Density,
Snow:
The stability of the snowpack is influenced by many factors,
but two of the most important is the strength of the weak layer
and the load it has to support.  The
weight of the snow resting on a weak layer is a factor of the
depth of the slab and its density. Snow density can be thought
of in technical terms and numbers (% density, kg/m3) but most
people have an intuitive feel for snow density, even if you
don’t realize it. Snow that is light and billows up in
your face while you’re riding is very low density, while
high density snow feels thick, heavy, or even wet.
New snowfall has an initial density, usually in the 3-20% range.
Once it accumulates on the old snow surface, metamorphism takes
over causing the snow to gradually become more dense. The rate
at which new snow becomes densifies depends on temperature,
among other things. We all know warm weather can quickly ruin
light powder snow, while cold temperatures can slow down the
densification process and preserve powder for quite some time.
Another important factor to consider regarding snow density
is trends during a storm. If the temperature is warm when the
snow starts falling, and then becomes colder, we have what we
call a “right side up” storm. The snow is light
and fluffy on top and becomes more dense with depth. A far less
desirable scenario is called an “upside down” storm
and is the result of increasing temperatures during snowfall.
The result is heavy, denser snow on top of lighter snow -- you
can see what we’re getting at can’t you? An upside
down storm can result in a slab (dense snow) over a weak layer
(less dense snow), providing the necessary ingredients for slab
avalanches.
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