Melt/FreezePosted: January 25, 2009
A few days ago I was asked a question about how the recent warm weather affected the snowpack at Lake Louise.
Warm days (+0C) and cold nights cause a melt/freeze cycle which forms a crust at the top of the snowpack. How strong and thick the crust is depends on daily maximum and minimum temperatures and the uninterrupted amount of time the air temperature stays below 0C. The longer and colder it is, the stronger the crust becomes. If it stays above freezing for too long, the snowpack becomes isothermal, which means it holds the same temperature from top to bottom and becomes completely unsupportive. There have been times when only the runs that have been groomed often (and the snow has therefore been packed and packed) will support the weight of a skier, even in flat areas. If one was to venture even a metre off the groomed trail, they’d find themselves sinking right through to the ground.
During avalanche control in long spells of warm weather, gently lobbing a snowball onto the top of a slope can be enough to get the whole thing to slide, which tends to happen in places that spend all day baking in the sun, like the Ptarmigan Chutes or above the Wounded Knee cat track on Saddleback. The photo below, from May 10, 2007 shows the latter, with lots of natural snowballing and even a small point release avalanche towards the right. The shaded area is the slope that usually avalanches in the spring.
Controlling Wounded Knee is actually a somewhat complicated affair, since it is usally performed with explosives sometime in the afternoon, when the day is at its hottest , and skiers are all over and can come from any direction. A snow cat is also on scene to clear the cat track of avalanche debris, and we can’t have a cat operating in an open area. To prepare, the gate into Saddleback is closed and manned, patrollers are stationed around the area to stop skiers approaching from other directions, and the control team does its stuff.
The short video below shows the use of ANFO at Wounded Knee a few years ago. As you can see, no skier could stand a chance getting hit by that avalanche of heavy wet snow. The sign we forgot to move stayed buried in snow until long after we closed for the season:
Melt/freeze cycles happen mostly in spring, but we had one last week when there were a few days in a row when the maximum temperature went above freezing for most of the day. As we get into April and May, they’re quite common. The temperature usually stays cold enough for a crust to bridge the weak snow underneath. That also explains the spring avalanche closures you’ll start seeing when it gets warm – runs will close before the end of the day if the crust is no longer supportive. The most obvious spring avalanche closure is the Grizzly Gully area, when an uninterrupted fence will run from the top of Paradise chair and down the front side, all the way along Home Run and down the right side of Wrong Turn, well below the top of the old Olympic chair. This closure protects areas like Flight Chutes, Mirkwood, Grizzly Gully, and Kernahan’s Folly.
Since a crust is a poor bonding surface, new snow above is more likely to avalanche, especially if the the crust is smooth and undisturbed. A rougher crust, like in moguls, will do a better job of anchoring the snow. New snow on top of crust doesn’t necessarily make for good skiing, since it has to be enough to keep you off the crust below. At the Lake, we can sometimes count on the wind to blow new snow around and pack on top of the crust, making a firmer base and improving skiing conditions.
So, since the sun can cause so much warming in the snowpack, it stands to reason that those slopes that spend most of their time in shade stay cooler, and can offer winter-like conditions when the south-facing aspects are melting down. Places like Whitehorn II and ER 7 are classic examples of great winter skiing well into spring.