A classic photo of layered, cup-shaped faceted snow grains, courtesy http://www.avalanche.org. These types of crystals are at the core of the local snowpack right now, leading to significant avalanche hazards on many backcountry slopes.
Still mild, maybe a dusting tonight, with the risk of backcountry avalanches continuing
By Bob Berwyn
SUMMIT COUNTY — There are some thin, high-level clouds floating around out there, but not much in the way of new snow in the area, as the latest storm rolled south into Mexico. The southern San Juans may pick up a couple of inches. Don’t look for much fresh powder in Summit County until sometime next week, although some cooler air dropping down from the north could set the stage for a dusting Thursday night into Friday.
Temperatures Thursday morning ranged from a balmy 23 degrees on the mountain at Copper down to 2 below zero at Kremmling, as always the cold spot. At sunrise, it was about 23 degrees in Frisco. Highs Thursday are forecast to reach into the 30s again, and it could get warmer if the sun shines for a few hours.
Clear skies and mild temperatures are predicted for the weekend, and some of the computer models are still calling for the westerlies to break through next week and deliver some moisture to the area.
But the National Weather Service is still holding back on predictions of moisture. Here’s an excerpt from this morning’s forecast discussion explaining the broader western weather situation:
“ALL OF OUR WEATHER THROUGH THE WEEKEND IS GOING TO BE DOWNSTREAM OF AN APPRECIABLE PRECIPITATION EPISODE OVER CALIFORNIA. THIS PATTERN IS LIKELY TO LEAVE OUR STATE FAIRLY DRY
UNTIL THE NEXT PATTERN SHIFT ALLOWS MORE MOISTURE TO MOVE INTO THE GREAT BASIN. THIS MAY OCCUR AT SOME TIME NEXT WEEK…BUT WILL HAVE TO WAIT A COUPLE DAYS TO GET A BETTER SENSE OF HOW MANY DAYS THE SYSTEM WILL BE FOCUSED ON SOUTHERN CALIFORNIA.”
See the rest here.
The Colorado Avalanche Information Center is holding the avalanche hazard at moderate. Natural slides are not expected to be a big problem, but triggered avalanches are still possible on steeper, slide-prone terrain. Visit the CAIC online for the latest forecast and snowpack discussion before you head into the backcountry.
Depth hoar: A short explanation
This season, avalanche experts have pointed out that the snowpack in our area is characterized by extensive formation of faceted crystals, sometimes also called depth hoar, sugar snow or temperature-gradient snow. When these types of snow crystals form in the snowpack and sit underneath new layers of fresh snow of wind-deposited slabs, it creates the perfect recipe for dangerous avalanches.
So what is depth hoar exactly? You don’t have to go far to see it. Just step into your backyard and dig down a few inches into the snow beneath the surface crust and you’ll find it: Larger loose grains that don’t stick together at all.
The crystals form when the temperatures at ground level are warmer than the top layers of snow and the air above it. The snowflakes lower in the snowpack give up some of their moisture to the crystals above through sublimation. That means the frozen water turns into water vapor and then refreezes without ever going through a liquid stage. As the process repeats itself, it can form large crystals that are beautiful to look at, but have absolutely no cohesion to surrounding crystals.
One researcher described the process as a “form of cannibalism,” in which “neither the density nor the thickness of the snowpack changes, but a weak, unstable and invisible layer forms nonetheless.”
The process is common when the snowpack is not deep and the air temperature is cold; in other words, when there is a big contrast between the temperatures at the base of the snowpack and the surface. These conditions are common in the local mountains. It doesn’t take a whole lot snow to insulate the ground and keep temperatures at the bottom of the snowpack near 32 degrees.
One of the best explanations of depth hoar formation is online at www.avalanche.org, including a good graphic explanation of the process.
The same web site also has a great library of avalanche images online here.
Filed under: avalanches, skiing and riding, snow, Snow and weather, Summit County Colorado | Tagged: avalanches, backcountry, depth hoar, faceted grains, skiing and riding, Snow and weather, sugar snow, summit county weather | 3 Comments »