Morphology dimensions
Snowflakes can be characterized by the snow-crystal morphology diagram, which shows crystal
shapes as a function of the temperature (x-axis) and humidity (y-axis) in which they grow. Here
the humidity axis actually refers to the supersaturation — the excess water-vapour density
above a humidity of 100%. The water saturation line (blue) shows the supersaturation that
would be found in a dense cloud of water droplets. The dramatic dependence of snow-crystal
morphology on temperature has yet to be explained.
Source: snowcrystals.com