The Ever-Changing Sky
by Gregory Crouch from GORP
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
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.
Thunderstorms are the
most dangerous type of local weather. Most injuries caused by weather in the
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
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:
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.
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 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
Indicators that a front may be moving toward you:
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.