There's dry snow and there's wet snow. Or at least that's how
most professionals think about the distinction. Although there's
really a continuum between dry and wet snow, it's only natural
to draw a hard line between them because they are such different
beasts altogether. Wet snow that has gone through repeated melt-freeze
cycles is often called Corn Snow. Under Corn Snow or Melt-Freeze
conditions, a crust forms on the surface that will support your
weight when frozen, but turns to deep slush during the heat
of the day.
Wet snow avalanches are caused by a completely different process
than dry snow avalanches. Although it’s a little overly
simplistic, dry avalanches are caused by overloading the strength
of buried weak layers while wet avalanches are caused by decreasing
the strength of buried weak layers.
To understand the difference between dry snow and wet snow,
imagine a bunch of grapes. In this analogy, the grapes are the
snow grains and the grape vines are the crystalline bonds between
them. Now, imagine that when you wash the grapes, the grape
vines dissolve, leaving you with nothing but free-floating grapes.
In the snowpack, when water percolates through the snowpack
it dissolves the bonds between crystals—the more saturated
the snow, the more it dissolves the bonds, thus, dramatically
decreasing the strength of the snow.
So, why doesn’t all wet snow instantly avalanche? Part
of the reason comes from the bonding power of water itself.
In the Lilliputian world of snow crystals, a tiny bead of water
usually clings between the grains, which act like a glue because
of the "surface tension" of water. Surface tension
means that water tends to cling to itself, which is why rain
comes down as discrete drops instead of falling as a fog. The
surface tension of water is actually a fairly powerful glue
that holds wet snow together.
But when the snow becomes saturated, all the surface tension
between the grains instantly disappears because we’ve
flooded the caverns with water—turning a snow cone into
a margarita. Not only have the bonds disappeared but millions
of tiny ice grains are now buoyant, free to slurp down the mountainside
like thousands of concrete trucks dumping their load at once.
That’s what makes wet slides especially tricky because
snow can loose its strength very quickly. Very stable snow can
turn into very unstable snow in a matter of an hour or even
Corn Snow becomes “ripe” when the bonds between
the snow grains just start to melt, providing a velvety surface
texture perfect for many types of riding. This usually occurs
in the morning hours, but the exact timing is very aspect dependent.
Seasoned corn harvesters know that predicting this timing is
an art form honed through experience. If you’re too early,
the frozen surface can rattle out your fillings. Worse is arriving
too late, after too many bonds have melted and the corn snow
has turned into deep, dangerous slush. The slope that may have
been perfect an hour ago is now prime for wet snow avalanches.