When high amounts of humidity are in the air, water molecules are more prevalent than on a cool, dry day. Water molecules, which have two hydrogen and one oxygen atom, are larger than oxygen and nitrogen found in the air, and the size of a molecule plays a significant role in what frequencies of light are scattered. When white light encounters a larger molecule or dust particle, larger wavelength light will be scattered, whereas if a smaller molecule is struck by white light, smaller wavelength light will be scattered. Snow, beaten egg whites, and beer foam look white for the same reason.

If the water or smoke is dense enough, then all light waves are scattered many times, and the cloud looks grey.