"Fetch" and "Deposition": How and where wind picks up and deposits snow
We all learn in the most basic of avalanche classes, as well and almost any book, that wind is important in moving snow from windward slopes onto leeward slopes. This is why we pay attention to cornices and other indicators of slopes where snow is deposited. In this article we look in more detail at how and why the wind picks up snow on some slopes and deposits it on others.
Snow is picked up by the wind in areas where it flows along the surface with a velocity that is zero the surface and increases with height. This is a type of laminar flow, and the difference in wind speed with height causes a shearing action. Along the surface this shear action (or "wind drag") picks up snow, or other particles and objects. [ If you are an avalanche center member you can log in and learn more about laminar flow, the boundary layer, and wind shear on the laminar flow sidebar. ]
When the wind goes over a surface with rapid changes in shape it loses this laminar characteristic and becomes turbulent, and it no longer has a steady direction or velocity. The flow generally slows and becomes disorganized with eddies. This reduces its carrying capacity, and snow is deposited in these areas. [One way to characterize flows as laminar or turbulent is with a special dimensionless number called the Reynolds Number (Re). This is described, along with the Froud Number (Fr) on the dimensionless numbers sidebar.]
The change from laminar flow to turbulent flow is important in airfoil design, although the change in shape (curvature) is not very abrupt. The length of the transition section varies with the roughness of the surface, which is also one of the factors in how much snow is picked up on a mountain slope. (For an example with a more striking difference between laminar and turbulent flow see the glossary entry.)
To the left we see a similar diagram for a mountain ridgeline. The windward side is labeled"high shear" - this is where the wind flow is laminar and snow is picked up. The lee slope is labeled"low shear," and here the flow will be turbulent and snow will be deposited.
This is a photo of an actual mountain ridgeline (on Austria's Wildspitze) with an abrupt change of shape. The prevailing winds are from left to right, and snow is picked up from the surface on the left and deposited on (or dropped off of) the right side. The cornices are evidence of this, although after resizing the photo for this page the cornices are not as obvious. There will be more on cornices in Part 3.
There are many other factors involved, of course. The amount of snow available for transport will depend on the surface roughness and how soft or hard the surface is. A hard icy surface will not provide much, if any, snow to be moved. A hardpacked surface will require more shear for snow to be picked up. A surface of soft new snow provides a lot of snow for wind transport. And this page has focused on how the wind picks up surface snow and moves it, but the distribution of new snow during a storm is determined in part by many of the same factors. We don't want to make things any more complicated here so any additional factors will be considered second order effects and left for another time and a more detailed analysis. (In academia they would say "this is left as a exercise for the student".)
Now that we have seen how snow is picked up and deposited lets go to Part 2 and look at the actual mechanisms through which the snow is moved.
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