Why do rain clouds look darker than white clouds?

Sunlight contains many colors, from red to violet. Cloud droplets are roughly 0.01 mm across, much larger than the wavelengths of visible light. Particles this large scatter all colors almost equally, and light mixed evenly like that looks white to our eyes.

A clear blue sky is the opposite case. Air molecules are far smaller than the wavelengths of light, so they scatter blue light more strongly than other colors. That's why white clouds stand out so clearly against a blue sky.

Rain clouds are often several kilometers thick and densely packed with droplets. Inside the cloud, sunlight bounces off droplets and changes direction over and over, and much of it escapes out the top or sides. Little light makes it down to the base we see from the ground, so it looks gray or almost black. Just before rain, the droplets near the base grow larger and absorb more light rather than scattering it.

Seen from an airplane above, the top of the very same rain cloud is dazzling white. The cloud hasn't changed color; the difference is whether you're looking from the side the sunlight enters or from the opposite side.

A dark gray cloud means a lot of water has gathered, so it often brings rain. But not always. Against a very bright sky or white clouds, a cloud in front can seem darker than it really is, and a thin cloud can look dull when the sky shows through it. On the other hand, even a thick cloud won't rain if its droplets don't grow as big as raindrops.

Cloud names reflect these observations too. In 1803, the English scientist Luke Howard classified clouds and gave rain-producing clouds the Latin word nimbus, which carries on in the names still used today, nimbostratus and cumulonimbus. Along with a cloud's color, checking whether its base is low and spread wide makes it easier to guess when rain is coming.