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Edgy Clouds

October 6, 2010

A listener asks why clouds have such well-defined edges.


Edgy clouds…I’m Bob Hirshon and this is Science Update.

Listener Dieter Deyke wrote to ask why water droplets form clouds with very defined edges, instead of just being dispersed in the atmosphere. We consulted University of Washington atmospheric scientist Robert Wood. He says clouds form when the concentration of water vapor in the atmosphere exceeds a certain threshold.

ROBERT WOOD (University of Washington):
But they tend to form in localized patches because where water vapor exceeds this threshold tends to occur locally, where air is being cooled, and air is cooled by raising it up, by lifting it up. So clouds really reflect where air masses are being cooled and that’s why you tend to get sharp edges to clouds, the edges of the motions that cause the clouds to form.

And if you have a question, give us a call at 1-800-why-isit. If we use your question, we’ll send you a Science Update mug! I’m Bob Hirshon, for AAAS, the science society.