Definition

Jet Stream is defined as a flat tubular current of air, quasi-horizontal, whose axis is along a line of maximum speed and which is characterised not only by great speeds but also by strong transverse (horizontal and vertical) gradients of speed (World Meteorological Organisation).

Description

The two major areas of Jet Stream activity in the upper troposphere are the Sub-Tropical Jet Streams near the 30 degree latitudes and the Polar Jet Streams in mid-latitudes. Both are predominantly west-east. There is also a jet stream of predominantly east-west flow over equatorial land masses, and a belt of strong winds in the stratosphere over the winter pole, the "polar night jet", at high latitudes and above FL600, or the 50 hPa level.