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A weak layer is used to describe a layer of snow within the snowpack that may be prone to causing a snow avalanche.

Weak layers are caused by changes in snow pack's temperature, snow crystal type and snow crystal deformation resulting from changes in atmospheric conditions and moisture content.
We dug a snow pit on Pucker Face and found a really weak layer that is sure to rip a avalanche on the next bone head who tries to ski it.
by The Snow Prophet May 12, 2010