Ice crystals are responsible for various atmospheric optics displays
in the atmosphere.
Exactly how and why ice crystals form in a "dendritic" pattern is still somewhat of a mystery to physicists. Up until recently the only theory we have had to use is the crystallization theory. Now scientists claim to have found a hypercrystallization process in ice crystals that help them to propagate as much as 10 times the normal rate.
Best view in high resolution... ;o)