JET STREAM

'Jet streams' are fast flowing, relatively narrow air currents found in the atmosphere at around 11 kilometres (36,000 ft) above the surface of the Earth. They form at the boundaries of adjacent air masses with significant differences in temperature, such as of the polar region and the warmer air to the south. The jet stream is mainly found in the tropopause, at the transition between the troposphere (where temperature decreases with height) and the stratosphere (where temperature increases with height)^[1].
The major jet streams are westerly winds (flowing west to east) in both the Northern Hemisphere and the Southern Hemisphere, although in the summer, easterly jets can form in tropical regions. 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. The theory of Rossby waves provides the accepted explanation for propagation of the meanders; Rossby waves propagate westward with respect to the flow in which they are embedded, but relative to the ground, they migrate eastward across the globe.

Contents

Description

Cause

Uses

See also

References

External links

Description

There are two main jet streams at polar latitudes, one in each hemisphere, and two minor subtropical streams closer to the equator. In the Northern Hemisphere the streams are most commonly found between latitudes 30°N and 70°N for the polar jet stream, (pilots remember that like people they go north in the summer and south in the winter), and between latitudes 20°N and 50°N for the subtropical stream. There are other flows in the atmosphere that are referred to as jets, such as the Equatorial Easterly Jet which occurs during the Northern Hemisphere summer between 10°N and 20°N, and the nocturnal poleward Low-Level Jet in the Great Plains. These are formed because of heating of
Tibetan plateau and subsequent anticyclogenesis . The equatorward divergence takes the form of easterlies, embeded in which are easterly jets. This jet stream is considered to play a crucial role in the SW monsoon of south Asia.
Jet streams are typically continuous over long distances, but discontinuities are common. Occasionally, a jet stream can even split its flow or cut off into a closed circular flow.
The wind speeds vary according to the temperature gradient, averaging 30 knots (55 km/h / 35 mph) in summer and 65 knots (120 km/h / 75 mph) in winter, although speeds of over 215 knots (400 km/h / 250 mph) are known. Technically, the wind speed has to be higher than 60 knots (69 mph / 111 km/h) to be called a jet stream.
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, usually next to or just below the axis of the jet.

Cause

Jet streams can be explained as follows. In general, winds are strongest just under the tropopause (except during tornadoes, hurricanes or other anomalous situations). If two air masses of different temperatures meet, the resulting pressure difference (which causes wind) is highest along the interface. 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.
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.
The polar front and subtropical jets merge at some locations and times, while at other times they are well separated. Historically, it was originally thought that the polar front was a structure that had an existence independent of the cyclonic eddies that, it was suspected, form as instabilities on this front. The modern perspective is that the cyclonic eddies are best thought of as growing from the store of potential energy in the broad north-south temperature gradient by a process known as baroclinic instability, and that the resulting extratropical cyclones then concentrate the gradient into a front, thereby creating the polar front jet stream.
Jupiter's atmosphere has multiple jet streams, forming the familiar banded structure. 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.

Uses

The location of the jet stream is extremely important for airlines. In the United States and Canada, for example, 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. On longer intercontinental flights, the difference is even greater, it is faster and cheaper (by flying the pressure pattern) or flying eastbound along with the jet stream and flying around the jet stream going west bound, than taking the shorter great circle route between two points.
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. In 2007, Britain experienced severe flooding as a result of an unusual diversion in the path of the North Atlantic Jet Stream.^[2][3]
Jet streams also play an important part in the creation of supercells, the storm systems which create tornadoes.

See also

★ Rossby wave

★ Surface weather analysis

★ Tornado

★ Wind shear

★ Japanese balloon bombing of the United States during World War II

★ Clear-air turbulence (CAT)

References

1. US Dept. of Energy, Ask a Scientist. 26 June 2002. http://www.newton.dep.anl.gov/askasci/wea00/wea00135.htm
2. Why has it been so wet?
3. Blackburn, Mike; Hoskins, Brian; Slingo, Julia: Notes on the Meteorological Context of the UK Flooding in June and July 2007

External links

★ Graphics and movies of the current jet streams

★ Ooishi's Observation: Viewed in the Context of Jet Stream Discovery, John M. Lewis, National Severe Storms Laboratory, 2001