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The jet stream is a river of wind that blows horizontally through the upper layers of the troposphere, generally from west to east, at an altitude of 20,000 - 50,000 feet (6,100 - 9,144 meters), or about 7 miles (11 kilometers) up.
This stream of air develops where air masses of differing temperatures meet. For this reason, surface temperatures determine where the jet stream will form. The greater the difference in temperature, the faster the wind velocity inside the stream. Jet streams can flow up to 200 mph (322 km/h), are thousands of miles long, hundreds of miles wide, and a few miles thick.
Cold polar air flowing down from the north meets the warmer air mass over the United States causing the polar jet stream to form. Areas of high and low pressure act like a moving riverbed, buckling and snaking the path of the air as it flows to the east. At times, it may dip further south into the U.S., bringing cold weather with it. At other times it retreats into Canada, leaving milder weather in the U.S.
During the winter, a second jet stream forms in the lower latitudes. It separates the air mass over the U.S. from warmer air masses rising from south of the equator. This is called the subtropical jet stream.
Within the streams of air, currents travel at varying speeds but are greatest at the core. Jet streaks are areas inside the stream where the velocity is higher than the rest of the air. These streaks cause air to rise, which lowers the pressure at the surface. When surface low pressures form, the rising air can cause clouds, precipitation and storms. Therefore understanding and observing the jet stream is instrumental in accurate weather forecasting.
The jet stream can also contain windshear, a violent and sudden change in wind direction and speed. Windshear can occur outside the stream as well, even at the surface. When vertical winds blast downward, it can cause an airliner that is in the process of take off to suddenly lose altitude and potentially crash. For this reason, all commercial planes in the U.S. since 1996 have been equipped with windshear warning systems.
In the final days of World War II, the jet stream was discovered when fighter planes flying to Japan found they were not making headway against the strong easterly winds. They ultimately changed altitude to make the flight. Today, trips to the east coast by commercial airliner are shorter in duration than trips to the west coast, due to the stream of air pushing the planes east.
There are those who have "walked" in the jet stream. Mt. Everest, at over 29,000 feet (8,839 m), is so high that its summit actually sits in the river of air with prevailing winds at about 118+ mph (190 kph). Standing on the summit is a dangerous business and one has to pick the right moment carefully. Most prefer to summit in early May or fall when the air pushes northward over Tibet.