People often underestimate how quickly snow changes under the influence of temperature and heating from the sun, and how different the conditions are up high compared to down low. You do not want to be on avalanche slopes when the snow is wet or slushy, and you want to stay off slopes where the sun is warming up cornices or steep slopes above.
Watch the clock—and the sun
Cold, dry snow and especially refrozen wet/slushy snow is much stronger, so avalanche danger is generally lower in the morning after a good freeze. Once the sun is up, conditions deteriorate—sometimes bringing the danger rating to high in minutes, certainly in hours. On sunny spring days, warming and avalanche activity follow a fairly predictable cycle. The first slopes to become unstable early in the morning are steep and southeast-facing, first in the alpine where the sun hits earlier and stays longer, then at treeline, and finally below treeline. This pattern then repeats on south aspects by mid-morning, then southwest in early afternoon, and finally west aspects in the late afternoon. As the sun leaves the slopes and temperatures cool off, improving conditions follow the same order as the snow begins to freeze up. When temperatures stay warm overnight and there’s no decent freeze, avalanche danger can remain elevated all night and still be at high in the morning, and if nothing else, high danger rises more quickly and arrives sooner in the day.
A detailed look
The slope pictured here is above treeline and faces southeast. The steep, 40-degree-incline upper slopes are unsupported (above cliff bands and rock outcrops) and hang above 30-degree lower slopes. Numerous cliffs and rock outcrops (I’ve circled some less obvious ones in red) collect and concentrate solar radiation, quickly creating localized weak snow. Cornices on the ridge high above (orange circle) get lots of sun for most of the day. The runout zone contains a significant gully, forming a classic terrain trap (yellow circle) where even small avalanches from above pile up very deeply.
My worry about this slope is that as soon as the sun hits the mountaintop at sunrise, there’s potential for cornice fall or a small slide from the steep, unsupported slopes above that could pile up deeply in the gully terrain trap in the runout zone (circled in yellow). A cornice or small slide hitting slopes below could trigger a larger, deeper avalanche, as occurred in this photo.
If the slope has a thick crust (strong enough to support a sled) from wet snow that froze up overnight and it’s a cool, cloudy day, I’d start early in the morning, keep other riders far away from the runout zone, and ensure only one person at a time goes on the slope. I’d keep a very close watch on what’s happening above, planning to get out ASAP if the sun comes out. On a clear day, my spring strategy would be to avoid the place completely, even if temperatures down below were cool and the snow was dry or frozen hard.
I’d consider heading for aspects where the sun hits later (with a close eye on solar radiation and time of day to assess when to move out) and/or go to more shaded aspects like north, northeast or northwest, assuming there are no other major problems there.
Karl Klassen is the public avalanche warning service manager at the Canadian Avalanche Centre.
