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Why are some clouds white, while others are dark?

John Hehr, state climatologist, associate dean of Fulbright College and professor of geosciences, replies:

The relative whiteness or darkness of a cloud is a function of cloud depth.

The small, puffy white cumuliform clouds that form on sultry summer days shine brightly because the sun reflects off the water droplets. As clouds become bigger and coalesce, light cannot reach the bottom of the cloud and is instead reflected off the top. The vertical extent of the cloud influences the darkness we see from the Earth’s surface. The darkest cloud formations occur during thunderstorms, where clouds can sometimes be 20,000 to 30,000 feet high!

A similar principle applies to stratiform clouds, layered clouds that extend from one horizon to the other. These clouds, which range from a few hundred to a few thousand feet in depth, cause a gray, “gloomy” cast to the sky. The thicker the clouds, the darker the sky will appear beneath the cloud bottom, where the sun’s light does not penetrate.