Avalanche Center Home Wind, Snow, and Avalanches - Part 3

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Wind Deposited Snow - Cornices and Windslabs

Part 1 - The "Fetch" and the "Deposition" - How and where wind picks up and deposits snow
Part 2 - Wind Transport of Snow - The Three Mechanisms of movement

We all learn in the most basic of avalanche classes, as well as almost any book, that wind is important in moving snow from windward slopes onto leeward slopes. This is why pay attention to cornices and other indicators of slopes where snow is deposited. In this article we look in more detail at how the wind deposits snow into windslabs and cornices.

After picking up snow in the "fetch" area where the wind flow is laminar the snow is deposited behind breaks such as ridgelines, which break up the flow. Behind these barriers the flow becomes turbulent with no steady speed or direction, forms eddies, and loses its capacity to carry snow. We most commonly think of ridgetops like the top diagram, but crossloading can also occur when the wind sweeps across gullies or couloirs like the bottom diagram. Similar principles can also be used in a beneficial manner when snowfences, jet roofs, and vegetation breaks are designed for the management of snow deposition.

Wind deposited snow is packed into a higher density and is deeper than snow in other areas, causing windslabs in the deposition areas. Along a ridgeline these are often assumed to be high on the slope just under the crest where a cornice may have formed, but sometimes the slabs can form lower on the slope if the wind is strong. Because of this, snowpits dug and analyzed in the area at the top of a slope are often not representative of other parts of the slope. It may be necessary to drop as far as 100m (~100 yds) below the ridgeline to get a representitive snowpit in some cases. Digging too close to the ridge is a common reason for misleading snowpit conclusions. (Of course, dropping too far off the ridge usually puts one at risk. This creates a dilemma - how much risk does one take in the course of assessing the danger?.)

Where the wind actually flows over the crest of a barrier such as a ridgeline a cornice often forms. Beyond the cornice the turbulent flow deposits snow, so cornices usually indicate windloaded slopes below. Cornices can break off, often further back than expected, and a cornice falling onto a windloaded slope below sometimes constitutes a slope stability test on a par with the use of explosives. Hopefully the falling cornice doesn't include a careless or unlucky climber, skier or snowmobiler! Traveling below cornices is usually dangerous as well, both because of the wind loading on those slopes and the possibility of the cornice falling. But there are exceptions to everything, and in the photo on the left the conditions were not conducive to the cornice failing and the slope is short and low angle with some visible anchoring.

So why is wind deposited snow more dense? And how do cornices grow "sideways"? One reason for the higher density is that the wind breaks up the snow particles into smaller pieces which can pack together closer and tighter. But this doesn't explain cornices. There is another interesting effect going on here in the formation of cornices that also plays a role in windslab formation. Members can learn more about this by logging in and reading the cornice sidebar.

A knowledge of how the wind effects snow deposition is also important for the design and location of various structures. Snow fences were mentioned above - with proper design a surprising amount of control can be exercised over where the snow is and is not deposited. Another application is the design and location of buildings and building features. The orientation and shape of a roof can play important roles in the snow load, both on the roof itself and on other parts of the structure. The location of doors and windows may also benefit from understanding snow deposition. The photo below shows an alpine hut in the Alps with a poorly located entry to the winter room. Considering that this room is intended for winter use, including potential emergency use, it is unfortunate that the entry is deeply buried. It could have been better located based on an understanding of wind, or some type of deflection structure could be installed to keep an area around the entry clear of drifted snow.

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