Jet streams are fast flowing, relatively narrow air currents found at the tropopause, the transition between the troposphere (where temperature decreases with height) and the stratosphere (where temperature increases with height), and are located at 10-15 kilometers above the surface of the Earth. They form near boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air to the south. The path of the jet typically has a meandering shape, and these meanders are one manifestation of Rossby waves. Rossby waves propagate westward with respect to the flow in which they are embedded, which translates to a slower eastward migration across the globe than smaller scale short wave troughs. The major jet streams are westerly winds (flowing west to east) in the Northern Hemisphere.
During the summer, low-level easterly jets can form in tropical regions. A southerly low level jet in the Great Plains of North America helps fuel overnight thunderstorm activity, normally in the form of mesoscale convective systems. A similar northerly low-level jet can form across Australia, instigated by cut-off lows which develop across southwest portions of the country.
Meteorologists use the location of the jet stream as an aid in weather forecasting. The main commercial use of the jet stream is during airline travel, as flight time can be dramatically affected by either flying with or against the stream. One type of clear-air turbulence is found in the jet stream's vicinity, which can be a hazard to aircraft. One future benefit of the jet stream could be to power airborne wind turbines, if technological hurdles can be overcome.
There are two main jet streams north of subtropical latitudes, with a weaker subtropical stream closer to the equator. The polar jet stream is typically located near the 250 hPa (7.38 inHg) pressure level, or to above sea level, while the subtropical jet is much higher, between and above sea level. Both upper-level jet streams form near breaks in the tropopause, which is at a higher altitude near the equator than it is over the poles, with large changes in its height occurring near the location of the jet stream. The streams are most commonly found between latitudes 30°N and 60°N, with the subtropical jet stream located close to latitude 30°N. The upper level jet stream is said to "follow the sun" as it moves northward during the warm season, or late spring and summer, and southward during the cold season, or autumn and winter.
Jet streams are typically continuous over long distances, but discontinuities are common. The path of the jet typically has a meandering shape, and these meanders themselves propagate east, at lower speeds than that of the actual wind within the flow. Each large meander, or wave, within the jet stream is known as a Rossby wave. Rossby waves are caused by changes in the Coriolis effect with latitude, and propagate westward with respect to the flow in which they are embedded, which slows down the eastward migration of upper level troughs and ridges across the globe when compared to their embedded shortwave troughs. Shortwave troughs are smaller packets of upper level energy, on the scale of to long, which move through the flow pattern around large scale, or longwave, ridges and troughs within Rossby waves. Jet streams can split into two due to the formation of an upper-level closed low, which diverts a portion of the jet stream under its base, while the remainder of the jet moves by to its north.
The wind speeds vary according to the temperature gradient, exceeding , although speeds of over have been measured. Meteorologists now understand that the path of the jet stream steers cyclonic storm systems at lower levels in the atmosphere, and so knowledge of their course has become an important part of weather forecasting. For example, in 2007, Britain experienced severe flooding as a result of the polar jet staying south for the summer.
In general, winds are strongest just under the tropopause (except during tornadoes, hurricanes or other anomalous situations). If two air masses of different temperatures or densities meet, the resulting pressure difference caused by the density difference (which causes wind) is highest within the transition zone. The wind does not flow directly from the hot to the cold area, but is deflected by the Coriolis effect and flows along the boundary of the two air masses. The polar front and subtropical jets merge at some locations and times, while at other times they are well separated.
All these facts are consequences of the thermal wind relation. The balance of forces on an atmospheric parcel in the vertical direction is primarily between the pressure gradient and the force of gravity, a balance referred to as hydrostatic. In the horizontal, the dominant balance outside of the tropics is between the Coriolis effect and the pressure gradient, a balance referred to as geostrophic. Given both hydrostatic and geostrophic balance, one can derive the thermal wind relation: the vertical derivative of the horizontal wind is proportional to the horizontal temperature gradient. The sense of the relation is such that temperatures decreasing polewards implies that winds develop a larger eastward component as one moves upwards. Therefore, the strong eastward moving jet streams are in part a simple consequence of the fact that the equator is warmer than the north and south poles.
The thermal wind relation does not immediately provide an explanation for why the winds are organized in tight jets, rather than distributed more broadly over the hemisphere. There are two factors that contribute to this sharpness of the jets. One is the tendency for developing cyclonic disturbances in midlatitudes to form fronts. A front is a sharp localized gradient in temperature. The polar front jet stream can be thought of as the result of this frontogenesis process in midlatitudes, as the storms concentrate the north-south temperature contrast into relatively narrow regions.
An alternative explanation is more appropriate for the subtropical jet, which forms at the poleward limit of the tropical Hadley cell. One can visualize this circulation as being symmetric with respect to longitude. Rings of air encircling the Earth move polewards beneath the tropopause from the equator into the subtropics. As they do so they tend to conserve their angular momentum. But they are also moving closer to the axis of rotation, so they must spin faster in the direction of rotation, implying an increased eastward component of the winds.
Jupiter's atmosphere has multiple jet streams, forming the familiar banded color structure, caused by internal heating. The factors that control the number of jet streams in a planetary atmosphere is an active area of research in dynamical meteorology. In models, as one increases the planetary radius, holding all other parameters fixed, the number of jet streams increases.
The location of the jet stream is extremely important for airlines. Commercial use of the jet stream began on November 18, 1952, when Pan Am flew from Tokyo to Honolulu at an altitude of . It cut the trip time by over one-third, from 18 to 11.5 hours. Not only does it cut time off the flight, it also nets fuel savings for the airline industry. Within North America, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream, or increased by more than that amount if it must fly west against it.
Associated with jet streams is a phenomenon known as clear air turbulence (CAT), caused by vertical and horizontal windshear connected to the jet streams. The CAT is strongest on the cold air side of the jet, next to and just underneath the axis of the jet. Clear air turbulence can be hazardous to aircraft, and has caused fatal accidents, such as with BOAC Flight 911 and United Airlines Flight 826.
The changing of the normal location of upper-level jet streams can be anticipated during phases of the El Niño-Southern Oscillation (ENSO), which leads to consequences precipitation-wise and temperature-wise across North America, affects tropical cyclone development across the eastern Pacific and Atlantic basins. Combined with the Pacific Decadal Oscillation, ENSO can also impact cold season rainfall in Europe. Changes in ENSO also change the location of the jet stream over South America, which partially effects precipitation distribution over the continent.