Most normal people have, at some time in their lives, laid on their back in the grass and looked up at the summer sky as clouds drift slowly over. Often we play games of finding familiar shapes in those clouds. But have you ever wondered how much a cloud weighs?
At first thought, that would seem to be a nonsensical question: obviously, it doesn’t weigh anything because it’s floating in the air. But if you think that through a bit more, you’ll see that this claim doesn’t hold water.
What are clouds made of? They’re mostly air and water in some form or another. Generally this would be water vapor. Think of it as cold steam. The droplets are so tiny, they can ride eddies and currents of air that constantly swirl about in our atmosphere. Dry air is also denser than water vapor, so it will buoy the clouds up until the vapor turns to larger droplets (rain) or freeze into snow or hail.
It is a common sight here in the Great Smoky Mountains to see fog banks that form overnight along creeks and rivers be lifted up the slopes of the mountains as the morning sun warms the trees, which warm the air, which rises. As the warmer air rise, it drags these fog banks with it, up the slopes, to the mountain crest, then they launch; changing from fog to cloud (which is essentially the same thing except for location).
Water has weight: 8.34 pounds per gallon at room temperature. So if clouds are made of water, clouds must have weight. Can we calculate the weight of a cloud?
Photo: Herzlich Rvvlich
The answer is yes, we can … but not very accurately. This inaccuracy comes from the variables involved in clouds. One is that the size of a cloud is very difficult to measure accurately. Another variable is the water vapor density. A very light, airy cloud like a Cirrus (we called them horse tail clouds when I was young) will have a fairly low water vapor density where a cumulonimbus cloud (storm cell) will be so dense it’s just pregnant with rain. Those lazy summer days were generally spent watching cumulous clouds:They look like cotton balls.
Photo: Doug Bittinger
One person who spent a fair bit of time cloud watching as a kid is Peggy Malone, and she too pondered the question of how much a cloud weighs. Fortunately for us she grew up to become a researcher at the National Center for Atmospheric Research, and she’s figured out an answer.
Scientists have measured the water density of a typical cumulus cloud as 1/2 gram per cubic meter – about a typical marble’s worth of water in a space approximately 3 feet by 3 feet by 3 feet.
Malone measures a cloud’s shadow when the sun is directly above it 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.
If you whip out your slide rule (you DO have your slide rule, don’t you?) and do the math with the density and volume you can determine the total water content of the cloud. In this case; 500,000,000 grams of water, or 1.1 million pounds, or just shy of 132,000 gallons of water in a cumulous cloud 1 kilometer, by 1 kilometer, by 1 kilometer.
So, the next time you’re looking for ducks and horses and your aunt Mable’s face on those cotton-ball clouds, bear in mind that each of the “average” cumulous clouds that are scuttling about up there weighs in the neighborhood of a million pounds.
Fortunately they are not prone to coming crashing down on you like some manmade flying contrivances!