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二级文章(题库未考查文章) Avalanche and Its Safety An avalanche is a sudden and rapid flow of snow, often mixed with air and water, down a mountainside. Avalanches are among the biggest dangers in the mountains for both life and property. All avalanches are caused by an over-burden of material, typically snowpack, that is too massive and unstable for the slope that supports it. Determining the critical load, the amount of over-burden which is likely to cause an avalanche, is a complex task involving the evaluation of a number of factors. Terrain slopes flatter than 25 degrees or steeper than 60 degrees typically have a low risk of avalanche. Snow does not gather significantly on steep slopes; also, snow does not flow easily on flat slopes. Human-triggered avalanches have the greatest incidence when the snows angle of rest is between 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. The rule of thumb is: A slope that is flat enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle. Additionally, avalanche risk increases with use ; that is, the more a slope is disturbed by skiers, the more likely it is that an avalanche will occur. | 
