Weather Signs of an Incoming Storm

How to Forecast Weather for Climbing

The warning signs of an oncoming storm.
Robert Alexander/Getty Images

When you are climbing in the high mountains, in wilderness areas, and even at your local crag, you should know how to read the weather and how to use some common indicators to predict what the weather will be in the next 12 to 24 hours.

If you have been in a few bad storms, pounded by rain, wind, and snow, then you realize how important it is to keep an eye on weather systems and know when to beat a retreat to avoid getting hypothermia or being benighted on the side of a mountain.

The good news is that there are lots of warning signs and signals that help you predict what's coming your way. Here are nine common signs of an impending storm.

Cumulus Clouds

Cumulus clouds, giant pillowy clouds that appear heaped up in the sky, are a common summer cloud formation that often portends severe thunderstorms accompanied by lightning, the usual afternoon threat to climbers and mountaineers.

Cumulus clouds grow quickly as the day heats up. They often grow faster vertically than horizontally into massive cumulonimbus clouds, which develop into black, anvil-shaped clouds with heavy thunderstorms accompanied by lightning. Building cumulus clouds are a good indicator that you need to break out the rain gear and get the heck off of a mountain summits and ridges.

Cirrus Clouds

Cirrus clouds, forming above 20,000 feet in the atmosphere, are high wispy clouds that portend a change in the weather, usually an incoming warm front and bad weather. These high clouds are one of your first warnings that the weather could change in the next 12 to 48 hours. Don't confuse cirrus clouds with condensation trails left by high-flying jet planes.

Lenticular Clouds

Lenticular clouds, also called wave clouds, are long smooth cloud formations that indicate high winds in the upper atmosphere. Lenticular clouds typically form over mountains and mountain ranges when the wind is forced upward as it reaches the mountain's windward side. The upward wind curls above the mountain, forming the lenticular cloud on the leeward side of the mountain crest. A localized low-pressure system often builds on the leeward side of the mountain. While the clouds appear stationary, they often indicate a larger incoming storm.

Moving Clouds

If you look up at the sky and see two layers of dark clouds moving in different directions, it is a good indicator that the atmosphere is the unstable and bad weather is coming. This is often a signal that a new weather front is moving against an existing front.

Southerly Winds

The air circulates counterclockwise around low-pressure systems in the Northern Hemisphere, meaning that strong winds out of the south usually indicate the impending arrival of a storm. Because the prevailing winds in the United States are westerly winds, low-pressure systems or storms move to the east, bringing southerly winds on their outer edges.

Do not, however, be deceived by localized winds in valleys or off mountains, since they are usually caused by heating and cooling during the day.

Warm Nights

Stratus clouds are high layered clouds that often cover the entire sky with a featureless gray cloudscape that blocks sunlight. These high clouds often indicate incoming storms. They also act as insulators, keeping the night warm and blocking heat from escaping into the atmosphere. If the stratus clouds are combined with southerly winds, the night can be very warm.

Decreasing Atmospheric Pressure

If atmospheric or barometric pressure decreases, it is a sure sign that the weather is deteriorating. A falling barometer usually indicates rain or snow, often within 12 to 24 hours. When you are out climbing, you do not need a barometer to determine barometric pressure. Use an altimeter on a GPS unit to figure out atmospheric pressure in the field.

If you check the altimeter and it shows an elevation change when you have not moved, then the pressure is changing. If the altimeter shows a rise in elevation, the barometric pressure is falling and a low-pressure system is on its way. If it shows a fall in elevation, then it indicates a rise in barometric pressure and an impending high-pressure system moving in.

When you're climbing, calibrate the altimeter if you know the elevation of the parking lot before you hike to the peak. Later in the day, check the elevation if you reach a point and know the elevation. Always recalibrate the altimeter whenever you can for accuracy.

Halo Rings

High clouds, often at night, will refract a halo or ring of light around either the sun or the moon. These halos can be a good weather predictor and often signal incoming moisture and fronts. Look at the moon at night. A halo around the moon indicates that a warm front is approaching, but plan on at least a couple days of good weather before it arrives. If the moon is bright and clear, a low-pressure system has blown dust out of the air, so plan on rain.

Low Cloud Base

If dark, thick clouds lower down and snug up against the mountain peaks and ridges, plan on precipitation. Low clouds are a clear indication that the dew point, or the temperature that the air becomes saturated with moisture, is dropping. Rain or snow, often lasting all day or night, is usually imminent. Plan on beating a retreat back to the trailhead or hunker down in your tent and play a game or two of cards.