Jet stream
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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, just under the tropopause. 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 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.
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[edit] 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, 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 55 km/h (35 mph) in summer and 120 km/h (75 mph) in winter, although speeds of over 400 km/h (250 mph) are known. Technically, the wind speed has to be higher than 111 km/h (60 Kts) 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.
[edit] Cause
Jet streams can be explained as follows. In general, winds are strongest just under the tropopause (except during tornados, 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 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.
[edit] Discovery
The jet streams were first noticed by atmospheric scientists in the 19th century using kites and, later, pilot balloons, but before widespread aviation the so-called "high winds" (or "strong westerlies") were of little interest, and many observers thought that individual observations were simply freak occurrences.
The first scientist to quantify jet streams was Japanese meteorologist Wasaburo Ooishi in the early 1920s by tracking weather balloons at a site near Mount Fuji. Between 1923 and 1925, Ooishi measured stratospheric westerlies over Japan at consistent speeds in all seasons. Although Ooishi had contacts with the International Meteorological Organization (IMO, now the WMO) and had traveled to Germany and the United States, his published work went largely unnoticed outside of Japan as he chose to write in the international language of Esperanto, which only had a small following in scientific circles, primarily among Asians like Ooishi. His observations were utilised during World War II by the Japanese military in the fire balloon attacks on the American mainland, although the Japanese scientist on the project, Hidetoshi Arakawa, doubted that Ooishi's measurements could be confidently projected across the entire Pacific Ocean.
In the 1930s, without Ooishi's data, international knowledge of the "high winds" grew slowly. The American aviator Wiley Post, who was interested in the low-friction environment of the stratosphere for increasing aircraft speed and range, worked for several years to perfect a rubber pressure suit that would allow him to breathe at higher altitudes.[1] In a flight across Siberia in the late 1920s, Post had flown high to avoid mountains due to poor maps, and had encountered a "strong river of air". On December 7, 1934, one of his test flights took him above 20,000 feet, where he found a strong tailwind. Because of this, Post has been widely credited with the discovery of the jet stream. In 1935, IMO member countries in Europe cooperated in an upper atmosphere study that was intended to help with cyclone prediction. The data show evidence of the jet stream, but were not recognized at the time for what they were. German meteorologist Richard Scherhag summed up the scientific view at the time by asking, "Why is there no front in the upper air?", in part based on the 1935 observations. His colleague H. Seilkopf is credited with the coining of the term "jet stream" (Strahlströmung) in a 1939 paper.
The jet streams finally became a major factor for aviation during World War II high-altitude aerial bombing. In 1944, a United States Army Air Force Boeing B-29 Superfortress bomber squadron encountered Ooishi's westerlies between Kyoto and Tokyo, measuring a 140-knot tailwind, and found the winds made precision bombing at those heights almost impossible. C.-G. Rossby independently coined the English term "jet stream" to describe these westerlies.
The general idea remained poorly understood and still had anecdotal qualities. In 1947, the Star Dust crash in the Andes Mountains probably resulted from this general ignorance. That same year, the theory of jet streams was explained by Erik Palmén and other members of the Chicago school of dynamical meteorologists, in a groundbreaking paper credited to "Staff members", and by the 1950s was widely accepted.
[edit] 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 about the same amount if it must fly west against it. On international flights, the difference is even greater, and it is often actually faster and cheaper flying eastbound along the jet stream rather 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. Jet streams also play an important part in the creation of supercells, the storm systems which create tornados.
[edit] See also
- Low-level jet
- Rossby waves
- Surface weather analysis
- Tornadoes
- Wind shear
- World War II
