When
the fracture of a lower snow layer causes an upper layer to
fall. Also called a whumpf, this is an obvious sign of instability.
Collapse:
Collapsing snow (sometimes mistakenly
called "settlement") is when the snowpack collapses
under you with a loud "whoomph." (Actually, whoomph
has been adopted as a technical term to describe collapsing
snow. Sounds funny but it's a great term.) Whoomphing is the
sound of Mother Nature screaming in your ear that the snowpack
is unstable and if you got a similar collapse on a slope that
was steep enough to slide it wouldn't hesitate to do so. Collapsing
snow occurs when your weight is enough to break the camel's
back and catastrophically collapse a buried weak layer, most
commonly faceted snow or surface hoar. Collapsing snow on a
flat valley bottom can easily trigger avalanches on steeper
slopes above and sometimes collapses can propagate very long
distances and trigger avalanches on more distant steep slopes.
Not surprisingly, collapsing snow means that the snow is extremely
unstable. The weak layer is already holding up the weight of
a significant amount of snow and just the wimpy addition of
your weight can collapse all the snow in sometimes a very large
area and can sometimes propagate long distances. Collapsing
snow is an obvious clue (this is repetitive with same phrase
used above) that you need to stay off of and out from underneath
avalanche terrain.
