Sun Crusts:
A frozen sun crust sometimes forms a hard bed surface for future
avalanches to run upon. However, a sun crust typically has a
fairly rough texture when compared to the more slippery rain
crust, and snow deposited on the crust tends to bond better
than you would imagine. However, a sun crust forms a strong
mechanical discontinuity in the snowpack, which tends to concentrate
any shear deformation within the snowpack to the layers just
above and below the sun crust. But probably more important,
crusts tend to concentrate temperature gradients above and below
them and they can grow a thin and almost invisible layer of
weak faceted snow. This probably accounts for most of the avalanches
that occur on sun crusts, especially in non-maritime climates.
Also, surface hoar that grows on top of a sun crust can be a
very nasty weak layer.
Sun crusts, of course, form only on sunny slopes and not at
all on the shady ones. So we find them mostly on southeast,
south, southwest and west facing slopes at mid latitudes in
the Northern Hemisphere (and conversely forms more uniformly
on all aspects in tropical and arctic latitudes). Instabilities
associated with sun crusts usually stabilize fairly quickly
after a storm, depending on temperature, except if faceted snow
has grown around the sun crust or surface hoar has grown on
top of the crust, it can produce much more persistent avalanche
activity.
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Hot
Tip:
When new snow falls on a sun crust, it's important to check
out whether the sun crust is wet or frozen when the snow starts.
If it's wet, the new snow will stick to it and you most likely
won't have any immediate avalanche problem, but if the crust
is frozen, then the new snow does not tend to bond very well.
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Sun Crust Summary:
Formed:
By strong sun on the snow surface.
Looks
like:
Shiny with slightly rough surface.
Distribution
pattern:
Forms only on sunny aspects, none on shady aspects - moderately
elevation dependent.
Persistence:
Instabilities usually last for 1-3 days, depending on temperature.
Forecasting considerations:
Is it wet or frozen when the next storm comes in? If it's wet,
it will bond well, if it's frozen it will bond more poorly.
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