The curious growth of a snow crystal
Temperature and humidity affects the shape of snowflake crystals. The temperature of formation determines the original crystal shape. Large (“dendritic”) flakes grow best between -10° and -12° C. Plates grow at warmer or colder conditions.
Humidity — water vapor pressure in the cloud — affects the growth rate due to deposition. A flake that passes through dry air may sublimate (lose mass by converting from solid to gas).
Because temperature and humidity change as a flake bounces around in a cloud, the basic shapes can blend into countless crystal shapes. An ice crystal may also collide with another crystal and aggregate into a snowflake of a different shape. Or crystals may grow as they accrete (collide and adhere) tiny drops of liquid water in the cloud.
The crystal shape of a snowflake depends on several parameters, including temperature. In this applet, you can explore how snowflake crystal shapes depend on temperature — click repeatedly in one of the temperature zones (between the lines). Then move your cursor to a different temperature zone and click some more. You may combine up to 3 different shapes before resetting (by clicking below the cloud).