A white cauliflower type cloud is made of the finest droplets of moisture. There is plenty of space between and countless trillions of them float freely in every cupful of filmy cloud material. They form a frothy mixture of snowy meringue which can cast a shadow on the ground. In a rain cloud, the particles of moisture are much bigger. The frothy cloud is now a burly burly of sizable raindrops.

These floating gobs of water are big enough to shut out the sun and each one can cast a shadow on its neighbors. The rain cloud becomes dark with the countless shadows of these watery gobs getting ready to come tumbling down onto the earth.