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Rising temperature during a storm accompanied by rising new snow density is one of the causes of avalanches, while falling temperatures have the opposite effect. New snow precipitation intensity is a significant factor, for it represents the rate at which the slopes are being overloaded. Values above 2.5 mm of water per hour warn of impending hazard. In practice this factor may not be measured directly; instead, new snow density and snowfall intensity are observed. The wind is also critically important, for soft slab avalanches seldom occur unless sustained average wind velocity exceeds 6 to 7 m/sec -1. The most reliable indicator of developing avalanche hazard is a sustained period of coincident high wind and high precipitation intensity.

Wet snow avalanches are generated by intrusion of percolating liquid water (rain or melting snow) in the snow cover. The rapid temperature rise-quickly alters snow behavior, while the water itself reduces snow strength. Liquid water accumulating at an impervious crust provides an especially good lubricating layer for slab release. The most extensive wet snow avalanching occurs during winter rains or the first prolonged melt period in spring, when liquid water intrudes into previously subfreezing snow. Snow melt by solar radiation is the commonest source of wet snow avalanching and this is amenable to quantitative prediction. It is essential., though., that the total snow surface energy balance be considered in estimating amount of melt, for longwave radiation, vapor exchange and sensible heat from the air all play an important part. A warm, windy, overcast day may produce more melting (and avalanche activity) than sunshine and cloudless skies.