One of the issues facing climbers is the frequent presence of high-speed winds. The peak of Mount Everest extends into the upper troposphere and penetrates the stratosphere, which can expose it to the fast and freezing winds of the jet stream. In February 2004, a wind speed of was recorded at the summit and winds over are common. These winds can blow climbers off Everest. Climbers typically aim for a 7- to 10-day window in the spring and fall when the Asian monsoon season is either starting up or ending and the winds are lighter. The air pressure at the summit is about one-third what it is at sea level, and by Bernoulli's principle, the winds can lower the pressure further, causing an additional 14 percent reduction in oxygen to climbers. The reduction in oxygen availability comes from the reduced overall pressure, not a reduction in the ratio of oxygen to other gases.