Jet Streams and Summit Window Explained

Many people have asked me: "What's the summit window and what are jet streams?" Well I'm no meteorologist and I don't work for the National Geographical Channel, but I'll try and explain this natural phenomena.

Summit Window

Mt Everest as we know, is the highest point on planet Earth. Because of the severe weather at the summit, climbers rarely attempt to complete the trek outside of May to June, when the jet stream is pushed north, causing the winds to calm and temperatures to warm enough for people to try to summit, this is called the 'Summit Window'. Even then, the weather can be pretty inhospitable, e.g. an average day in May 2008, saw a high temperature of minus 17 degrees Fahrenheit (-27 degrees Celsius) and winds of 51 mph. This is similar wind speeds to what Paul has been experiencing. Add this to the fact that there's less than one-third the amount of oxygen in the air compared to sea level and you can see why the summit window is so tricky to judge and why Paul is experiencing this delay with his summit attempt. Every element of the climb has to have a green light or you just don't risk it and at the moment we waiting for a couple of lights to change.

View of Mt Everest and Lhotse from the Western Cwm. It's early in the season and the jet stream is creating interesting cloud patterns over the summit. (Pic taken 28th April 2011)

The Jet stream sits on Everest almost all year round and at other times during the year (either side of May-June) it passes directly by the summit and winds can blow at hurricane strength, e.g. 118 pmh and temps can dive as low as minus 100 degrees Fahrenheit (-73 degrees Celsius).

Jet Streams

The Jet Stream is a current of rapidly moving air that is usually several thousand miles long and wide, but is relatively thin. They are found in the upper levels of Earth's atmosphere at the tropopause - the boundary between the troposphere and stratosphere.

Atmospheric Levels

Jet streams are important because they contribute to worldwide weather patterns and as such, they help meteorologists forecast weather based on their position. In addition, they are important to air travel because flying in or out of them can reduce flight time and fuel consumption.

Jet streams flow from west to east in the upper portion of the troposhpere

How Do They Form?

Warm air masses in the south meet cool air masses from the north and create temperature and air pressure gradients. Essentially, you can compare a 'gradient' to a ski slope. The steeper the slope, the faster you will reach the bottom due to a large difference in the grade of that slope.