No one visits Mt. Everest for the weather, but every climber with summit aspirations spends lots of time watching the sky.
That's because there is usually just a small window of time in May when conditions are as good as they're going to get for climbing. No one can predict exactly when those weather windows will open, and they can close quickly and without warning.
Over the last 20 years, forecasting techniques and communication technologies have improved enough to give teams a better sense of when to set out for the top. Nevertheless, weather remains a major cause of trouble on the world's highest peak.
"When climbing is condensed to a really limited time frame because of tiny weather windows, it creates a lot of problems on the mountain," said Michael Fagin, lead forecaster at West Coast Weather, a high-altitude weather forecasting service. "Everyone wanting to go up at exactly the same time creates all sorts of issues."
Every mountain creates its own weather, said Jeff Masters, director of Meteorology at Weather Underground in Ann Arbor, Mich. One reason is that air rises up slopes as the sun warms it over the course of the day.
When that air reaches altitude, it cools off, and any water held in it condenses and falls as rain or snow. The taller a mountain is, the more air is forced over it, creating even more potential for precipitation.
Winds can also exacerbate the severity of high-elevation storms if they pass over major bodies of water on their way to a mountain. Peaks in the Cascades, for example, get precipitation from winds that blow in from the Pacific. On Everest, winds can carry water in from the Bay of Bengal to the south.
Everest is always formidable but for most of the year, it is also un-climbable. The monsoon season from June through September brings drenching rains down low and heavy snow up high. Conditions dry out in October, and some climbers choose to make a summit push then, but by late fall, short days and frigid temperatures increase the difficulty of climbing.
The majority of expeditions arrive at Base Camp in April, when temperatures warm up enough to make climbing possible. Then, all eyes turn to the jet stream.
Jet streams are fast moving air currents that circle the globe at high speeds in the upper atmosphere. For most of the year, the meandering jet stream passes directly over the mountain, which is tall enough to come close to the moving air highway. Winds regularly blow at speeds of 100 miles per hour and up.
Only when the jet stream shifts far enough to the north do winds calm down to a more reasonable range of 10 to 20 miles per hour, Fagin said. When he forecasts for the mountain, he compares six weather models, blends them together and puts the most faith in the models that appear to be the most accurate at the time. He also factors in real-time feedback from clients on the mountain.
Fagin recommends climbing only when there is a good chance that the jet stream will stay far enough to the north for at least five days in a row -- long enough for teams to get from Base Camp to the summit.
Most seasons get an average of 12 summit-worthy days, said Alan Arnette, a mountaineer and respected Everest blogger based in Fort Collins, Colo. But during the spring of 2012, there were just five good days for summiting and 500 people on the mountain, leading to dangerous crowds and notorious images showing long lines on the way up.