Rain is water. Water is light in color. Rain clouds are full of water. Therefore, rain clouds should be light. Impeccable logic, but wrong.
Obviously, there are always water particles in clouds. But, when the particles of water are small, they reflect light and are perceived as white. When water particles become large enough to form raindrops, however, they absorb light and appear dark to us below.
From Why do clocks run clockwise? and other imponderables. – by David Feldman
