snow crystal[′snō ‚krist·əl]
a crystal of ice that falls from the clouds and mists in which it formed. Snow crystals occur in two basic forms: tabular and prismatic. Tabular crystals include hexagonal plates and stellar crystals with three, six, or 12 rays. Prismatic crystals include hexahedral columns and needles, as well as clusters of columns called spatial dendrites. Snow crystals often have irregular shapes. The dimensions and shapes of snow crystals vary greatly, depending on the physical conditions of formation and growth (most importantly, the air temperature and humidity). At higher temperatures, the dimensions of the crystals and the proportion of tabular shapes increase. Snow crystals most often have the form of stars and spatial dendrites. Stars with acicular rays reach 6-8 mm across; those with tabular rays are 4–5 mm across. Spatial dendrites are 2-3 mm across; plates, 1–2 mm across; columns, 1–2 mm long; and needles, 3-5 mm long. Large snow crystals falling from clouds are called snowflakes; they most commonly take the shape of stars.
Snow crystals result from the sublimation of water vapor on frozen drops of clouds and mists or on crystalline dust particles. Small cloud crystals have the simple shapes of plates or columns. Shapes usually become more complex when the crystals reach dimensions of 0.1–0.2 mm. Strong winds break snow crystals down to dimensions of 0.1 mm or less. Graupel forms when snow crystals fall through a cloud containing supercooled drops of water. [23–1885–]