Corn Snow

Wet Snow:

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 minutes.

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.
Source http://www.fsavalanche.org/Encyclopedia/cornsnow.htm