OCR Text |
Show Climate and weather in connection with hazard forecasting and snow safety planning are discussed in Chapters 5 and 7. The purpose here is to describe them in general terms. If terrain offers a kind of firing platform for avalanches, climate and weather supply the ammunition- snow. To complete the analogy, man provides the targets. Snowfall When moisture- bearing winds strike the high mountains in winter they dump their loads of water in the form of snow. The results are impressive and beautiful but occasionally destructive. Snowfall at the rate of an inch an hour and storms which deposit up to three feet of new snow in one continuous downpour are not uncommon. Snowfall of the alpine zones is typically " dry,"; that is, it does not contain enough free moisture so that the particles will adhere when squeezed lightly in the gloved hand. It is varied in form: crystalline, granular, and pellet, in any size and combination. Its average density is about 0.08 inch of water per inch of snowfall, but this varies widely with individual storms. Average density is influenced by the form of snowfall, higher usually for granular and pellet; and this in turn is governed by temperature conditions and wind action. Temperature Temperature fluctuates widely and rapidly in the alpine zones. It often rises above freezing during daylight hours in fair weather and nearly always goes below freezing at night. Prolonged spells of sub- freezing temperature, fair weather or foul, occur and there are occasional intrusions of warm air masses, nearly always in connection with a storm. Rainfall may occur in winter at any altitude and is always dangerous. The temperature curve in the alpine zones is mostly below 32 degrees Farenheit. Wind Wind action during alpine storms is generally strong and its influence on snow is the most important of all the contributory avalanche factors. It transports snow from one exposure to another during storms and fair weather, thus promoting overloads on certain slopes. It modifies the size and shape of snow particles. Slab formation does not take place in the absence of wind. Storm Characteristics The typical alpine storm begins at moderate temperatures which gradually fall, and is followed by a period of cold. Snowfall in quantity at temperatures below ten degrees is uncommon except in the high alpine zone. The coastal alpine zone specializes in the inverted storm. This begins at low temperatures which then rise and the storm ends with wet snow or even rain. Storm characteristics lead directly to the types of avalanche hazard which predominate in various zones. - 26 - |