Ninety percent of all avalanches occur on moderate slopes with an angle of 30° to 45° (snow tends not to accumulate on steeper slopes). Avalanches occur when the gravity pushing the collection of snow at the top of the slope is greater than the strength of the snow itself. A change in temperature, a loud noise, or vibrations are all that are necessary to trigger one of these snowfalls that begin at a "starting zone." The avalanche continues downslope along the "track" and ultimately the avalanche fans out and settles in the "runout zone."
Internationally, the Alpine countries of France, Austria, Switzerland, and Italy experience the greatest number of avalanches and loss of life annually. The United States ranks fifth worldwide in avalanche danger. The states of Colorado, Alaska, and Utah are the most deadly.
Avalanche prevention and mitigation involves a variety of methods. Snow fences are built to prevent the buildup of snow in starting zones, structures are built to stabilize snow. deflecting walls are built to divert avalanche flows away from buildings and even entire towns. Sheds build across roadways that pass through persistent avalanche paths can help to protect motorists from avalanches. Additionally, the reforestation of slopes with trees helps to prevent avalanches.
Sometimes avalanche control experts actually desire to create smaller, controlled avalanches to prevent larger, uncontrolled ones. Percussion guns, explosives, and even artillery have been used to produce these controlled avalanches when people are kept away.
Although a variety of recreationists spend time in snow-covered mountains - snowmobilers are those most often killed by avalanches in the U.S. Most avalanches in the U.S. occur during the months January, February, and March and on average, 17 are killed annually nationwide. Backcountry explorers are strongly advised to not only know how to identify avalanche hazard areas but also to carry a avalanche beacon/transceiver and a shovel in case of emergency.