WeatherQuesting with April Holladay, Article printer-friendly version

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

Super cell — the king of thunderstorms. [NOAA]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:

  • Thickness. The thicker the cloud, the more light it reflects so less light makes it to a viewer below.
  • Size of the water droplets inside the cloud "It is evident that droplet size plays a role," says Craig Bohren, author of Clouds in a glass of beer. Because, we cannot see through misty droplets of a thick cloud. We can, however, see through big water drops of falling rain. "Yet, the rain came from the cloud. What’s the difference? Droplet size."
  • The amount of water. If two clouds have the same size droplets and the same physical thickness, then the cloud containing more water reflects more light.
  • Amount of light that falls on the cloud. A cloud can be gray because it’s in the shadow of another cloud.
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    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)

    Find this article at: http://weatherquesting.com/dark-clouds.htm