A snow drift is a mass of snow that has been moved and deposited by wind, creating an accumulation distinct from the original snowfall. This phenomenon is a function of meteorological conditions, specifically wind speed, and the presence of obstructions or variations in the terrain. Snow drifts take on sculpted shapes as the wind removes snow from one location, known as the source area, and deposits it in another.
The Mechanics of Snow Transport
Before snow can form a drift, wind must first mobilize the particles from the surface, a process that occurs through three distinct modes of transport. The first mode is creep, where snow particles are rolled or slid along the surface, typically only traveling a distance equal to their own diameter. This movement requires the lowest wind energy and stays entirely within the surface layer.
The second mechanism is saltation, which involves snow grains following ballistic, bouncing trajectories just above the ground. Saltating particles can reach heights of 10 to 15 centimeters and are launched either by the direct force of the wind or by the impact of other falling grains in a process called splash. This bouncing and impacting is responsible for the bulk of snow movement near the surface.
The third mode is suspension, where very fine snow particles are lifted high into the air and travel for long distances, following the wind’s turbulent flow. This transport occurs when wind speeds are high enough to overcome the particle’s weight and keep it aloft. All three modes require dry, cold snow conditions and wind speeds that exceed a minimum threshold to initiate movement.
How Snow Drifts Form
The formation of a snow drift is a direct consequence of the sudden reduction of wind speed in the vicinity of an obstacle or terrain feature. Wind carrying a load of snow particles loses its ability to keep the snow in motion when it slows down. This drop in velocity causes the airborne and saltating snow to fall out of the airstream and accumulate.
Drifts typically form on the lee side, or the downwind side, of an obstruction such as a building, fence, or cut in the landscape. As the wind passes the obstacle, it creates an area of sheltered air and turbulence where the velocity drops rapidly. The snow load is then deposited in this low-pressure zone, building up the characteristic mound shape of a drift.
On the upwind side of an obstacle, the wind often accelerates, causing a scouring or erosional effect that removes snow from the area closest to the obstruction. This differentiation between windward erosion and leeward deposition illustrates that snow drifting is a localized redistribution process dictated by aerodynamics. The size and shape of the resulting drift are determined by the wind’s direction, the speed of the air current, and the geometry of the obstruction itself.
Common Forms of Snow Drifts
Snow drifts manifest in several distinct shapes depending on the nature of the terrain and the obstruction. The most common form is the leeward drift, which is the large, wedge-shaped mound that accumulates behind any solid object that breaks the wind, such as a wall or a snow fence. These drifts characteristically have a right triangular profile, with a gentle slope facing the wind and a steeper slip face on the downwind side.
Another form is the snow cornice, an overhanging mass of snow that forms on the leeward edge of sharp terrain features like mountain ridges or rooftops. Cornices develop as wind blows snow over the edge and deposits it onto the downwind side, building out horizontally over empty space. These structures can grow to be massive and are inherently unstable.
Windward drifts are accumulations that can form directly against the upwind face of a structure, such as a building. They can also refer to the “scoop” or clear area of erosion that the wind creates directly in front of an obstruction. This erosional scar can sometimes act as a ramp, allowing snow to be transferred to an elevated surface like a roof.
Impact and Mitigation
Snow drifts pose practical challenges, primarily by blocking transportation routes and creating hazardous conditions for travel. A significant drift can completely engulf a roadway, rendering it impassable and disrupting emergency services and mobility. Drifts accumulating on rooftops also present a structural hazard due to the concentrated weight of the packed snow, which can exceed the building’s design load.
To mitigate these issues, a common strategy involves the strategic use of snow fences. These barriers are not designed to stop the snow completely, but rather to force the snow to deposit in a controlled area before it reaches the protected object, like a highway. The fence works by reducing the wind speed immediately downwind of its structure, which causes the snow load to drop out of the air current.
A properly installed snow fence is typically situated upwind from the area needing protection, creating a zone of deposition where the drift can accumulate harmlessly. For maximum effectiveness, the fence is positioned at a distance that is often calculated as many multiples of its height away from the roadway. This technique effectively manages the wind’s kinetic energy to control where the snow is ultimately stored.