Why Are Clouds White?
Clouds appear white because of the nature of the incident light and the way this light is scattered by them.
Sunlight looks white but contains all the colors of the rainbow. Each color has a different wavelength—it increases as we go from the violet end of the visible spectrum toward the red. When all the colors reach our eyes together, our brain interprets the mixture as white light.
When sunlight reaches clouds, it interacts with millions of water particles that make up the clouds. These droplets are about 20 micrometers across—significantly larger than the wavelengths of visible light. Because of their larger size, these particles scatter the different wavelengths of visible light approximately equally in all directions. This process is called Mie scattering. As a result, the light emerging from the cloud still contains all the colors of incident sunlight mixed together. When our eyes receive these visible wavelengths in roughly equal amounts, we perceive the combined light as white.
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The color of a cloud, however, may change depending on how sunlight travels through it. For example, when it is about to rain, the water droplets inside a cloud clump together into larger drops. These drops scatter light upward and outward through the top and sides of the cloud, allowing less sunlight to reach the bottom, which causes the clouds to appear dark from below.
The color of the cloudless sky is governed by the phenomenon called Rayleigh scattering. It is the dispersion of light or other electromagnetic radiation by particles that are much smaller than the wavelength of the incident light. Gas molecules in the atmosphere are smaller than the wavelength of visible light, and they scatter light of shorter wavelengths, especially blue, more strongly than longer ones. This process spreads blue light across the sky and makes the sky appear blue.
Clouds take on an orange or reddish hue during sunrise and sunset because sunlight travels through thicker atmospheric layers—owing to the Sun’s position low on the horizon—where Rayleigh scattering removes much of the shorter-wavelength blue light. This leaves only the red and orange colors with longer wavelengths to strike and illuminate the clouds. At night clouds over towns and cities may appear yellow or orange if they reflect artificial lights from below.