Dark storm clouds look gray or black because they are thick enough to block sunlight, not because they contain dirt.

The color of a cloud is determined by a process called Mie scattering. In a typical white cloud, small water droplets scatter all wavelengths of visible light equally, creating a bright white appearance. However, as a cumulonimbus cloud develops, it can reach heights of over 12,000 meters (about 40,000 feet).As the cloud becomes taller and more packed with liquid water and ice, it becomes more efficient at absorbing and scattering light before it reaches the base. By the time sunlight travels through several kilometers of dense moisture, very little light energy remains to pass through the bottom. This creates a shadow effect that makes the base appear dark gray or even black to an observer on the ground.According to the National Oceanic and Atmospheric Administration (NOAA), this phenomenon is purely optical and has nothing to do with the purity of the water. Even the darkest storm clouds are composed of clean water vapor and ice crystals. The contrast is often heightened by the background light or the brightness of the cloud's own edges, which may still be receiving direct illumination.In extreme cases, such as supercell thunderstorms, clouds can even appear green. This happens when the blue light scattered by the atmosphere combines with the red light filtered through the massive water content of the cloud. Research by meteorologists like Dr. Frank Gallagher has shown that these color shifts are reliable indicators of high liquid water content and potential hail.