Major causes of avalanches are weather, snowfall, temperature, wind direction, snow pack conditions, slope angle, slope orientation, terrain, and vegetation.
Weather: Avalanches are more likely to occur after a heavy snowstorm. The 24 hours following a storm are the most critical.
Snowfall: Recent snowfall puts extra stress on the existing snow pack, especially if the new snow does not adequately bond with the layer of snow already there. Snowfall amounts of one foot or more create the most hazardous situation, producing avalanches that are large enough to block highways and cause major destruction.
Temperature: Because snow is a good insulator, small changes don't have as much effect as larger or longer changes do. Changes that last several hours or days, such as a warm front moving through can seriously weaken some of the upper layers of snow.
Wind Direction: Wind usually blows up one side of the slope of the mountain(the windward side), and down the other(the leeward side). When blowing up the windward side, wind will scour snow off the surface and drop it on the leeward side. This packs snow unevenly on the leeward side, making it more prone to avalanches. Cornices are caused by wind scouring.
Snow Pack Conditions: Perhaps the most significant factor is how the snow pack has developed over the season. Understanding the history of snow pack for that season can reveal several clues about slope stability. Snow pack conditions can change not only over the course of the winter season but over the course of a day. This is why constant awareness and frequent slope testing are necessary.
Slope Angle: Most avalanches occur on slopes between 30 and 45 degrees. You can measure the slope angle with an inclinometer, or you can "eyeball" it by dangling a ski pole by the strap and estimating the angle.
Slope Orientation: Although avalanches will run on slopes facing any direction, most avalanches run on slopes facing north, east, and northeast. Because the sun is at such a low angle, particularly during the winter, a colder and deeper snow pack develops.
Terrain: 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. Even if you can see that an avalanche has already run, be wary. Avalanches can fall in a "piecemeal" fashion, where one avalanche will run and leave the rest of the slope weakened, and the slightest provocation can cause subsequent avalanches on that same slope. Smaller depressions or shallow gullies in the mountainside can also be hazardous. During an avalanche, these "terrain traps" serve as accumulation points for snow and debris where a victim could be buried.
Vegetation: On a snow covered slope, heavily forested areas are much safer than open spaces. 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. Tree line (above which conditions make it to harsh for trees to grow) and smooth surfaces such as a rock face or grassy slope can effect the probability of an avalanche.