Weather is, in many ways, the hardest factor to ignore while looking for a good day of skiing. Things like bad light, bitter cold, high winds, and miserable, miserable rain (can ya tell I am writing this in March?) can turn a fun day into a day spent on the ever-fickle margins of suffering in winter. Ain’t nobody can ignore the weather when it is making you suffer. On the other hand, it is amazing to see how these same sort of weather events can get ignored when the promise of bitchin’ turns is beckoning. Things that may be kicking your butt into sufferland while you are headed to the top of a run can also be altering avalanche conditions considerably and rapidly, and should be heavily weighed at the time when planks are attached and the fall line is under your nose.

Weather, at least the way we like to talk about it in terms of snowpack and avalanches, is a function of three main variables: precipitation, winds, and temperatures.

Precipitation
Precipitation is anything that is made of water and falls from the sky. In the mountains in winter, people tend to talk (and bitch) most about the solid, crystalline form of water that falls from the sky, namely snow. When it is the liquid form, namely rain, people tend to just bitch.

Periods of snowfall are what snowriding is all about. The yin to this yan is that periods of snowfall can also be what avalanche danger is all about. A snowpack is stable, which is to say unlikely to avalanche, when all of the layers in the snowpack are stuck together well, or bonded, instead of sliding on eachother. When new snow falls out of the sky, it takes time for it to bond to the previous snow surface. Until the new snow has bonded, it will be unstable. If it is falling as part of a bigger dump, the falling snow can build up into a slab faster than it is bonding to the existing snowpack. This combo can create conditions for scary slab avalanches to happen. For most snowfalls, bonding usually transpires in a period of time lasting from hours to days. In a snowpack that has lingering weak layers, like facets or surface hoar or depth hoar, this can take weeks. In this case, bonding is sketchy and slow, so that even a small amount of new snowfall can put extra load on an existing slab and make it unstable.

The bottom line is that, during and just after a dump, new snowfall will increase avalanche danger. Big dumps will increase danger rapidly while it is snowing, and are more likely to remain an unstable slab after a dump. So if it is snowing hard, be thinking about choosing safer lines and avoiding big terrain over your head. If you wake up to a big fresh dump, it is wise to give it a few days to bond (and to let you watch for avalanche activity) before charging into the backcountry, especially on bigger, steeper lines.

Snow’s evil, warmer cousin, rain, is pretty much always bad news if you are out riding. Not only will it make for lousy turns, but rain also adds a lot of heat and load (liquid water is denser than snow!) to the snowpack in a hurry, which makes stability deteriorate rapidly. If it starts raining, pick a very safe route down and out and leave right away, because it is going to get ugly in a hurry!

Winds
Winds go with mountains just as much as rock and snow. They form part of the reality of backcountry travel, ditto avalanche conditions. At speeds greater than around 25km/hr, winds pick up snow from slopes facing into the wind (windward) and drop it onto sheltered (leedward) slopes nearby. The faster the wind, the greater this rate of wind transport. In fact, during a snowfall, winds commonly deposit snow on leeward slopes at three to five times the rate it is snowing. In times of little or no snowfall, winds can still pull this off. Just ask anyone getting pow turns at a ski hill in the Rockies when it has not snowed for a couple of months!

The downside of all this wind action is how it often presses the snow it is moving around into a windslab in leeward areas. By the way, whenever you hear the word “slab”, it should be making you think “avalanche”. Windslabs are often harder, deeper, and slower to stabilise than storm snow slabs, which makes recent or ongoing winds something to watch for in the backcountry!

The effects of wind, of the kind that may be building slabs, can be seen in many ways. The most obvious are flapping flags, swaying trees, blowing snow, spindrift, or plumes jetting from ridgetops. All of that airborne snow has to be coming back down somewhere, and building slabs. Cornices and lip features are formed by winds, on the top or sides of leeweard slopes. If cornices are big, or noticeably bigger than the days before, the winds have been moving snow onto the slopes below. Riming, which is when snow gets plastered to stuff like rocks or lift towers or trees, is caused by recent high winds. Snow that has surface patterns and then is variably crusty, soft, or punchy, or grabby, or just feels funny has probably been smacked by the wind. Let me be a little more clear on the word “funny” here.. funny is when the spots that look soft suddenly stop you in your tracks, because they are unexpectedly deep and heavy and firm. Funny is when you are tromping along, be it bootpacking or skinning or snowshoeing, and some steps sink deep, then the next stay on the surface, then punch in again randomly in a few more steps. If tracks are filled in and it has not snowed much, wind transport is going on.

Temperature
Temperature has incredible effects on the mountain snowpack. These effects are seen in two distinct time frames.

First, long-term temperature trends govern the evolution of the fallen snow crystals that make up the snowpack, over periods from days to weeks. Without getting overly snow geeky here, it suffices to say that long-term cold trends will weaken the snowpack, especially in areas that are thinner (such as over rock outcrops or wind-scoured slopes, or like most of the Rockies). Long-term warm trends will strengthen a snowpack, but only to the point where the whole snowpack has warmed to zero degrees celsius. Then it becomes like trying to hold a slurpee in your hands without a cup; a pile of mush that wants to run downhill. Not good to be there!

Of equal consideration are short-term temperature trends, which have different effects. Again, I will try to go easy on the geek. A rapid rise in temperatures or sun pounding on a slope can quickly reduce the stiffness of the surface snow slab, so that it gets easier for the stress applied by a skier to get deeper into the snowpack, and thus trigger a weak layer. Think of standing on a board suspended over a gap filled with hot coals, then standing on a trampoline over the same gap. The board is stiff, so only bends a little and keeps your feet comfy; the tramp is way less stiff, and so has a way better chance of roasting yer little piggies! Warming may reduce the slab stiffness enough for things to pop naturally, no trigger required! Rapid warming is easy to feel in yer bones, and may also be seen when the sun comes out and feels hot, tree bombs fall off trees, the snow surface gets soft and moist, snowballs start rolling down slopes, or point release avalanches start (they make triangle patterns). A rapid cooling will have the opposite effect, stiffening the slab and making it harder to trigger in the short term.

So, before a trip and while you are out seeking turns, it pays to keep an eye on the weather. It not only affects the quality of skiing, but also the chances of getting in avalanche trouble. Weather warning signs should never be ignored, and must always play an important role in all of your backcountry go/no go decisions!

-TC



Previous articles by Tom Chalmers
Avalanche Bulletins
Tree Wells
Terrain Management
Testing Avalanche Beacons
Season's Greetings Biglines!

An Interview with Tom Chalmers
Backcountry Evac
Septic to Epic
Remembering a Legend: Craig Kelly
Addicted to Porn