Jet streams are wind streams that reach great speeds in narrow zones at a high altitude. They occur where atmospheric pressure gradients are strong. Along a jet stream, the air moves in pulses along broadly curving tracks. The greatest wind speeds occur in the center of the jet stream, with velocities decreasing away from it.
There are three kinds of jet streams. Two are westerly streams, and the third is a weaker jet with easterly winds that develops in Asia as part of the summer monsoon circulation. The most poleward type of jet stream is located along the polar front. It is called the polar-front jet stream (or simply the “polar jet”). The polar jet is generally located between 35° and 65° latitude in both hemispheres. It follows the boundary between cold polar air and warm subtropical air). It is typically found at altitudes of 10 to 12 km, and wind speeds in the jet range from 75 to as much as 125 m/s.
The subtropical jet stream occurs at the tropopause, just above subtropical high-pressure cells in the northern and southern hemispheres. There, westerly wind speeds can reach 100 to 110 m/s, associated with the increase in velocity that occurs as an air parcel moves poleward from the Equator.
The tropical easterly jet stream occurs at even lower latitudes. It runs from east to west—opposite in direction to that of the polar-front and subtropical jet streams. The tropical easterly jet occurs only in summer and is limited to a northern hemisphere location over Southeast Asia, India, and Africa.