Snow patch

A snow patch, is a geomorphological pattern of snow and firn accumulation which lies on the surface for a longer time than other seasonal snow cover. Snow patches are known by a wide range of synonymous terms including snowpatches, snow beds, snow banks, and ice patches. Snowpatches are categorised by their longevity. Seasonal snowpatches melt prior to the return of the following winter's snowpack, semi-perennial snowpatches last occasionally to the following snowpack, while perennial snowpatches habitually last until the next winter's snowpack.

Snow patches often start in sheltered places where both thermal and orographical conditions are favourable for the conservation of snow such as small existing depressions, gullies or other concave patterns. The main process that creates these accumulations is called nivation. It is a complex of processes that includes freeze–thaw action (weathering by the alternate freezing and melting of ice), mass movement (the downhill movement of substances under gravity), and erosion by meltwater which is the main agent of the surroundings' influence.

A seasonal snowpatch on the south east side of Mount Kosciuszko, Australia.

There is high soil moisture around the snow patch that supports growing of specific vegetation. Snow patch vegetation is very distinctive. It is usually dominated by species that tolerate a shortened growing season and is predominantly herbaceous. With increasing duration of snow persistence, non – vascular plants predominated over vascular plants for example Salicetum herbaceae, Salix herbacea etc.

At times water can be seen flowing downslope from the margin of snowpatches. The origin of this water may be from the melting of the snowpatch itself, from groundwater reaching the surface in slopes next to the snowpatch or from groundwater being forced to surface by obstructing permafrost. In areas with permafrost the active layer may be lacking under snowpatches, with the permafrost extending all the way to the firn at the base of the snowpatch.