An avalanche occurs when snow packed on a mountain or slope becomes loose and slides down. Several factors, including human ones, contribute to the loosening of snow and ice and the potential for danger to people and property.
Cold temperatures keep snow and ice packed tightly. When a warm front causes snow to loosen, the weight of snow at the top of the slope can start a domino sliding effect. Heavy rain, sleet or snow can also trigger the loosening of packed snow and the start of an avalanche. When snow is more powdery, heavy winds can cause it to loosen and slide.
Movement on and around the slope also triggers avalanches. Earthquakes that shake the ground beneath the snow can cause them. When packed snow is ripe for loosening, even the simple jostling at the top of a slope from skiers and snowmobilers can start the process. Trucks and equipment used in road construction and tree removal at ground level can generate reverberations felt on a snowy slope.
The level of a slope impacts the propensity of an avalanche as well. The snow packed on a high-pitched slope is much more vulnerable to triggers than snow on a flatter incline.