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Hexagonal plate snowflake, light micrograph. Snowflakes are ice crystals that form in air that has a temperature near the freezing point of water. If the air is calm, then a symmetrical, hexagonal snowflake can form. The two main growth patterns observed are faceting and branching. When growth is fast and unstable, branching patterns create a dendritic snowflake. Slower growth allows the straight lines and hexagonal shapes of the plate snowflake seen here. There is also a bullet snowflake at right. (KEYSTONE/SCIENCE PHOTO LIBRARY/KENNETH LIBBRECHT/SCIENCE PHOTO LIBRARY)