Jet streams are very fast moving air currents flowing in narrow paths in the Earth´s atmosphere. Jet streams are several hundreds of miles long and less than one mile thick. Jet streams form in a region between the troposphere and the stratosphere called the tropopause, situated between 10 and 14 km (6-9 miles) above the surface of the Earth. There are two jet streams that are usually used to forecast weather; the tropical jet stream located near 30 degrees latitude, and the polar jet stream located at 60 degrees latitude. Jet streams are produced by two air masses of contrasting temperature at the tropopause and the rotation of the Earth.
The polar and subtropical jet streams
Jet streams are created at the region in the tropopause where strong winds are deflected into narrow bands of fast moving winds. The polar jet stream, which is associated with the polar front, travels in a west to east direction. The polar jet stream becomes strongest in the winter when the variations in temperature along the polar front are greatest. The subtropical jet stream forms in a region between 20° and 30° latitude, where warm air is carried toward the pole by the Hadley cell. The polar jet stream forms at about 10 km (6.2 miles) of altitude, while the subtropical jet stream is produced at a higher altitude, close to 13 km (8.8 miles).
What causes jet streams
Warming at the equator produces a low pressure system, while at the poles excessive cooling produces a high pressure system. Cold surface polar air travels toward the equator driven by a horizontal pressure gradient, while warm air at the equator flows towards the poles. The warm air at the equator rises to the tropopause, where it is forced to move toward the poles. The Coriolis effect acts on the winds flowing toward the poles, deflecting them toward the right, creating westerly winds high in the atmosphere in both hemispheres. These westerly winds produce jet streams at 30° and 60° latitude, when these winds reach maximum speed.
Polar jet stream seasonal variations
Due to temperature variations along a cold front during summer and winter, the polar jet stream´s winds are greatest during the winter, making the jet stream move farther south, while in the summer, the polar jet stream´s winds are weaker, and are produced at higher latitudes on Earth. The subtropical jet stream forms on the winds flowing poleward in a Hadley cell, usually at higher latitudes than the polar jet stream. The warm air carried toward the poles by a Hadley cell creates temperature contrasts and strong winds.
Jet streams influence the transfer of heat globally. The zigzag path of the polar jet stream in the northern hemisphere moves cold air toward the equator and carries warm air toward the poles. Moreover, the wavy motion of jet streams distributes pollutants, volcanic ash and many other particles around the world. Sometimes, volcanic ash that is ejected in one part of the world is deposited thousands of kilometers away, carried by jet streams. Meteorologists use the jet streams, especially the polar jet stream, to forecast weather.
The polar and subtropical jet streams are the most frequently recognized jet streams in the media; however, there are other jet streams that form at other altitudes. A low level jet stream forms in the central plains of the U.S. during the summer. The tropical easterly jet is produced at the base of the tropopause at the subtropics, and the stratospheric polar jet forms at the top of the stratosphere during the dark polar winter. Squall.sfsu.edu contains an up to date forecast of the jet streams in both hemispheres.