Reading the Weather


The Ever-Changing Sky

by Gregory Crouch from GORP

Eldorado Canyon. May. Casey Newman and I are two pitches off the ground on "Rewritten." The morning's white puff clouds have doubled in size and taken on a gray tint since we left the ground. Is there enough time left to finish the route before those swelling clouds become thunderheads? We think yes and go for it.

Casey's leading the crux traverse two pitches later when the first drops of rain strike. We thrash up the last two pitches as lightning flashes and thunder echoes through the canyon. We scramble through the trees and down the trail at a dead run. By the time we get to the car, Casey looks like a cat in a bathtub with his gray hair matted against his scalp. I shiver in my shorts and T-shirt. We're wet through to the skin and our gear is soaked. If we had paid more attention to the clues the sky was offering, we could have avoided some risk and misery.

To me there are two types of weather: climbing weather and not climbing weather. The scientists also have two kinds of weather: frontal and local. Frontal weather occurs when two different masses of air—say cold, dry air from Canada and warm, moist air from the Gulf of Mexico—collide and cause a storm that sweeps across the United States. Local weather is caused by local conditions like temperature, humidity, winds, and mountains.

Those puff clouds over the foothills billowing up into thunderheads? That's local weather. A slow steady rain that lasts all day? That's frontal weather.

The sky constantly offers us clues about its future behavior. If we learn to recognize those clues and what they portend, we can make ourselves safer climbers and reduce the amount of suffering we have to endure.


Local Weather

Thunderstorms are the most dangerous type of local weather. Most injuries caused by weather in the United States are thunderstorm-related. Since the sun's heat energy is a major factor in thunderstorm generation, thunderstorms typically occur in the spring and summer. Winter sunshine simply doesn't deliver enough warmth. For thunderstorms to develop, the air needs to be warm, moist, and unstable—typical spring and summer conditions.

Keep an eye on those small, fleecy, white puff clouds (cumulus) drifting through the sky. They can grow into thunderstorms in a few hours. These clouds have flat bases and rounded tops and are not a threat as long as the cloud tops are less than one cloud width above the cloud bases. In the morning and early afternoon of a warm, sunny day, these clouds indicate that the atmosphere is unstable enough to generate thunderstorms. If these are the only clouds in the sky by mid- to late afternoon, there probably isn't much risk of thunderstorms developing. When these puff clouds become taller than they are wide, they change to swelling clouds (cumulus congestus) and the chance of thunderstorms rises greatly.


Swelling clouds: Watch these clouds carefully. They are the first stage of thunderstorm growth and can become dangerous in a matter of minutes. These white or gray clouds with flat bases are taller than they are wide. The earlier in the day that swelling clouds appear, the greater the probability that they will develop into thunderstorms. Their growth can usually be seen with the naked eye, and the faster the swelling clouds are growing the more threatening the situation.

Since the weather in the United States generally moves west to east, swelling clouds to your west are much more dangerous than swelling clouds to your east. This west to east airflow doesn't always hold true, especially in mountain ranges, so try and determine if the swelling clouds are headed toward you. If they are, an orderly retreat before the storm develops might be your best option. It's always better to get to safety before a storm hits.


Local Thunderstorms

Swelling clouds will become thunderstorms if they continue growing. As the rising tops of thunderstorms surge into the jet stream, these strong upper-level winds drive the top of the cloud downwind, forming the characteristic "anvil" shape of a mature thunderhead. Violent thunderstorm weather includes rain, hail, sleet, or snow with accompanying lightning, flash floods, strong winds, and plummeting temperatures—any one of which can ruin your whole day.

At times it is difficult to identify thunderstorms in confusing and unstable skies. Some indicators of their approach are:

  1. Pouch-shape clouds hanging down from a higher cloud layer (mammatus clouds). These pouch clouds look like dozens of gray or black half-basketballs suspended from the cloud layer above. Pouch clouds are only formed as a result of thunderstorm activity. If they're in the distance and not approaching, the thunderstorm that produced them is probably not a problem. If they're large and approaching, prepare to get hit by a storm.
  2. A sudden change in the direction and strength of the wind and a sharp drop in temperature. These "gust fronts" are caused by cool air flowing out from the base of a thunderstorm, and sometimes form a long, horizontal, arc-shaped cloud just in front of the thunderstorm. The arrival of the gust front is often the best sign that a thunderstorm is about to strike. Rain and lightning usually follow the gust front. Take immediate action.

If conditions are very favorable for thunderstorm development, the gust front of one thunderstorm can actually act as a "cold wedge" and push up the warmer air in front of it, leading to the birth of yet another thunderhead.


How Mountains Help Spawn Thunderstorms

Mountains and ridges can help generate thunderstorms by providing a "push" to set air rising into the atmosphere. Air is forced up by mountain slopes by the wind, and moisture condenses out of the air to form puff clouds. These clouds can continue growing into thunderheads if enough heat and moisture are present. Mountain (orographic) effects make mountain weather more unstable than flatland weather.


Other mountain clouds (orographic): These clouds indicate strong winds at high elevations.

  1. Sierra wave: A long line of clouds with smooth edges and features that remains stationary over or downwind of mountains or ridges.
  2. Lenticular clouds: Lens-shape clouds over or downwind of mountains or ridges.
  3. Cloud caps or pilot clouds: Wispy clouds that sit right on a mountaintop. Although the cloud stays stationary, violent winds can usually be seen inside the cloud—strong winds are striking the summit.


These clouds alone in the skies do not necessarily mean a weather change is imminent, but mixed skies with orographic clouds and other high clouds moving into the area can indicate the approach of a front.


Frontal Weather

Frontal weather occurs when different air masses meet and cause a storm. The stronger the differences between the colliding air masses in temperature and moisture content, the stronger the front and the more severe the weather associated with it.

Fronts move across the United States from west to east, and clouds in the atmosphere often give us clues that indicate a front is heading our way. If you pay attention to the warnings, you can make an educated guess as to how much time you have remaining before the front arrives.


Indicators that a front may be moving toward you:

  • Mare's tails: Thin, white to light-gray streamer clouds high in the sky (cirrus clouds). Isolated mare's tails aren't a strong portent of bad weather to come, but dense and widespread mare's tails can indicate the approach of a front from as far as 24 to 48 hours away.
  • Uniform, featureless, white or gray clouds at high altitude covering most or all of the sky (cirrostratus): These clouds usually mean that a significant amount of moisture is moving into the area. They aren't spectacular, but they are important. Monitor the sky for a change in the weather.
  • Halos: Rings of light around the sun or moon caused by light refracted through high altitude ice crystals. The old adage about halos preceding storms by 24 to 48 hours generally holds true in the moister regions of the United States, but in the drier climates, halos often pass with no ill-effects.
  • Gray, water-droplet clouds at medium altitude (altostratus clouds): A solid coverage of these gunmetal-gray clouds usually precedes major weather systems by less than ten hours.
  • A layer of dark clouds with noticeable blurring below the cloud bases (nimbostratus clouds): These clouds bring rain or snow, and in the warmer months can develop into thunderstorms. Typically associated with the arrival of a front. The weather is likely to stay bad for 24 hours.

Frontal thunderstorms: These storms are formed when the leading edge of a cold-front wedges up warmer air in front of the moving cold front. This forms lines of thunderheads stretching from horizon to horizon (squall lines) and very severe weather.


Remember: This information is meant to help you make informed decisions. Weather forecasting is a very complicated discipline. When weather threatens, exercise good judgment and err on the side of safety.


Three months after our dousing in Eldo, Casey and I were halfway up the Longs Peak Diamond. When the puff clouds started swelling, we retreated. The sky became ever-more threatening as we descended. Peels of thunder echoed through the range as we rappelled the North Chimney. Just as we reached the snowfields, the swelling clouds passed overhead to reveal a calm, blue sky.