Towering clouds darken skies

Q: When a thunderstorm approaches, the sky turns black. Why? If it’s filling with rain, it should be white, right? What causes the black or gray in clouds?  Teresa, Cape Coral, Florida

A: You’re right, it is white — from above. A big cloud is like a big white marshmallow. It reflects light and therefore doesn’t let much sunlight through. If we’re above the cloud in an airplane, the cloud appears white because we see the bright light reflected by the cloud. But, to folks below, the same cloud appears dark because they see only the little light that doesn’t bounce off the cloud surface and does slither its way through the cloud mass to their eyes.

Super cell — the king of thunderstorms.  Photo courtesy of NOAA.

In fact, a cloud’s brightness depends on four things:

Thunderstorm clouds tower 8 to 11 miles (13 to 18 km) up— as high as two Mount Everests. Little light shines through to our eyes. Thus, they look dark from below. If thunderclouds crowd in, the whole sky darkens.

Further reading:

Bohren, F. Craig. Clouds in a glass of beer. New York: John Wiley & Sons, Inc., 1987.

Lynch, David K. and William Livingston. Color and light in nature. New York: Cambridge University Press, 1995.

Minnaert, M. The nature of light & color in the open air. New York: Dover Publication, Inc., 1954.

Williams, Jack. The Weather Book. New York: Vintage Books, 1997.

Wood, Elizabeth A. Science from your airplane window. New York: Dover Publication, Inc., 1975.

(Answered Jan. 21, 2005; updated Oct. 10, 2007)

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