The Avalanche Review, Vol. 1, No. 3, December 1982
Copyright All Rights Reserved; AAAP


by Rand Decker

Mountain weather has some unique aspects. One of those features is the phenomena of blowing snow. That is: the erosion, suspension, transport and eventual "wind-aided" redeposition of snow by the wind.

The source of snow for suspension and transport is two-fold; previously deposited snow and atmospheric precipitation. Both sources can be active singly or in combination. Where the snow is eroded from and redeposited to depends essentially on the "shape" or mapping of the wind very near the ground over and around the mountains.

The radical changes in the shape of the windflow in the lee of a mountain ridge and the attendent loss of snow load carrying capacity results in the wind-aided deposition of an asymmetric "wedge" of snow on the lee slope. This wedge of snow is in excess of that which would be deposited on a flat area. The consequence of this is that this wedge of snow does not have a "toe," i.e. this wedge of new snow is bonded to the old snowpack essentially by its bed surface alone. Furthermore, snow particles which have been in blowing snow transport tend to be fine-grained and when they are deposited on a lee slope the final depositional density is generally higher than the surrounding snow density. This tends to give the wedge of wind-aided deposition a cohesiveness relative to the surrounding slope. So now you have a top-heavy wedge of cohesive snow resting on its bed surface alone, a prime candidate for avalanching.

It should be emphasized that wind-aided deposition and the formation of wedge-shaped wind-slabs does not have to take place at the summit crest of a mountain system. Any place where the windflow has an opportunity to entrain snow and then is confronted with a divergent slope angle change, there is an opportunity to wind-load that slope. Hence, a windflow blowing transverse to a mountain will load the sides of gullies and couloirs with
wind-aid deposition. Even a windflow blowing straight down a mountain will load a
slope if it encounters a dramatic slope break.

 depth vs distance(15183 bytes)

In mountainous areas that are heavily used, there are often observations and instrumentation which clue winter alpinists and professionals that blowing snow transport and wind loading of lee slopes is active. But how can a back-country traveler determine if windaided deposition has been active prior to his arrival? One clue is to measure the shape of the newest snow deposition on a lee slope. If the newest snow deposition has the asymmetric wedge-shape, with its maximum 0 to 20 meters down the lee slope and gradually decreases to the average new snow depth in 60-100 meters down the lee slope, then avalanche potential due to wind-slab on lee slopes certainly may exist. A warning goes with the test: you are measuring for the presence of the unstable wedge-shaped wind-slab, perform the measurement on a small, indicative test slope. Getting caught and killed in the avalanche whose possible existence you were trying to determine is not a successful test.

    Sue Ferguson photos

    Bitsy (72556 bytes) Sophia (61227 bytes)