Cirrus Stuff: The Weight Of A Cloud

Q: How much does a cloud weigh?

A: Heavily on Why Gal's mood, come a Seattle January. Or more precisely, a cumulus cloud weighs about half a gram per cubic meter.

To put that in American, imagine a fluffy white cloud about as big as downtown Seattle between Yesler and Stewart and as high as it is wide. It will weigh roughly 550 tons, or more than the 416 tons of a fully loaded Boeing 747-200.

Wispy cirrus clouds are far lighter, while drenching storm clouds can be 10 times heavier per volume.

So why don't the dang things fall down?

Peggy LeMone, a cloud expert at the National Center for Atmospheric Research in Boulder, Colo., explained that clouds typically form when warm, moisture-laden air rises and cools to its dew point, the water vapor turning to water droplets about twice the width of a human hair in diameter.

Such droplets are so small they fall at a rate of about 2 feet per minute against the pressure of still air. "It's like a piece of paper," said LeMone. "If you crumple it in one piece it will drop quickly, but if you tear it up, the smaller the pieces of paper, the longer it takes to fall to the ground." Why Gal has noticed this with her lottery tickets.

Meanwhile, air may rise as fast as 3 feet per second, meaning a typical cumulus cloud is often puffing up in its center yet falling on its edges where further cooling condenses droplets into ice crystals. With such turbulence, most individual clouds live only a half hour or so, the scientist noted.

Given the right moisture, temperature and particulates, of course, the droplets form into drops 100 times larger in diameter and fall about 14 mph. So clouds do fall down. Living in Seattle, you knew that.

Why Gal tackles readers' science questions and welcomes more. You may write to her c/o Scene, Seattle Times, PO Box 70, Seattle, 98111.