OCR Text |
Show time for protective measures to do some good, a matter, experience has taught us, of at least 2 hours at Alta. Since a major avalanche hazard can disappear in five minutes, strictly accurate predictions are impossible. It frequently happens that a storm of dangerous proportions will unexpectedly end within the 2-hour limit, 3, The new snow depth illustrates another and different type of fluctuation. Strictly speaking, any amount of new snow at all is a factor favorable to avalanches. But the snow observer finds that there is an amount beyond which danger becomes acute, other factors being favorable. At Alta, this amount of dry snow is about 12 inches. It is one of the best guide posts the snow ranger has. When the 1-foot mark on his snow stake disappears he can start mulling over the other factors in earnest* Remembering his 2-hour factor he has probably been uneasy for some time anyway. U. A standard snow terminology has been constructed for Alta to aid in study and record keeping. Every snow observer must either adopt a terminology or construct one of his own. Records are worthless unless they are definite and consistent. The Alta snow terminology is included in the Annex. The type of snow is classified according to form-crystalline (powdery), granular, pellet; size-fine, medium, coarse; and weight-dry, damp or wet. No type of new snow can properly be called stable. For the observer it is a question of marked departure one way or the other from its average ability to stay in place. On this basis both granular snow and pellet snow of average water content are unstable and favorable to avalanches. All types of dry crystalline snow and mixtures of one or more types are normal and negative in their effect. 5. The influence of water content brings us to snow weight, a factor closely allied to type. For analysis of its effect on avalanches, it is again a question of departure from normal. Average snow weights at Alta range between .07 and .09 of an inch of water per inch of snow. The familiar dualrole performance comes in. Damp snow is sticky and its type is therefore unfavorable to avalanches. Damp snow is also abnormally he^vy and therefore its weight is a favorable factor. 6. Rate of accumulation of new snow is chiefly important in combination with other factors, principally wind action. Like the depth of new snow it is always favorable to slides in some degree. But from the standpoint of slides of dangerous proportions, it often has a negative or even unfavorable value. At Alta snowfall rates above .6 of an inch per hour are in the danger zone. 7. Wind action, force-plus-direction, is undoubtedly the most important variable in the list of contributory avalanche factors. If the snow observer is careful to remember that in action wind force and wind direction are inseparable, they can be analyzed one at a time. Force is a modifying and direction a selective factor. Acting favorably they are present in an overwhelming majority of dangerous avalanche cycles resulting from major storms. Snow falling in quiet air is distributed evenly over the surface of the area. Wind modifies this process in two ways. First it removes snow from parts of the area and dumps it in others. This uneven distribution increases -30- |