Q. Why are clouds white?
A. When clouds are white (and many are not), “it’s because light from the sun is white, a combination of all the colors of the rainbow,” said Michael Kaufman, professor of physics and astronomy at San Jose State University in California. As light moves through the clouds, it is dispersed by water droplets. These are comparable to or bigger than the wavelengths of light, Dr. Kaufman said, and there are lot of droplets in the average cloud.
The result is that all the wavelengths are scattered more or less uniformly; to our eyes, no color predominates.
“Looking at a cloud from any other angle, you see a mixture of all the scattered colors, so it looks white,” Dr. Kaufman said. “White light in, white light out.”
This effect is called Mie scattering, he added, “and is what you get when the wavelength of light and the particles it hits are comparable in size. It also works when particles are bigger than wavelengths of light.”
When light is moving through cloudless atmosphere, on the other hand, our perception of it is very wavelength-dependent. The small gas particles in the atmosphere scatter blue light about 16 times as efficiently as they scatter red light, an effect called Rayleigh scattering. That is why the sky is blue. C. CLAIBORNE RAY