Extratropical cyclone

Japanese: 温帯低気圧 - おんたいていきあつ（英語表記）extratropical cyclone

A low pressure system that occurs and develops in mid- and high-latitudes. The wavelength from east to west is several thousand km. In the atmosphere on a rotating sphere like the Earth, when there is a temperature gradient from north to south, there is a vertical shear of the mean flow (the rate at which the wind speed and direction change in a certain direction, for example vertically or horizontally). This state is fluid-dynamically unstable, and waves (vortices) with wavelengths of several thousand km develop, which have the highest development rate (wave strength doubles in 2 to 3 days). This is the development mechanism of mobile high and low pressure systems in mid- and high-latitudes, and is called baroclinic instability. Along with such waves (vortices), warm air in the south rises and moves north, and cold air in the north descends and moves south. In other words, when warm and light air in the south and cold and heavy air in the north are lined up at the same altitude at first, they have excess potential energy, but during the process of unstable wave development, some of the potential energy is released and converted into kinetic energy, further strengthening and developing the vortex. These vortex motions exchange air between the north and south, transporting heat from the equator to the poles, creating the north-south temperature gradient in the mid-latitudes today. In other words, these high- and low-pressure vortices can be called horizontal convection. The theory of baroclinic instability, or the development of extratropical cyclones, was independently developed by Jules Charney in the United States in 1947 and Eric Eady in the United Kingdom in 1949, and led to the development of meteorological dynamics after World War II.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

中・高緯度で発生，発達する低気圧。東西の波長は数千km。地球のような回転球面上の大気では，南北に温度傾度があると鉛直に平均流のシアー（風速，風向がある方向，たとえば垂直または水平に変化している割合）が存在する。このような状態は，流体力学的に不安定で，最も発達率の大きい（2～3日で波の強さが 2倍になる）波長数千kmの波動（渦）が発達する。これが中・高緯度の移動性高・低気圧の発達機構で傾圧不安定性といわれる。このような波動（渦）に伴って南側の暖気は上昇しつつ北上し，北側の冷気は下降しつつ南下する。すなわち，最初同じ高度に南側の暖かくて軽い空気と，北側の冷たく重い空気が並んでいる状態は位置のエネルギーを余分にもっているが，不安定波動発達の過程で位置のエネルギーは一部解放されて，運動エネルギーに変わり，さらに渦を強め発達する。これらの渦運動は南北の空気を入れ替えることによって，熱を赤道側から極側に運び，今日の中緯度における南北温度傾度を実現している。つまり，この高・低気圧渦は水平対流ということができる。傾圧不安定理論，すなわち温帯低気圧の発達理論は，1947年アメリカ合衆国のジュール・チャーニー，1949年イギリスのエリック・イーディにより，それぞれ独立に完成され，第2次世界大戦後の気象力学の発達につながった。

出典　ブリタニカ国際大百科事典小項目事典ブリタニカ国際大百科事典小項目事典について　情報

<<: Ondake - Ondake

>>: Temperate rainy climate - Ontai tau ki ko