Tom Ross: Surface hoar sings a siren’s song
February 2, 2010
Just when I thought I'd skied every kind of snow condition known to "Jackrabbit" Johannsen, I encountered something new Saturday. I'm calling it "whispering surface hoar."
I was fortunate to get patched into a group of nine hybrid snowmobile skiers for an outing up Buffalo Pass during the weekend. Snowmobiles aren't exactly my deal — they thrill me and scare me a little bit all at the same time. However, when some old friends and interesting new acquaintances beckoned to me to come slice up some of the best powder we've had in almost four weeks, I signed up. The skiing Saturday was almost as good as the company I kept.
What we encountered at about 10,000 feet on the lower flanks of Soda Mountain wasn't just fresh snow — it was beautiful skies and remarkably calm air.
What I could not have predicted was that we'd be skiing on a fluffy blanket of sparkling frost crystals that made an audible swooshing sound as we danced between the fir trees.
It was a sound I'd never heard before — somewhere between a dry sizzle and a freshly poured glass of bubbly Prosecco.
The skiing conditions Saturday were actually a complicated mix of four kinds of snow. On treeless hilltops, the snow was windblown. Dropping down the north side of a gentle hill, we began to float through 6 or 7 inches of perfect powder.
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Everyone in our party but myself was either riding a snowboard or skiing on exceptionally wide powder boards. My skis occasionally were sinking into a layer of condensing snow that had fallen a week earlier, and I had to stay light on my feet. Twice, I allowed the deeper layer to snag my left ski and I took a tumble.
It was only after we dropped down the hill to where the snow was barely kissed by the winter sun that we encountered fields of large surface hoar that whispered indiscreetly to us with every turn.
Surface hoar is the most common form of hoarfrost, according to scientists at the California Institute of Technology. Hoarfrost forms whenever it's cold outside and there is a source of relatively warm water nearby.
During a late November 2009 cold snap, when the Yampa River had yet to freeze, it frosted shrubs and trees at Rotary Park with a thick coating of crystals.
Surface hoar typically forms overnight on snow banks. The scientists at Cal Tech say it results when snow banks warm up during the day and then cool at night. Water begins to evaporate from the inside of the snow bank and re-crystallizes on the surface.
Often, hoarfrost dissipates again with the morning sun. But not always. And that can create dangerous conditions for backcountry skiers.
It turns out that the intriguing sound created when our skis cut through the surface hoar on Buffalo Pass was something akin to a siren's song.
Retired avalanche forecaster Art Judson, of Steamboat Springs, reminded me Monday that layers of surface hoar often contribute to instability in the snowpack.
"That layer is now likely preserved and waiting to serve as another potential sliding layer after it is buried by cohesive snow more than a foot thick," Judson said.
I'll never forget the enchanting sound that surface hoar made as we skied it. But I'll never give in to it, and I'll always take Judson's admonishment to heart.
— To reach Tom Ross, call 871-4205 or e-mail firstname.lastname@example.org
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