Why are rain clouds dark when other clouds are white?

A clear sky looks blue because tiny particles of air scatter blue light more than other colors. Water droplets in clouds are much larger, so they scatter red, green, and blue almost equally.

When light bounced around by countless droplets mixes together again, it becomes white, like the original sunlight. Clouds therefore show the color of sunlight rather than the blue of the sky.

Light scattered inside a cloud mostly escapes through the top or sides. Those parts are bright, while the base receives less light. Rain clouds build up high and thick, with more and larger droplets, so far less sunlight reaches the base. Droplets that grow just before rain also absorb a little light, making the cloud look darker.

When we look up from the ground, it is this shaded base that we see.

Fly above the clouds, and even rain clouds have brilliantly white tops. That is the side receiving direct sunlight. The same cloud is dark from below and white from above.

Gray clouds on an overcast day do not mean that the clouds themselves are gray. They mean we are standing on their shaded side. If you're out on a walk and the cloud base keeps getting darker, the cloud is growing thicker, which makes it a good cue to grab an umbrella.

Dark clouds can look as if they're full of dust or smoke, but the main reason for the gray is thickness. NOAA, the US weather and ocean agency, explains that as clouds thicken, less sunlight passes through them, so they look gray. Dust and haze can tint clouds yellow or red, but they aren't what turns clouds dark.

Our eyes play a part, too. Against a brightly lit sky or other white clouds, even a slightly less bright cloud can look quite dark. In the morning and evening, when the Sun is low, blue light is scattered away along its long path and red light remains, so clouds take on that light and glow orange.