snowflake
© Kenneth LibbrechtThis snowflake has fernlike stellar dendrites - so called because the crystals have so many sidebranches that they look like ferns. Follow the link in the text to see many more types of snowflake.
As anyone who has tried to examine them under a microscope will appreciate, you've got to be quick - or in a very cold place - to investigate snowflake structure. It doesn't help if you live somewhere like California where there isn't much snow to start with.

Kenneth Libbrecht at the California Institute of Technology has come up with a solution. His team have built a machine that creates snowflakes in conditions similar to those in the atmosphere. They hope that their freefall convection chamber will shed light on the mechanisms responsible for generating the amazingly diverse shapes of snowflakes.

See a slideshow of snowflake photographs by Kenneth Libbrecht.