Jet stream

An intense, narrow air current that flows along a nearly horizontal axis in the vicinity of the tropopause. In its central part it reaches the maximum wind speed, never below 110 km/h and frequently exceeding 200 km/h. Jet streams can reach several thousand kilometers in length and usually take on a sinusoidal shape. Their width is several hundred kilometers and their thickness a few kilometers. Around their core there is strong horizontal and vertical wind shear, which generates areas of strong turbulence. Commercial aircraft manage to save fuel and shorten the duration of their journeys by flying, with a tailwind, in the core of the jet. The only precaution pilots must take is to penetrate that central part while avoiding passing through the peripheral zones where turbulence is dangerous. To that end, maps are drawn up daily with very precise information on the position and altitude that the various enroute jet streams are expected to occupy. Their position and orientation vary from one day to another. Occasionally, one of these currents arises further down in the atmosphere (low-level jet). On Earth there are five main jet streams. Due to the spherical symmetry of our planet, in each terrestrial hemisphere there is a polar jet and a subtropical one (both westerly), while at the equator an easterly jet stream flows. Both in the Arctic and in Antarctica the so-called stratospheric jets sometimes arise. Jet streams were discovered by the Japanese meteorologist Wasaburo Oishi (1874-1950) in 1926, although the first scientific reference is later; due to the German meteorologist Heinrich Seilkopf (1895-1968), who in 1939 used the expression strahlströmung ("jet stream") to refer to them.