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White Clouds

October 12, 2010

If water droplets are clear, why do the clouds that form from them appear white?


When is water white?…I’m Bob Hirshon and this is Science Update.

Why are clouds white when the water they’re made up of is clear? We put the question to atmospheric scientist Robert Wood of the University of Washington in Seattle. He says clouds form from tiny, transparent water droplets that reflect sunlight.

ROBERT WOOD (University of Washington):
The clouds appear white because these little tiny droplets, they essentially act like mirrors, lots and lots of little mirrors together, and they reflect solar radiation, and sunlight. Sunlight is pretty white, really. And so they scatter it in all kinds of different directions and the eye sees that as being essentially a white material.

He adds that clouds cool the planet by reflecting much of that sunlight back into space. And if you have a science 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.