When snow falls from the sky, you don’t usually see individual ice crystals, but rather clumps of crystals stuck together.

As ice crystals grow and clump together, they become too heavy to stay aloft. With the help of gravity, they begin to fall back down through and eventually out of the cloud.

As a professor of atmospheric and planetary sciences, I’ve studied how ice crystals floating in the sky become the snow that coats the ground. It all starts in the clouds. Clouds form when air ...