Watch Out for That Cloud

Dave Thurlow, Host

NOAA Historical Photo Collection

Imagine a typical cumulus cloud, the puffy cotton ball clouds you see almost every summer afternoon. Let's say this cloud takes up the space of a cubic kilometer. That's about half a mile across and half a mile deep. A cloud like this may look like a big pile of cotton candy, but looks can be deceiving. The water in this fluffy cloud weighs 500 tons!

Why doesn't the whole thing fall to the ground? It's because the water droplets in the cloud are dispersed across such a big area. If you boiled enough water to fill your house with steam, you'd have about the same concentration of water vapor as you do in a cloud.

There's something else that helps keep our heavy cloud from falling; it's the wind. A cloud forms because the air is rising, and the water condenses out as the air rises and cools. The same updraft that forms the cloud also helps keep the water vapor suspended in space. That is, until the droplets start bumping into each other and combining to form raindrops. Then you'd have tiny pieces of the cloud falling to the earth in the form of a rain shower. However, even thunderstorms drop only a small part of their weight as rain or hail. Most of it evaporates back into the air, waiting for a chance to be part of a new cloud.

Thanks to contributing writer Bob Henson. Our show is underwritten by Subaru, the beauty of all wheel drive with major support provided by the National Science Foundation.

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Soul of the Sky - "Exploring the human side of weather"  NOW AVAILABLE Compiled and edited by: Dave Thurlow & Ralph Adler. North Conway: Mount Washington Observatory, 1999. Paperback, 150 pages.   Soul of the Sky is a different kind of weather book. It's not preoccupied with charting fronts, defining what an isobar is, or trying to get you to memorize the conversion formula from degrees Centigrade to degrees Fahrenheit. Instead, it's a collection that illustrates how the weather can inspire and terrify, connect us and urge us on to new adventures, and invite us to gain a deeper appreciation of how weather and climate affect our everyday lives.