Backcountry Essentials: Wind Loading
Analyzing wind direction, speed, and duration
Sean Zimmerman-Wall is a full-time ski patrolman at Snowbird, an avalanche educator, and an Andean mountain guide. Check in on Tuesdays for resources and education that will help you have a safe and good season exploring terrain beyond the boundary line.
With a storm comes wind. And with wind come complications for backcountry skiers. Windslab requires that we exercise extreme caution. Wind direction, speed, and duration are important in determining where windslab will be found and how dangerous the resulting avalanche conditions will be.
Wind direction: The prevailing wind direction in the Western U.S. and Canada is from the west. Sure wind will blow from other directions, but the western wind is a component in all storms tracking through the continent. Pay attention to which way the wind blows and you can determine where the windslab problem will be most pronounced. Often it is east-facing terrain that requires the most attention. Top-loading over a ridgeline or cross-loading across a terrain feature add further complications in dealing with windslab.
Wind speed: The intensity of the wind will vary depending on elevation and terrain features. During frontal passage the mountains usually experience the highest wind speeds. Standing on a ridgeline during a storm is like facing a hurricane from the beach. Speeds of 100mph are not uncommon in the Sierra Nevada. The optimal speed for transporting snow is around 25-40mph. Snow can be moved up to 10 times faster than it can fall at these speeds and the development of thick drifts and windslabs is accelerated.
Wind Duration: Paying attention to how long the winds were sustained is perhaps the most important factor in analyzing the hazard that windslabs pose. Longer duration wind events can completely strip the windward slopes and load up the lees with windslabs several feet thick. Any wind sustained for more than 12 hours should be considered a major player in building avalanche hazard. It is also important to note if the winds spiked during a storm cycle because windslabs can be buried by new snow and become even trickier to spot.
You’re the most likely to find windslab along ridgelines and pronounced terrain features. However, large openings in a forest or meadow can also harbor wind-affected snow. The most dangerous combo is when you find windslab over faceted weak snow. The telltale hollow-drum sound of windslab is a sure sign you are in the danger zone. Approach these slabs with caution and know that if they do break, it will often be once you are in the middle of the slab, when you are powerless and exposed.
|Last week’s backcountry tip: Be Your Own Forecaster →|
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