From Glossary of Meteorology
One of the many forms in which ice crystals are found in the atmosphere.
This particular crystal habit of ice is characterized by hexagonal cross section in a plane perpendicular to the long direction (principal axis, optic axis, or c axis) of the columns. It differs from that found in hexagonal platelets only in that environmental conditions have favored growth along the principal axis rather than perpendicular to that axis. Growth by vapor deposition at temperatures of from -3° to about -8°C and also at lower temperature below -25°C leads to growth of columnar crystals, though other crystal features (needles, scrolls) also appear in this temperature interval, depending on the degree of water vapor supersaturation and the crystal fall velocity.
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