There’s an old saying in the Presidential Range: “If you don’t like the weather, wait a moment.” Funny how that saying also crops up in the Sierra, the Rockies, the Appalachians, and most other mountain ranges. In the backcountry, weather can—and will—change quickly and dramatically. In our new book, BACKPACKER’s Predicting the Weather ($13, falcon.com), excerpted here, Lisa Densmore teaches you how to read the skies for safer, drier outings.
Elemental Ingredients 1. Air temperature While ground temps determine the number of layers you wear, the mercury up high dictates whether you’ll need raingear. As warm air rises, it cools off and approaches its dew point (the temperature at which water vapor turns to droplets). Heavy droplets then fall to earth as precip.
2. Wind The stronger it is, the colder it feels (e.g., a 30°F day with 30-mph winds feels like 15°F). For a windchill chart, go to backpacker .com/windchill. Wind also signals change.
3. Humidity Relative humidity (RH) is the amount of moisture in the air divided by how much water the air can hold at that temp (times 100). So an RH of 100 percent means the air is saturated (aka, at its dew point), and rain is coming. High humidity makes cold feel colder and heat hotter, via conduction. As air rises and cools, relative humidity increases.
4. Barometric pressure This is the weight (per unit area) that the air exerts on the earth. A warm air mass is always lighter (less dense) than a cold air mass, and thus exerts less pressure. If the barometric pressure is falling, a warm front is coming in. If the barometric pressure is rising, a cold front is approaching.