How to Predict Mountain Weather
by Leif Wefferling.
The old folk saying "Red sky at night, sailors delight..." may be well and good if you are a sailor, but how can hikers read the changing and variable weather conditions of the mountains? Checking weather forecasts before leaving home and carrying a NOAA weather radio is always a good idea, but paying attention to clouds and rapid changes in air pressure while on the trail can alert one to changing weather conditions in the field.
READING THE SKY
Clouds offer the most accessible reading of weather changes since they are formed by changes in the atmosphere. Lenticular clouds, those lens-shaped clouds hovering over a mountain peak like a UFO, indicate strengthening winds and the approach of moisture-laden air, both indicative of an advancing weather system from six to eighteen hours away.
Similarly, the approach of fronts and low-pressure storm systems may be signaled by the appearance of scattered, wispy cirrus clouds high in the atmosphere. Condensing moisture can also sometimes be seen as a halo, or ring around the sun or moon. As a weather system moves closer, watch for a thickening, lowering cloud cover. If, over the course of a day or two, you see the progression from high cirrus to a low blanket of clouds obscuring the sun, precipitation may be only three to six hours away- time to head back to the car.
A handy sign to watch for as a storm draws near is the base of the cloud cover. A non-precipitating cloud will have a sharper, more defined cloud base, while a precipitating cloud will have a blurry, indistinct base that can often be seen from some distance, giving you time to take cover.
Air Pressure Conditions
Rapidly falling barometric pressure is another clue to changes in the atmosphere. A pocket altimeter, if used correctly, can be a useful aid in foretelling the approach of a low-pressure storm system. Because air pressure naturally drops as you go up in the atmosphere, an altimeter actually registers a change in altitude by measuring the change in air pressure. So if your altimeter shows a rapid gain in elevation when none has taken place (or it steadily rises above your known altitude), it is actually measuring a drop in air pressure.
Suggested Reference Materials
Learning about how weather works will greatly help in your reading of environmental clues while on the trail. Two books by local authors are worth looking into. Northwest Mountain Weather, by Jeff Renner gives a good introductory understanding of the forces at play in mountain weather. More comprehensive in scope and full of useful pictures is The Weather of the Pacific Northwest by UW Atmospheric Scientist Cliff Mass.
Consult with the Professionals
Before heading to the mountains, check current conditions. NOAA's National Weather Service is an excellent source for everything from point forecasts and weather warnings, to current radar images. The Northwest Avalanche Center provides a mountain weather forecast for our region as well as essential information for anyone venturing into snow country.