151 | Constructive Metamorphism

If you're wondering why I've avoided the backcountry recently, you can blame constructive metamorphism, something so elaborately detailed by Peter Marchand, professor of winter ecology (click here for a more concise explanation of this phenomenon as provided by the University of Alaska).

Taken from the UAF link above, as the winter season progresses and snow accumulates on the ground, the snow itself becomes an important insulating layer. The blanket of snow helps retain the latent heat of the ground. Temperatures at the ground's surface tend to hover within several degrees of freezing. Conditions can be significantly warmer on the bottom of the snow pack than at the snow's surface, where it is exposed to the chilly arctic winter air. If the temperature gradient is at 0.18° F/cm, then the process of constructive metamorphism (where new crystals are actually formed) dominates.

The water vapor present near the warmer ground surface is under higher pressure than the cool water vapor at the top of the snow pack. Natural dispersion of energy causes water vapor to move from warmer to cooler, and from higher vapor pressure to lower. The greater the temperature gradient from bottom to top, the more quickly the vapor transfers. As the water vapor contacts cooler temperatures it crystallizes directly from vapor to frozen solid ice crystals in a process called sublimation. In a snow pack with a significant temperature gradient, large six-sided, cup shaped depth hoar crystals form a loosely packed layer at the bottom. This "sugar snow" can often be the weak and unstable layer that causes avalanche hazards.



Now that we all understand constructive metamorphism, let's examine a real life scenario by taking a closer look at the Berthoud Pass SNOTEL site. From December 3rd to the 13th, zero precipitation fell while the combined average temperature over that time was -0.4° F. For nine of those eleven days, the average daily temperature was less than 14° F or well within the required temperature gradient of 18° F per meter of snow pack that causes depth hoar to form.

I was hoping to ski Cameron Pass last weekend until I read a recount from December 10th in which 30cm of fresh snow was found on top of 40cm of sugar. Similar sketchy conditions were reported at Berthoud Pass throughout this past week. Everything I read hinted towards imminent disaster which ultimately occurred yesterday when several slides occurred near heavily-traveled terrain - fortunately, no one was injured. If we digest further, 13" of snow had fallen at Berthoud Pass between Monday and Friday - that's 13" of dense snow forming a slab on top of what is effectively 20" of air. Add extra force on top of that such as a skier, and you have the perfect recipe for avalanches to occur.