Snow forms when water vapor turns directly into ice crystals without becoming liquid first.

The longer version

Deposition is the process by which water vapor in a cloud turns straight into ice without becoming liquid first. It happens on tiny particles when the air is saturated and below freezing. The vapor molecules arrange themselves into a hexagonal lattice, and the crystal grows arm by arm.

The exact shape depends on the temperature and humidity the crystal passes through as it falls, which is why snowflakes show such variety. If the air below the cloud stays cold the whole way down, the crystals reach the ground as snow, while warmer layers turn them to rain.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.