Why before it rains do the clouds turn black?
The tiny water droplets and ice crystals in clouds are just the right size to scatter all colors of light, compared with the smaller molecules of air that scatter blue light most effectively. When light contains all colors, we perceive it as white. When clouds are thin, they let a large portion of the light through and appear white. But like any objects that transmit light, the thicker they are, the less light makes it through. As their thickness increases, the bottoms of clouds look darker but still scatter all colors. We perceive this as gray. If you look carefully, you will notice that the relatively flat bottoms of clouds are always a little grayer than their sides. The taller the clouds become, the grayer their bottoms look.
