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How does snow form?

Snowflakes, the basic unit of snow, originate as tiny ice crystals within "cold clouds." Cold clouds are clouds that exist within air that is at, or below, the freezing point. As an ice crystal is blown back and forth between the top and bottom of the cloud, it grows in two ways: by coalescence and by deposition. In coalescence, the ice crystal collides, and sticks to the cold water droplets it encounters in the cloud. In deposition, water vapor molecules (particles made by the combination of two or more atoms) within the cloud freeze directly onto the ice crystal.

As the ice crystal grows, it bonds with other ice crystals and takes on the six-sided shape of a snowflake. When the snowflake becomes heavy enough, it falls to the ground.

Sources: Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment, 5th ed., pp. 201-3; Engelbert, Phillis. The...

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