The familiar blue of a clear sky is a constant of fair weather, yet a gray, overcast canopy is just as common a sight. This shift from a bright blue background to a subdued gray ceiling marks a significant change in how sunlight interacts with the Earth’s atmosphere. Understanding why the sky turns gray requires a look at the basic physics of light and how it behaves when encountering different particles in the air.
Setting the Scene Why the Sky is Usually Blue
The blue color of the daytime sky is a consequence of Rayleigh scattering. Sunlight, which is white light, is composed of the entire spectrum of colors, each with a different wavelength. When this light enters the atmosphere, it encounters tiny gas molecules, primarily nitrogen and oxygen, which are much smaller than the wavelengths of visible light.
This size difference means that shorter wavelengths of light, like blue and violet, are scattered in all directions far more effectively than the longer wavelengths, such as red and orange. Because blue light is scattered so strongly by these small air molecules, it appears to come to our eyes from every part of the sky. This selective scattering of the shorter blue light is the reason we perceive the sky on a clear day as blue.
Cloud Composition and Light Interaction
The mechanics of light interaction change when clouds are present. Clouds are not composed of tiny gas molecules; instead, they are vast collections of liquid water droplets or minuscule ice crystals. These water and ice particles are significantly larger than the nitrogen and oxygen molecules responsible for the blue sky.
When sunlight hits these larger cloud particles, a different process called Mie scattering takes over. Mie scattering occurs when the scattering particles are roughly the same size as or larger than the light’s wavelength. Crucially, this type of scattering does not favor one wavelength over another.
As a result, the water droplets and ice crystals scatter all colors of the visible light spectrum almost equally. When all colors of light are scattered uniformly and reach the eye together, the mixture is perceived as white. This is why thin, less dense clouds or the edges of thicker clouds appear bright white.
Density and Depth The True Reason for Gray
The transition from a white cloud to a gray or dark gray sky is a function of the cloud’s physical size and density. A gray sky is the view from underneath a deep cloud layer. As a cloud accumulates more water droplets and ice crystals, it grows both thicker and denser.
The sunlight that manages to penetrate the cloud’s top surface is repeatedly scattered and reflected internally by the countless water particles. This multiple scattering causes the light to diffuse and travel a much longer, more chaotic path through the cloud. With increasing depth, a progressively smaller amount of light manages to make it all the way through the cloud base to the ground.
The gray color we see from below is not a different color of light, but rather the shadow cast by the volume of water and ice above. The light that reaches the bottom of the cloud has been so diminished by scattering and partial absorption that it appears dark. The grayness is a measure of blocked transmission, indicating that the cloud is too thick for significant sunlight to pass through.