By Robert Lucke
All winter recreationalists should be aware that weather factors play a big part in avalanche activity in most all mountains. The U.S. Forest Service has some weather related tips.
Wind - Even during clear weather, sustained winds of over 15 mph may cause danger to increase rapidly when loose surface snow is available for transport. Snow plumes from ridges and peaks indicate that snow is being moved onto leeward slopes. This can create dangerous situations. Leeward slopes are dangerous because wind-deposited snows add depth and may create unstable wind slabs. Windward slopes generally have less snow and the snow is compacted and usually more stable toward the leeward slopes.
Storms - A high percentage of all avalanches occur shortly before, during, and after storms. Be extra cautious during these periods.
Rate of Snowfall - Snow falling at one inch per hour or more increases avalanche danger rapidly.
Crystal Types - Observe snow-crystal types by letting them fall on a dark ski mitt or parka sleeve. Small crystals, needles, and pellets often result in more dangerous conditions than the classic star-shaped crystals.
New Snow - Be alert to dangerous conditions with a foot or more of new snow. Remember that new snow depth may vary considerably with slope elevation and aspect.
Old Snow - When snow depth covers natural anchors, such as rocks and brush, new snow layers slide more readily. The nature of the old snow surface is important. For example, cold snow falling on hard refrozen snow, such as rain crusts, may form a weak bond. Also a loose underlying snow level is more dangerous than a compacted one. Check the underlying snow layer with a ski pole, ski, or probe.
Temperature - Cold temperatures will maintain an unstable snowpack while warm temperatures (near or just above freezing) allow for snow settlement and increasing stability. Storms starting with low temperatures and dry snow, followed by rising temperatures, are more likely to cause avalanches. The dry snow at the start forms a poor bond to the old snow surface and has insufficient strength to support the heavier snow deposited late in the storm.
Temperature Inversion - It may be warmer at higher elevation when warm air moves over cold air trapped near the ground. This weather situation can occur in avalanche terrain throughout the Northwest and may produce dangerous and unpredictable changes in local snow stability.
Wet Snow - Rainstorms or spring weather with warm winds and cloudy nights can warm the snow cover. Percolating water may cause wet snow avalanches. Wet snow avalanches are more likely on south slopes and slopes under exposed rocks.
Danger Signs - Old slide paths. Generally, avalanches occur in the same areas. Watch for avalanche paths. Look for pushed-over small trees or trees with limbs broken off. Avoid steep, open gullies and slopes.
Recent Avalanche Activity - If you see new avalanches, suspect dangerous conditions. Beware when snowballs or "cartwheels" roll down the slope.
Sounds and Cracks - If the snow sounds hollow, particularly on a leeward slope, conditions are probably dangerous. If the snow cracks and the snow cracks run, this indicates a slab avalanche danger is high.