Why Are Clouds White?
Clouds appear white primarily because sunlight is white. When sunlight enters a cloud, it interacts with countless water droplets. These droplets are significantly larger than the particles scattering light in the clear sky. This interaction causes the sunlight to scatter equally across all wavelengths of visible light – red, orange, yellow, green, blue, indigo, and violet. Because all colors are scattered equally, the resulting light reaching our eyes appears white.
- Sunlight's Composition: Sunlight is a mixture of all the colors of the visible spectrum.
- Scattering by Water Droplets: The water droplets in clouds scatter this incoming light in all directions. The size of these droplets is crucial; they're large enough to scatter all colors equally. This is different from Rayleigh scattering which causes the blue sky.
- Equal Scattering = White: The even scattering of all colors results in white light being reflected back to our eyes. This is why we see clouds as white.
Why Some Clouds Aren't White
While most clouds are white, this isn't always the case. Clouds can appear gray or even dark depending on their thickness and composition. Thicker clouds absorb more light, leading to a grayer appearance. In some instances, atmospheric conditions like dust or haze can also affect cloud color.
This scattering effect is well-explained by sources like the Met Office, which states that in a cloud, "sunlight is scattered equally, meaning that the sunlight continues to remain white and gives clouds their distinctive white appearance." (https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/why-are-clouds-white). Other sources highlight the role of water droplets in this scattering process (https://www.nytimes.com/2013/03/12/science/why-do-clouds-appear-white.html).
