Several factors may affect the likelihood of an avalanche, including weather, temperature, slope steepness, slope orientation (whether the slope is facing north or south), wind direction, terrain, vegetation, and general snow-pack conditions. Different combinations of these factors can create low, moderate or extreme avalanche conditions.

The following factors often occur in combination to produce an avalanche, but if a slope is unstable in any way, it may take only the weight of one skier to set off an avalanche.

Weather

Avalanches are most likely to run either during or immediately after a storm where there has been significant snowfall. The 24 hours following a heavy snowstorm are the most critical. If there has been heavy snowfall the day or night before your trip, it may be wise to postpone the trip in order to avoid the increased avalanche danger.

Snowfall

Recent snowfall puts extra stress on the existing snow-pack, especially if it does not adequately bond to the pre-existing surface layer. The extra weight of new snow alone can cause a slab to break off and fall down the slope, particularly in storm-induced avalanches.

Temperature

Changes that last several hours or days, such as a warm front moving through, can gradually increase temperatures that cause melting within the snow-pack. This can seriously weaken some of the upper layers of snow, creating increased avalanche potential, particularly in combination with other factors.

When temperatures rise above freezing during the daytime and drop back down again at night, melting and re-freezing occurs, which can stabilize the snow-pack. This is particularly common during the springtime. When temperatures stay below freezing, especially below zero degrees Fahrenheit, the snow-pack may remain relatively unstable.

Wind direction

  Wind usually blows up one side of a slope or mountain (the windward side), and down the other (the leeward side). Blowing up the windward slope, wind will "scour" snow off the surface, carry it over the summit, and deposit it on the leeward side. What this does is p. In the Northern Hemisphere, storms generally move from west to east. Consequently, the leeward slopes are most often the northeast, east, and southeast facing slopes. These slopes become easily wind-loaded and will more readily avalanche. Many ski areas are built on slopes with these orientations and must use prevention measures to counteract the natural avalanche conditions that build up on these slopes.

Snow-pack conditions

Perhaps the most significant factor (but not the only one) is how the snow-pack has developed over the season. The snow-pack as a whole may change not only during the course of the winter season, but throughout the course of a single day, due to changing weather and temperature conditions. This is why constant awareness and frequent slope testing are necessary. Snow-pack conditions are extremely important because many layers of snow build up over the winter season. Each layer is built up under different weather conditions and will bond differently to the subsequent layers.

Slope angle

Most avalanches occur on slopes between 30 and 45 degrees, but can occur on any slope angles given the right conditions. Very wet snow will be well lubricated with water, meaning it might avalanche on a slope of only 10 to 25 degrees. Very dry or granular snow will most likely avalanche on a slope close to the 22 degree angle of repose.

Slope orientation

Although avalanches will run on slopes facing any direction, most avalanches run on slopes facing north, east, and northeast (also the slope directions that most ski areas are located on). Because the sun is at such a low angle, particularly during the winter, a colder and deeper snow-pack develops. Slopes that are under shadow throughout most of the day are suspect because the snow-pack remains cooler, without much of the melting and bonding that can make the snow layers stronger.

Remember also that certain slope orientations are much more affected by wind-loading, particularly northeast, east, and southeast (similar to the orientations mentioned above).

Terrain

Paying attention to where you are in the grand scheme of things can offer clues about avalanche likelihood. Bowls and gullies are suspect at any time, regardless of other conditions. Snow can accumulate deeply and quickly in these areas, increasing the possibility of an avalanche.

Vegetation

On a snow-covered slope, heavily forested areas are much safer than open spaces, but don't assume that any vegetation at all will be protective. Lone trees, bushes, or large rocks on a mountainside can sometimes weaken the stability of the snow-pack. A fracture line (the break-off point for an avalanche) may run from a lone tree to a rock to another tree. Also, during avalanches, trees and rocks catch debris and cause excessive snow pile-up, as well as provide lethal obstacles for anyone caught in an avalanche.