A jet stream is a narrow channel of strongest winds near the tropopause (Section 12.4). A typical example is a few thousand kilometres long and a kilometre or two deep within the upper westerlies, with winds faster than 30 m/s (108 km/h). It is only about a hundred kilometres wide at any moment, but meanders from high latitude to low and back within the Rossby waves, and covers a range of latitudes within a month, so that monthly mean averages of wind speed show a broad band of strong wind (Figure 12.10). The instantaneous position of a jet stream is often indicated by a characteristic long band of cirrus.
Jet streams are discontinuous and vary with the weather. They may slow down, split or join, and contain patches, called jet streaks, where winds exceed 50 m/s (180 km/h), sometimes reaching twice that. Jet streams were first discovered in the course of high-altitude flights to Japan during wartime in the early 1940s. A pilot going in the same direction tries to catch a ride for extra speed, whilst one flying in the opposite direction steers well clear.
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