How do Snowflakes Form?
It turns out that "pure" snow is made up of snowflakes which are made up of from 2 to 200 separate snow crystals. Snow crystals are crystals that have formed around tiny bits of dirt that have been carried up into the atmosphere by the wind. So snow crystals are really soil particles that have been dressed up in ice.
Scientists think that there are really four different shapes of snow crystals. The graphic below shows, in a general way, the kinds of snow crystals that form at various temperature ranges.
The simplest shape is a long needle shaped like a spike. The other shapes all have six sides. One of them is a long, hollow column that is shaped like a six-sided prism. There are also thin, flat six-sided plates. And lastly there are intricate, six-pointed stars.
The shape that a snow crystal will take is dependent upon the temperature at which it was formed. The temperature in the highest clouds is around -30°F and they are made up exclusively of ice crystal columns. The other three shapes are formed in a narrow temperature range. When the temperature in the clouds is 3° to 10°F the star shaped crystals form. From 10°-18°F the plates form, and from 18°-23°F columns form. From 23°-27°F needles form and from 27°-32°F the plates reappear. As the snow crystals grow they become heavier and fall towards Earth. If they spin like tops as they fall then they may be perfectly symmetrical when they hit the Earth. But if they fall in a sideways fashion then they end up lopsided. Falling snow crystals clump together forming snowflakes. Each snowflake is made up of from 2 to about 200 separate crystals. The many things that happen to snow crystals as they fall, such as collisions, partial melting and colliding with water drops that freeze to them, create even more shapes. This is why irregular crystals with no easily identifiable form are the most common. Some times crystals are a combination of more than one form.