Drifting snow is a common winter phenomenon where strong winds redistribute snow that has already fallen and settled on the ground. This wind-driven movement is a form of aeolian transport, carried by the air, and is responsible for the formation of large snow deposits known as drifts. The necessary meteorological conditions include loose, dry snow and a wind speed that exceeds a certain threshold, often around 11 miles per hour. This redistribution can occur even when no new precipitation is falling, such as during a “ground blizzard.”
Defining Drifting Snow Versus Blowing Snow
The terms drifting snow and blowing snow describe different states of wind-mobilized snow, primarily distinguished by the height to which the snow particles are lifted. Drifting snow refers to the movement of snow close to the ground, typically remaining below a height of about 6 feet (2 meters). This low-level movement is a surface event that results in the accumulation of snow into drifts, posing a hazard to ground transportation by rapidly covering roads and pathways.
Blowing snow involves snow particles being lifted higher into the atmosphere, often reaching 6 to 8 feet or more above the surface. Because these particles are suspended in greater concentration, blowing snow significantly reduces horizontal visibility, creating whiteout conditions. While both involve wind transporting snow, blowing snow is an atmospheric event that impacts visibility, whereas drifting snow is a ground-level process that precedes drift formation.
The Mechanics of Snow Transport
Wind moves snow crystals across the surface through aeolian transport, which involves three distinct physical modes of movement. The first mode, known as creep, involves snow particles rolling or sliding along the snowpack surface, generally only a few millimeters above the surface.
The dominant method is saltation, derived from the Latin word for “jump.” During saltation, snow grains bounce or skip along the surface, typically reaching a maximum height of 4 to 8 inches (10 to 20 centimeters). These particles are lifted by the sheer force of the wind or ejected upward when a moving particle impacts a stationary one, a process called splash.
The third mode, suspension, occurs when the finest, lightest snow particles are lifted higher into the air by turbulent eddies. These suspended particles travel long distances until the upward force of the air turbulence is no longer sufficient to counteract gravity. The constant movement and impact in all three modes tend to break down the snow crystals into smaller, denser grains, which increases the cohesion of the resulting snow drifts.
Why Snow Drifts Form Around Obstacles
Snow drifts form when the wind carrying the snow encounters an obstacle that causes a sharp reduction in air speed. When the velocity of the snow-laden wind drops, the air can no longer support the transported snow, causing the particles to fall out and deposit. This deposition most frequently occurs on the leeward side, or downwind, of an object.
Obstacles like trees, buildings, and snow fences create a localized area of calm air called a wind shadow or eddy. Within this turbulent region, the wind speed decreases rapidly, and the accumulated snow forms the characteristic ramped shape of a drift. The size and shape of the resulting drift are determined by the size and permeability of the obstacle, along with the sustained wind direction and speed.