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Jet streams are fast flowing, narrow air currents found in the upper atmosphere or in troposphere of some planets, including Earth. The main jet streams are located near the altitude of the tropopause. The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet. The strongest jet streams are the polar jets, at around above sea level, and the higher and somewhat weaker subtropical jets at around . The Northern Hemisphere and the Southern Hemisphere each have a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round. Jet streams are caused by a combination of a planet's rotation on its axis and atmospheric heating (by solar radiation and, on some planets other than Earth, internal heat). Jet streams form near boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air towards the equator.〔(【引用サイトリンク】University of Illinois "> url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/jet.rxml )〕 Other jet streams also exist. During the Northern Hemisphere summer, easterly jets can form in tropical regions, typically in a region where dry air encounters more humid air at high altitudes. Low-level jets also are typical of various regions such as the central United States. Meteorologists use the location of some of the jet streams as an aid in weather forecasting. The main commercial relevance of the jet streams is in air travel, as flight time can be dramatically affected by either flying with the flow or against the flow of a jet stream. Clear-air turbulence, a potential hazard to aircraft passenger safety, is often found in a jet stream's vicinity, but it does not create a substantial alteration on flight times. ==Discovery== After the 1883 eruption of the Krakatoa volcano, weather watchers tracked and mapped the effects on the sky over several years. They labelled the phenomenon the "equatorial smoke stream".〔See: # Bishop, Sereno E. (17 January 1884) "Letters to the Editor: The remarkable sunsets," ''Nature'', 29: 259-260; on (page 260 ), Bishop speculates that a rapid current in the upper atmosphere was carrying the dust from the eruption of Krakatau westward around the equator. # Bishop, S.E. (May 1884) ("The equatorial smoke-stream from Krakatoa," ) ''The Hawaiian Monthly'', vol. 1, no. 5, pages 106-110. # Bishop, S.E. (29 January 1885) ("Letters to the Editor: Krakatoa," ) ''Nature'', vol. 31, pages 288-289. # Rev. Sereno E. Bishop (1886) "The origin of the red glows," ''American Meteorological Journal'', vol. 3, pages (127-136 ), (193-196 ); on pages 133-136, Bishop discusses the "equatorial smoke stream" that was produced by the eruption of Krakatau. # Hamilton, Kevin (2012) ("Sereno Bishop, Rollo Russell, Bishop's Ring and the discovery of the "Krakatoa easterlies"," ) ''Atmosphere-Ocean'', vol. 50, no. 2, pages 169-175. # Krakatoa Committee of the Royal Society (London ), ''The Eruption of Krakatoa and Subsequent Phenomena'' (London, England: Harrison and Sons, 1888). Evidence of an equatorial high-speed, high-altitude current (actually, the quasi-biennial oscillation) is presented in the following sections: :: * Part IV., Section II. General list of dates of first appearance of all the optical phenomena. By the Hon. Rollo Russell., (pages 263-312 ). :: * Part IV., Section III. (A). General geographic distribution of all the optical phenomena in space and time; including also velocity of translation of smoke stream. By the Hon. Rollo Russell., (pages 312-326 ). :: * Part IV., Section III. (B). The connection between the propagation of the sky haze with its accompanying optical phenomena, and the general circulation of the atmosphere. By Mr. E. Douglas Archibald., (pages 326-334 ); that Rev. S.E. Bishop of Honolulu first noticed a westward circulation of dust from Krakatau is acknowledged on page 333. :: * Part IV., Section III. (C). Spread of the phenomena round the world, with maps illustrative thereof. By the Hon. Rollo Russell., (pages 334-339 ); after page 334 there are map inserts, showing the progressive spread, along the equator, of the dust from Krakatau.〕 In the 1920s, a Japanese meteorologist, Wasaburo Oishi, detected the jet stream from a site near Mount Fuji.〔John M. Lewis (2003) ("Oishi's Observation: Viewed in the Context of Jet Stream Discovery," ) ''Bulletin of the American Meteorological Society'', vol. 84, pages 357–369.〕〔Ooishi, W. (1926) ''Raporto de la Aerologia Observatorio de Tateno'' (in Esperanto). Aerological Observatory Report 1, Central Meteorological Observatory, Japan, 213 pages.〕 He tracked pilot balloons, also known as pibals (balloons used to determine upper level winds),〔Martin Brenner. (Pilot Balloon Resources. ) Retrieved on 13 May 2008.〕 as they rose into the atmosphere. Oishi's work largely went unnoticed outside Japan because it was published in Esperanto. American pilot Wiley Post, the first man to fly around the world solo in 1933, is often given some credit for discovery of jet streams. Post invented a pressurized suit that let him fly above . In the year before his death, Post made several attempts at a high-altitude transcontinental flight, and noticed that at times his ground speed greatly exceeded his air speed.〔Acepilots.com. (Wiley Post. ) Retrieved on 8 May 2008.〕 German meteorologist Heinrich Seilkopf is credited with coining a special term, ''Strahlströmung'' (literally "jet streaming"), for the phenomenon in 1939.〔Seilkopf, H., ''Maritime meteorologie'', which is volume II of: R. Habermehl, ed., ''Handbuch der Fliegenwetterkunde'' (of Aeronautical Meteorology ) (Berlin, Germany: Gebrüder Radetzke (Brothers ), 1939); Seilkopf coins the word "Strahlströmung" on page 142 and discusses the jet stream on pages 142-150.〕〔Arbeiten zur allgemeinen Klimatologie By Hermann Flohn (p. 47 )〕 (Modern German usage is ''"Strahlstrom"''.) Many sources credit real understanding of the nature of jet streams to regular and repeated flight-path traversals during World War II. Flyers consistently noticed westerly tailwinds in excess of in flights, for example, from the US to the UK. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「jet stream」の詳細全文を読む スポンサード リンク
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