The color and brightness of clouds depend on the reflection, scattering, and transmission of sunlight. When sunlight reaches atmospheric particles, blue light is scattered more than other colors, and as a result, the sky appears blue during the day. However, when sunlight reaches a cloud, all colors of light became scattered equally. The equal scattering of light is caused by the large water droplets contained within a cloud. As a result, when clouds are viewed from Earth they appear white in color against the background of a blue sky. Therefore, clouds are white because light from the Sun is white.
The Science Behind the Scattering of Sunlight
Sunlight contains a full spectrum of colors that can make up a rainbow. However, each color has a different wavelength, which is measured in nanometers. Blue has a wavelength of 400 nanometers, which is the shortest, while red has the longest wavelength at 700 nanometers. Small particles can scatter short wavelengths more efficiently than long ones, while bigger particles, such as water droplets, scatter all wavelengths of light almost equally. Since there are millions of water droplets in the atmosphere, sunlight is scattered into various colors that combine to form the color white, which gives clouds a white color.
The scattering of sunlight by water droplets that form clouds occurs through a process known as Mie scattering, which is essentially refraction. When sunlight hits a water droplet, it is redirected or scattered through refraction in a forward direction. Similar redirection occurs in the millions of water droplets in the sky. Since Mie scattering is independent of wavelength, a random scattering of sunlight occurs. As a result, it is the equal scattering of all colors that makes clouds white in color.
Why Are Some Clouds Gray?
White clouds are formed as a result of the equal scattering of sunlight in all directions. However, when the scattering of light does not reach all parts of a cloud, especially the base and sides, the cloud appears gray in color. The color gray is often visible in large rainclouds, which have bigger water droplets than typical clouds. The top of a rain cloud is still white since its receives adequate white light from the Sun. This is evident when viewing rain clouds from the window of an airplane, as they appear bright white in color. Therefore, gray clouds are a sign of inadequate sunlight reaching the bottom and sides of a cloud.