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 we 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 actually moves the snow.
There are three ways in which the wind shear can move snow - rolling along the surface, saltation within 10cm or so of the surface, and turbulent suspension up higher in the air stream.
A rolling motion along the surface is also known as "creep", but should not be confused with viscoelastic creep within the snowpack on an inclined surface, which results in mechanical deformation of the snowpack. Rolling occurs at low windspeeds (under 5 m/s or 11 mph) close to the surface (within 1 cm or .4 in) and is responsible for only about 10% or less of the snow transported by wind.
Stronger winds eject particles from the surface causing saltation. These particles are subject to aerodynamic (wind) drag and also to gravitation, and the result is a sort of bouncing within about 10cm of the surface. There are also additional forces including a form of lift called "Magnus Lift", but the implications of these are more important later (in Part 3 and the Cornice sidebar) than they are to an understanding of saltation.
Saltation occurs within about a meter (3 ft) of the surface at moderate wind speeds (5-10 m/s or 11-22 mph) and is responsible for 80% or more of the snow transported by wind.
In higher winds (over 15 m/s or 30 mph) snow particles get taken up into higher levels of the windflow and carried a long distance. This is called turbulent suspension (or turbulent diffusion). Like rolling (creep) this is generally responsible for less than 10% of the transport. In many cases a lot of the snow involved in turbulent suspension may end up being lost to sublimation.
Now that we have seen how snow is picked up and moved lets go to Part 3 and look at the deposition of snow including windslabs and cornices.
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