Snowflake patterns. 19th-century illustration of different patterns of snowflakes observed under a microscope, ranging from dendritic to plate snowflakes. 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, but due to slight differences in the conditions in which they form, no two snowflakes are identical. The arms of a snowflake form due to instabilities that cause branching growth from a snowflake's corners. This illustration is from 'Physique Populaire' (Emile Desbeaux, 1891).