Backcountry winter activities such as skiing and snowmobiling are exhilarating adventures, but can also have serious risks.  This winter, avalanches in the Unites States have claimed the lives of seventeen people, including four this past weekend.

A product of weather and topography, avalanches depend on the steepness of a terrain. A slope between 25° and 60° is considered most favorable for a dangerous slide.  A steeper gradient will cause more frequent, but smaller events.  Snow, the other key ingredient, accumulates on mountain slopes in layers, forming a snow-pack over time.  Each layer is different in texture and weight, depending on the moisture content of the snow. Once on the ground, the snow remains subject to atmospheric conditions.  As temperatures rise and fall, the process of melting and re-freezing can form seams of instability in the snow-pack. Eventually, something will trigger an unstable slab to overcome the delicate friction holding it in place.

Triggers can be natural or man-made.  Some natural triggers include the weight of additional snow, sudden changes in temperature, and falling trees.  People can set off avalanches when their activities traverse an unstable snow slab.  Once a slab breaks away, it will cascade down the mountain engulfing anything in its path. So, if you are heading out to the back woods, it is vital to heed the avalanche warnings in your area.

On the Ice

Winter is usually the time for outdoor activities like ice-skating or cross-country skiing. This year, however, the unseasonably mild conditions have limited the opportunities for many traditional winter sports across the U.S.

A brief cold blast of winter, like the one forecast for the New York City area this weekend, is generally not enough to form ice capable of supporting significant weight.   Below is a guideline on how thick ice needs to be to support different activities. The measurements refer only to new, clear, solid ice.  White ice usually has air trapped inside it and is consequently not very strong.

The thickness of ice can vary dramatically from spot to spot on the same pond or lake. Therefore, it is always best to follow the instructions of local officials and posted signs.  When in doubt, stay off the ice.  It is better to be safe than sorry.

Chart Source: Minnesota Department of Natural Resources