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, 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. 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 explanantion for propagation of the meanders; Rossby waves propagate westward with respect to the flow in which they are imbedded.
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 latitude 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, but these are not typically included in the term jet "streams".
The wind speeds vary according to the temperature gradient, averaging 55 km/h or 35 mph in summer and 120km/h or 75 mph in winter, although speeds of over 400km/h or 250 mph are known. Technically the wind speed has to be higher than 90km/h or 55 mph 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.
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. 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" (Strahlströhmung) in a 1939 paper.
The jet streams finally became a major factor for aviation during World War II high-altitude aerial bombing. A 1943 Royal Air Force raid on Gironde, France, encountered tailwinds that sped them to their target, but returning the same headwinds, estimated at 380 km/h, caused their aircraft to stall, and the crews were forced to parachute into occupied Vichy France, where they were captured. 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.
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 super cells, the storm systems which create tornados.
See also
An unrelated weather phenomenon is the low-level jet which forms above a temperature inversion in the lower atmosphere.