Jet stream
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The main jet streams flow from the west in the upper atmosphere |
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; this is due to the
Coriolis effect caused by Earth's rotation. The paths of the flows typically show a meandering shape, and these shapes themselves propagate east, at lower speeds than that of the actual
wind within the flow.
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 is also the Equatorial Easterly Jet which occurs during the Northern Hemisphere
summer between 10°N and 20°N.
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), which is the result of massive disturbances of air, 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.
Jet streams can be explained as follows. In general, winds are strongest just under the tropopause (except during
tornados,
hurricanes or other exceptional situations). If two air masses of different temperatures meet, the resulting pressure difference (which causes wind) is highest at those altitudes. If one of the air masses lies north of the other one, then the wind will not flow directly from the hot to the cold area as one would expect, but is deflected by the
Coriolis force and flows along the boundary of the two air masses.
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" (
Strahlströmmung) in a 1939 paper.
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Airline great circle track at top, jet stream track at bottom |
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.
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.
An unrelated weather phenomenon is the
low-level jet which forms above a
temperature inversion in the lower atmosphere.
*
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
