Jet streams move around the Earth in a narrow band. They are created by the difference in temperatures between two air masses, usually cold polar air and warm tropical air. The temperature variance creates gradients in air pressure, which in turn affects the strength of the winds in the jet stream. The greater the variance, the greater the wind speed. Jet stream winds normally are 100 to 200 mph but can reach speeds as high as 300 mph.
A jet stream develops where air masses of differing temperatures meet, so surface temperatures help determine where they will form. The jet stream is snakelike, undulating like a river, because of the pressures on either side from the warm and cold air masses.
When the jet stream is pushed south by a cold air mass, it allows high pressure to sink and create colder-than-normal weather in the South. In the opposite situation, when northern regions get warmer than normal, the jet stream has been pushed north by tropical air.
Jet streams travel from west to east in both hemispheres.
Jet streams were discovered in the 1920s by meteorologist Wasburo Ooishi, who was using weather balloons for his study of high elevation wind patterns over Japan. In 1939, German meteorologist H. Seilkopf was the first to use the term "jet stream" in a published scientific paper.
However, it wasn't until World War II, when the Japanese used the jet stream for fire balloon attacks on the American mainland, that the upper-level winds gained public recognition. Wiley Post, an American aviation pioneer, is credited with being the first person to fly within a jet stream. He and other WWII military pilots flying Boeing B-29 Superfortress bombers found the jet stream made high altitude flights difficult.
Currents travel at varying wind speeds within a jet stream, with the greatest speeds at the core. A jet stream contains jet streaks, where the wind velocity is higher than the rest of the stream. The jet streaks cause air to rise, lowering the air pressure at the Earth's surface. When surface pressures are low, the rising air can form clouds, precipitation and storms.
Jet streams play a major role in air travel. Eastbound flights usually take less flying time than westbound flights because of help from the fast-moving air. Jet streams can contain wind shear, a violent and sudden change in wind direction and speed, which is a major threat in air travel. Wind shear has caused airliners to suddenly lose altitude, putting them in danger of crashing. In 1988, the FAA decided that all commercial aircraft must have wind-shear warning systems, but it wasn't until 1996 that all airlines had them on-board.
The most common jet streams in the United States are the polar and subtropical jets. Polar streams occur year-round, but subtropical streams are formed during the winter when there are greater differences in temperature.
A jet stream can occur anywhere in the atmosphere where winds exceed about 57 mph. A tropical easterly jet stream occurs between the equator and India during the months of July and August. There are also low-level jet streams that travel at slower speeds and occur at lower levels than the major jet streams.
