Secrets of the Snow: Visual Clues to Avalanche and Ski Conditions

Snow in Trees

Snow in Trees The way that snow lies (or does not lie) in trees contains many clues to the character of recent snowfall and weather. These clues add to the store of visual snow information available to the alert observer. When predominantly stellar crystals fall in light wind, the deposited snow exhibits canopying as the crystals interlock during accumulation. Cushions of snow begin to build up on exposed objects, notably including tree branches, until the enveloping snow grows to a much larger diameter than the object it covers. Moderate canopying is seen in figure 47, evidence that a limited amount of snow has fallen, probably less than 30 cm. With each branch still freshly covered with snow cushions in bright sunshine, the observer can recognize that the snowfall has just ended and deduce that the air temperature is far enough below freezing to inhibit any immediate melting. Only a moderate snowfall is suggested because not enough snow has built up on the cushions to start sliding away, and the cushions remain fat and rounded. Figure 48, on the other hand, shows the canopying associated with a deep snowfall. Here the weight of the accumulated snow has begun to bend the branches downward, and the snow has piled up deep enough to start shedding loose sluffs from the cushions, so that many of the snow cushions on the branches have lost their rounded character and now have a wedge-shaped appearance. Once the snow is deposited in cushions, metamorphism immediately begins to break up the stellar crystals and weaken the interlocking bonds. When this happens the snow loses cohesion and starts falling from the trees (fig. 49). As a lump of snow descends, it often 59 47 Wasatch Mountains, Utah Snow in Trees 61 48 Cariboo Mountains, British Columbia sweeps other lumps along with it until a regular cascade of snow arrives at the bottom, pocketing the surrounding snow with impact craters like those in the foreground of fig. 50. Freshly fallen in new snow, these “tree bombs” offer little resistance to skiing. Once agehardened for a few hours, they become tough, resisting lumps lurk62 Snow in Trees 49 Bugaboo Mountains, British Columbia ing under the next snow accumulation. Falling lumps and tree cascades can easily serve as avalanche triggers if they occur in a release zone. Below timberline, the post-snowfall period of initial metamorphism and snow dumping from trees raises the danger level in an unstable snow cover. Snow in Trees 63 50 Bugaboo Mountains, British Columbia External forces, such as wind, also cause trees to shed snow. When a strong wind gust strikes a snow-laden forest, large amounts of snow are dumped from trees as well as carried into the air as dust clouds (fig. 51). When this happens, avalanche triggers abound, and avalanche release in unstable snow becomes highly probable. Gusty winds following deep new snow should always be regarded with caution , especially in forested terrain. Even modest amounts of heat on a sunny day can also start the shedding process. In figure 52 this process is underway, with slender branches already showing gaps in the festoons of new snow, and the pocketed appearance of the sur64 Snow in Trees 51 Bugaboo Mountains, British Columbia face below showing where the departed snow has gone. When strong solar heating, common in late winter and spring, follows a new snowfall , it can rapidly cause melt instability in the deposited snow and at the same time generate many avalanche triggers as trees shed their loads. Not all snow accumulation in trees comes about through canopying as snow crystals interlock. Near the freezing point snow becomes wet and sticky, so that it readily adheres to every branch and twig and then may freeze in place. In this case snow sticks to each separate branch and twig without building wide cushions (fig. 53). Snow in Trees 65 52 Mount Teine, Hokkaido 53 Cascade Mountains, Washington Freezing rain occurs when liquid droplets fall through a layer of air colder than the freezing point and become supercooled. These supercooled rain droplets, very large compared with the tiny supercooled cloud droplets that form rime, spread out and freeze to form a layer of clear ice when...