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Sectored plate star 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 sectored plate snowflake seen here, which has central star-shape elements. (KEYSTONE/SCIENCE PHOTO LIBRARY/KENNETH LIBBRECHT/SCIENCE PHOTO LIBRARY)