bit by bit, as rain.
