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The length of time between the start of a snowfall and danger of avalanche, and the further length of time be- fore the snow will have settled sufficiently to become safe again, depends on the weather conditions and slope exposure. If the slope is exposed to sunshine soon after the fall, dry new-snow avalanches may occur within a few hours, and if the sunshine continues, the danger from avalanche will be over within two or three days, and will be greatly reduced after one full day of warm sunshine. If the temperature is very low, however, and if the snow is not exposed to sunshine, the period before avalanches may occur is extended, and it may be two or three weeks before safe conditions are reestablished. Danger from this type of avalanche should therefore be suspected from almost any time during a heavy snowfall until not less than two or three days later, and the length of time that danger should be suspected is governed by the exposure of the slope and the weather conditions known to have existed subsequent to the fall of snow. Generally speaking, south and west slopes will avalanche first and become safe again first, whereas north and east slopes, receiving less radiation from the sun, will remain dangerous for a longer period. This period may be affected, however, by wind. Wind may pack the new fall of snow sufficiently to remove the hazard; if the wind is humid, it may form a wind slab, which introduces the hazard of a different type of avalanche; wind may form large snowdrifts which may be hazardous. A humid wind above freezing temperature may make the snow wet and introduce the hazard of a wet avalanche. |
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