Winter Weather

Beginner's Guide to Backpacking

Winter Weather
In the continental United States in winter, bad weather is usually produced by major low-pressure systems tracking west to east. If your trip will last only a couple of days, you should be able to get a decent weather forecast before you start. The National Weather Service, with its far-flung network of observing stations, will always be able to do a much better job of predicting the weather than you will by observing conditions in your immediate vicinity. However, a knowledge of the typical pattern of clouds generated by an approaching low-pressure system can be helpful, particularly if you're out for several days and the forecast you got at the beginning of the trip has become outdated.

About 24 to 48 hours before a storm hits, high cirrus clouds usually began to cover the sky. These feathery or fibrous clouds are composed of ice crystals and found above 23,000 feet. As the storm draws closer, the cirrus thicken to cirrostratus, a more continuous, sheet-like cloud that often causes a halo around the sun or moon. Stratus means stratified or layered. This sequence of cirrus followed by cirrostratus is important, for cirrus alone is not necessarily a sign of bad weather. As the storm gets closer, the cirrostratus lower and thicken further to altostratus, layered middle-level clouds between 6,500 and 23,000 feet. When the storm is imminent, the altostratus thickens and descends to form nimbostratus, a continuous blanket of low-level (below 6,500 feet) clouds, and snow begins to fall. Low pressure systems less than 300 miles across usually die out within 36 hours. Bigger ones, from 350 to 1,800 miles across, often last three or four days. This idealized portrait of a low-pressure system doesn't correspond perfectly to every storm you'll encounter, but at least it gives you some clues on what to look for.

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