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Glossary Index

Avalanche Institute

 

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Crystal

Also: Ice Crystal, Snow Crystal, Crystal Habit, Column Crystal, Needle Crystal, Plate Crystal, Dendrite Crystal, Stellar

A crystal is any substance, usually a solid, whose atoms or molecules are arranged in an orderly array.

Ice Crystal , Snow Crystal

Any segment of ice in which the molecules are arranged in the same orderly array. Snow particle growth in the atmosphere begins in the form of small ice crystals. If the ice crystal continues to grow by sublimation a snow crystal is formed. This is a larger particle, often with an intricate shape, which is visible to the naked eye.

Ice crystals have three symmetric axes in the basal plane (the a-axes) and one axis perpendicular to that basal plane (the c-axes). The three a-axes are separated by 120 degrees which leads to hexagonal symmetry in this plane.

Crystal Habit

"Habit" refers to the crystal shape which results from differing rates of growth in the different axis directions.

Column Crystal , Needle Crystal

An ice crystal, such as precipitated snow, in the shape of a needle or a hexagonal column with major growth along the c-axis. Some forms of surface hoar also have a needle shape.



Plate Crystal

An ice crystal, such as precipitated snow, shaped like a hexagonal plate with major growth along the a-axis.



Dendrite Crystals, Stellar Crystals, Stellars

Precipitated flat snow crystals with needle-like growth primarily along the a-axis. At high growth rates growth on the a-axes occurs more rapidly at edges and corners. The result is ore complicated forms such as dendrites. The dynamics of such growth is relatively poorly understood.



All micrographs are by the USDA and as government products made with public funds are in the public domain.

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