Jet streams

Jet streams are particular evolutions of high altitude winds. They are rapidly moving air currents caused by pressure differences that result from the temperature divergences that occur when big air masses meet. They were discovered fortuitously by American aeroplanes flying towards Japan during the Second World War. To be called a jet stream, the wind speed has to be higher than 50 knots, about 90 km/h, however, jet streams usually have much greater speeds ranging from 160 to 250 km/h, with peaks of 320 km/h. Generally, these winds are stronger during the winter because temperature differences are more marked. They form at altitudes of about 10-14 km. They tend to form at the boundaries between hot and cold masses of air, along the so-called fronts: in these areas distinct variations in the isothermal and isobaric surface gradients can be found and it is along these surfaces that the winds tend to move with greater speed. Jet streams surround the globe forming kinds of ‘belts’ around the planet following the parallels. Their trend is not straight (see graph “Jet streams” for more information) and this causes descending movements towards the equatorial zones (wave troughs) and ascending movements towards the Poles (wave ridges). Ridges and troughs move, change and evolve continually.
The effects of jet streams
In our hemisphere, there are two main jet streams in winter: the Polar Front jet stream, above Canada and northern United States, and the Subtropical jet stream, over northern Mexico. Both these winds move from West to East. A third wind, that moves from East to West, forms in summer, instead, over Africa and India and is partly responsible for the summer monsoon. In the Southern Hemisphere, on the other hand, only two westerly jet streams are present.
Jet streams influence air navigation, making the same route take more or less time depending on whether the aeroplane travels with a head or tail wind: the return journey from the United States towards Italy is generally approximately one hour shorter respect to the outward trip. But the most important feature of jet streams resides in their influence on climate. Where there are wave troughs, in fact, high pressure areas form; where there are the so-called regions of convergence, there are zones where the speed of the flow decreases. As a result of the Coriolis effect, the latter are transformed into anticyclonic cells, with dry, fair weather at ground level. On the contrary, where there are wave ridges, a low pressure area forms; where there are regions of divergence, there are areas where the speed of flow increases. These zones are transformed into cells with a cyclonic circulation that bring bad weather at ground level.

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