Wind
After precipitation, wind is the next most important weather variable affecting avalanche potential. It is fundamental to slab development and determines where wind-transported snow accumulates.
Wind speed:
Wind speed determines the amount and rate of wind loading. Winds between 20 and 60 mph are optimal for transporting snow, while those less than 20 mph can only transport very low-density snow (< 5%) on the ground.
Very high winds (> 60 mph) often disperse snow beyond the starting zones of avalanche paths, depositing it farther down the slope. There it will stay, beyond the point where it can form dangerous wind slabs on critical slope angles. Very high winds, especially during periods with low relative humidity (such as when skies are clear) also cause snow to sublimate into the atmosphere before it can fall out and form a thick slab layer.
Average wind speeds between 20 and 60 mph transport and deposit snow of almost any density onto the upper reaches of avalanche starting zones. Average speeds between 30 and 40 mph have the greatest potential to build dangerous slabs in these areas.
Remember that new snow is not needed to form an avalanche. As long as there’s snow on the ground that the wind can transport, new slabs can form on leeward slopes.
Wind and snow accumulation rates: The combination of new snow and wind-deposited snow during a storm increases the snow accumulation rate significantly, easily doubling or even quadrupling it on leeward slopes. For example, if snow falls at a rate of 1 inch (2.5 cm) per hour for 8 hours, you’d expect 8 inches (20 cm) of accumulation on the ground. But if ridge-top winds average 30 to 40 mph from a consistent direction throughout that period, you might get snow depths of 16 to 32 inches (40 and 80 cm) on leeward slopes.
Wind
direction: Wind direction indicates which slope aspects are susceptible to snow being
deposited from wind loading.
Say, for example, that a storm begins with strong southwesterly winds that load leeward, northeast-facing slopes. If it is followed by northwesterly wind flow after, for example, the passage of a cold front or an upper-level trough of low pressure (in the Northern Hemisphere), southeast-facing aspects will become the leeward slopes and get the most loading.
When wind blows over a ridge top and transports snow from the windward to leeward side, it’s known as top-loading. Wind blowing parallel to a ridgeline may dump wind-deposted snow on avalanche paths. This cross-loading may be difficult to detect or monitor without observations from mid-slope and valley anemometers.
If the wind is strong enough to transport snow, the longer it blows from a steady direction, the greater the buildup of snow on favored leeward slopes.
If the wind is strong enough to transport snow, the longer that it blows from a steady direction, the more snow will build up on favored leeward slopes.
Summary table:
Select Operational Information at the top.
Data: Wind data should come from anemometers on an exposed, representative ridge top or atop several peaks in the area. Naturally, the more reports, the better. Hourly or more frequent data make it easier to determine the persistence of wind speed and direction. Be aware that remote unheated anemometers are susceptible to collecting rime ice, which can cause data and reporting errors. (Rime ice forms when supercooled water droplets strike a cold, sub-freezing surface.)
Wind speed: Gather wind data for the previous 24 hours, noting periods when the average wind speed is optimal for transporting snow (between 20 and 60 mph). Notice if there’s fresh or low-density snow on the ground that can be easily transported by the wind. If it snowed in the last 24 hours while the wind speed was between 20 and 60 mph, consider doubling or quadrupling the estimated snowfall depths on leeward slopes.
Wind direction: Gather wind data for the previous 24 hours, noting the average direction and how long it blew from various directions. Pay particular attention to long periods with a consistent wind direction.
Summary table: