Backpacker Magazine – October 2010

The Manual: Forecast the Weather

Can crickets tell you the temperature and a cup of coffee forecast storms? Here's how to become a backcountry meteorologist.

by: Excerpted from Predicting the Weather: Forecasting, Planning, and Preparing by Lisa Densmore

Tags:
• Skills
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• Weather
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Prevailing Patterns
Mountains In a process called adiabatic cooling, air cools by 5.5°F for every 1,000 feet of elevation gained (if there’s no moisture). Add humidity, and the rate slows to 3.2°F per 1,000 vertical feet. This can create precipitation on a peak even if the plains below are dry. Also: Wind flows upslope during the day as the air heats up, then downslope in the cool evening. So the earlier you summit, the less windy and cloudy it likely will be. Still, widespread clouds or strong prevailing winds can neutralize mountain effects.

Valleys Since cold air sinks, valleys are usually cooler than surrounding hillsides.

Ocean, sea, or lake It takes a huge body of water to impact the weather significantly. Water changes temperature more slowly than land, so during the day, breezes blow inland as air flows from the colder water toward the warmer land. At night, gusts travel from the cool land toward the warmer water. Thick clouds cancel this effect because they prevent a significant temperature differential between the lake or sea and the land. So coastal wind on a cloudy day signals an approaching front and likely a storm.

Glaciers and snowfields These create downslope breezes that travel about a third of a mile below them.

Deserts One big weather danger here? Thermals: columns of rising air that occur over hot spots on land or water. Air rushes to fill the column’s low-pressure zone, spawning sandstorms with up to 75-mph winds. Thermal action builds during the day, making sandstorms more likely in the afternoon. They also interfere with electronic transmissions like cell phones and radio. Wear goggles, a windshell, and a bandanna over your mouth and nose; seek shelter.



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