How do snowflakes form different shapes?

Snowflakes are ice crystals that form in clouds when the air temperature up there gets below freezing. Water molecules are in the air as a gas called water vapor. In the cloud, these molecules come out of the air and stick together.

In any crystal, molecules are lined up in some special way so that their atoms are arranged in a pattern. In an ice crystal, water molecules line up and form six-sided figures, or hexagons. That explains why all snowflakes have a six-pointed structure.

Snowflakes are also known for having great variety in their structures. This is because each ice crystal forms as it falls through layers of air. Each layer has a slightly different temperature and a little more or less water available for adding to the crystal structure. Ice crystals form differently in these various kinds of air.

Because air currents are always changing and snowflakes are always swirling around, each snowflake takes a unique downward path through the air layers. So each snowflake has a unique process that builds up its crystals, and each seems to be unlike any other.