I skied powder for the first time in the 1980s, at Jackson Hole, Wyo., and it was quite an experience.

New England rarely sees powder. If it falls, it doesn't last long enough for most of us to develop skill at cruising through it.

One day on that trip to Jackson Hole the temperature climbed to the 50s, and the snow warmed into peanut butter; by mid-afternoon the east-facing slopes were in shadow and the snow surface had started to set up into the crunchy snow New Englanders consider normal.

I skied around a corner and saw below me a slope of moguls, each with a Western skier frozen atop it.

For the first (and only) time on that trip the advantage was mine. I skied down the hill like a pro, setting edges, carving turns, and hearing a chorus of "How does he do that?" follow me down the trail.

Point of this story isn't that I'm a great skier; I'm not. It's that snow conditions change constantly.

The Northeast has seen lots of lovely snow this winter, and very little rain, but one rain event in early December left quite a thick crust atop a couple of feet of light snow in the woods.

Over the next few weeks that crust just disappeared, a phenomenon I had noticed in past years.

What's going on? I wondered.

Avalanche Scientist Karl Birkeland with the U.S. Forest Service National Avalanche Center in Bozeman, Mont., explained.

"What's happening is a grain-to-grain transfer of water molecules," Birkeland told OnTheSnow. "When there's a temperature gradient, or difference, water molecules go off one snow crystal, go into the vapor phase, migrate, hit the next crystal above or below them, and deposit onto that crystal.

"The ice crust is providing water molecules, which move onto an adjacent crystal above or below them, eroding both sides of that ice crust," Birkeland said.

The migration goes one way at night, when the air is colder than the snow, and the other way during the day, when the air is warmer than the snow, he said.

The water molecules move from warm to cold, so come off the bottom of the ice crust during the day, and off the top of the ice crust at night, he said.

The process happens faster when daytime temperatures approach 32, and temperatures at night are in the single numbers, he said. The alternation of warm days, cold nights, warm, cold, warm, cold, over a period of time drives the changes in the snowpack, resulting in the thinning and eventual virtual disappearance of the ice crust, he said.

The question of ice crust evolution is not simple, Birkeland said, in part because of the complex nature of water.

"Water is the only material on earth that exists in nature in all three of its phases: solid ice, liquid water, and gaseous water vapor," he said. "All materials have these three phases, but water is the only one that occurs naturally in all three."

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