Wanting a Wide Weather Window
The weather window exists when the calm right before the monsoons arrive coincides with the migration of the powerful jet stream to the north. The jet stream is located at 30,000 feet and typically sits right on top Everest, very near its summit at 29,035ft. Fortunately, a battle between cold air from the north and warm air from the south takes place each year; during the weather window, the south is winning, pushing the jet stream north and off the summit. These conditions bring about favorable weather conditions.
Everest climbers count on the weather window. Without it, the likelihood of summiting is very low. Fortunately, the weather window has existed on Everest with great consistently for many years. The length of window can vary. In some cases the duration is a mere 4 days while in others years it can last a much preferred two weeks. Climbing conditions within the window are favorable, winds ranging between 10-20mph and little to no precipitation. With these conditions, the mountain is easier to climb. On my recent climb of the volcano Cotopaxi in Ecuador, we faced 50-70mph winds during our summit push. Indeed, the climbing was really tough. With no shelter, the high speed gusts made the risk of being blown off the mountain very real.
Whereas the weather window on Everest is counted on, there is no guarantee it will exist. In the fall of 2007 and on the neighboring 8000 meter peak Cho Oyu, climbers were counting on the existence of a similar weather window. It never materialized. Consequently, most groups did not summit. In fact, the weather was very poor with multiple, bad storms hitting the mountain. Whereas there is no evidence, the mountain's conditions and the high number of tropical depressions bring into question whether global warming is having an impact.
Perhaps The Clash's song "Should I Stay or Should I Go" comes to mind as this is the key question climbers face when dealt with marginal weather conditions. Some groups roll the dice, betting the weather will improve or that they are strong enough to endure the conditions. This can either lead to summit success or disaster. Other groups, in contrast, play it safe — but this can result in a few weeks sitting around camp and no summit attempt made at all. Whereas the safe path may seem best, some groups taking this route realize later that the weather did in fact improve — they could have made it. But instead they will be returning home without trying.