High pressure - Koukiatsu

Japanese: 高気圧 - こうきあつ

At the same altitude, an area with higher pressure than the surrounding area is called a high pressure area, and an area with lower pressure is called a low pressure area. In this case, the pressure is relative, so there is no set rule that a certain number of hectopascals is high pressure and a certain number of hectopascals is low pressure. Depending on the surrounding conditions, there can be a low pressure area with a central pressure of 1020 hectopascals, and a high pressure area with a central pressure of 1010 hectopascals. This is similar to how, for example, the same height of 800 meters can become a mountain in the Kanto Plain, and a valley or basin in the Alps.

On a weather chart, high pressure areas are surrounded by a number of closed, roughly elliptical isobars, with the inner isobars indicating higher readings. Air in a high pressure area blows out, swirling clockwise from the center to the periphery across the isobars. This causes a decrease in the amount of air near the ground in the center, and a downdraft occurs to compensate. This downdraft makes it difficult for clouds to form in the high pressure area, and existing clouds disperse, resulting in generally clear skies. Above the high pressure area, air is blowing in from the surroundings to compensate for the downdraft, causing an increase in the amount of air. This increase in air volume is also caused by the formation or movement of a cold, heavy layer of air within the area. If this increase in air volume exceeds the decrease in the amount of air in the lower layers, the pressure in the lower layers of the area increases and the high pressure develops, or if the opposite is true, it weakens. Usually, when we talk about high pressure, we do not simply mean the formal pressure distribution, but rather the overall picture, including the wind system described above, its maintenance mechanism, and weather distribution.

Compared to low pressure areas, the pressure gradient within high pressure areas is gentler, and the winds are weak and unpredictable, especially near the center. The characteristics of a high pressure wind system (clockwise spiral blowing) are clearly evident in the distribution of wind in the peripheral areas.

[Atsushi Kurashima and Takashi Aoki]

kinds

High pressure systems are classified into various types based on their structure and movement, as follows:

(1) Warm highs and cold highs A high pressure area with a higher temperature than the surrounding area is called a warm high, and one with a lower temperature than the surrounding area is called a cold high.

(2) Tall and Short Highs The higher you go, the clearer the shape of the high pressure system becomes. When the high pressure system is high even above the middle of the troposphere (5-10 kilometers or higher), it is called a "tall high pressure system." Conversely, when the shape of the high pressure system becomes unclear the higher you go, and when it becomes a low pressure system or a trough above a certain height (usually about 3 kilometers), it is called a "short high pressure system." Generally, warm high pressure systems are "tall high pressure systems" and cold high pressure systems are "short high pressure systems." In warm high pressure systems, the mechanism that causes the increase in surface pressure is in the upper troposphere, and the warm part is formed by descending air currents. On the other hand, in cold high pressure systems, the increase in surface pressure can be considered to be maintained by the weight of a cold air mass about 3 kilometers thick above the earth's surface.

(3) Mobile highs and stationary highs A mobile high is one that moves in roughly the same direction with a relatively regular pattern, whereas a stationary or stationary high is one that covers roughly the same area for several days to several weeks. Mobile highs often move eastward in the mid-latitudes with the westerly winds above them, and the eastern half of the highs has a low-profile cold high-pressure system. On the other hand, the western half has a high-profile warm high-pressure system with a high-profile ridge. Mobile highs, together with preceding and succeeding lows, often create a weather distribution as shown in the model in Figure A. As can be seen from this figure, when a mobile high passes south of a certain point (point A), the period of clear weather at that point is long, whereas when it passes north (point B), the clear weather at that point does not last long and it soon becomes cloudy or rainy. In the front half of the mobile high pressure system (especially the northeastern part), cold air masses flow in and radiation cooling is strong at night on clear days, making crops susceptible to damage from late frost and early frost in spring and autumn. In the lightly cloudy areas behind the mobile high pressure system, "sunshine" and "moonlight" appear, or a "hazy moon" appears, which is said to be unique to spring scenery. And as the weather distribution as shown in Figure A moves eastward as a whole, a periodicity appears in the weather changes.

Representative examples of stationary high pressure systems include subtropical high pressure systems (such as the Ogasawara High) ( Figure B ). These are tall warm high pressure systems formed by the general atmospheric circulation between the subtropics and the equator. Continental high pressure systems, such as the Siberian High, are also stationary high pressure systems ( Figure C ). These are large high pressure systems formed on continents by stationary long-wavelength westerly wind waves that appear in the westerly wind belt in the sky due to the influence of land-sea distribution and large topography (such as the Tibetan-Himalayan massif and the Rocky Mountains), and cold air masses created by radiative cooling of the continent's land surface during the cold season. Structurally, the eastern half of the system is a low-profile cold high pressure system, and the western half is a tall, relatively warm high pressure system. Within the zone of stationary high pressure systems, the atmosphere is in contact with the continent or ocean for a relatively long period of time, so the properties become almost uniform over a wide area, and air masses are formed. In addition, within a stationary high pressure area, sunny days with little wind continue for several days, causing the air near the ground surface to stagnate and creating a temperature inversion layer near the ground surface that weakens the diffusion phenomenon, which can lead to serious air pollution in areas where there are pollution sources.

(4) Blocking high pressure When the westerly wind waves in the sky become stationary, their amplitude increases, and the waves become unstable, a warm cut-off high pressure system separated from the westerly wind belt is formed in the north, and a cold cut-off low pressure system is formed in the south ( Figure D ). In such cases, the high pressure system on the ground below the cut-off high pressure system in the sky also becomes extremely stationary, stopping the movement of the following moving high pressure system and low pressure system. As a result, in sunny areas, this continues for a long time, sometimes resulting in drought, and conversely, in rainy areas, it becomes prolonged rain, and furthermore, air masses from the north and south are mixed on a large scale, often resulting in abnormal cold waves and abnormal heat waves. This phenomenon is called blocking, and the cut-off high pressure system in the sky and the surface high pressure system below it are called blocking high pressure systems. Blocking high pressure systems are tall, warm, stationary high pressure systems. The Okhotsk Sea high that often appears during the rainy season is often accompanied by a ridge or cut-off high caused by westerly wind waves that have increased in amplitude and become stagnant, and has strong characteristics of a blocking high.

(5) Tibetan High The high pressure system that appears in the upper troposphere (8-15 km) above the Tibetan Plateau in summer is called the Tibetan High. It is formed when the Tibetan and Himalayan mountains are heated by strong solar radiation in summer, warming the air above. The heated air column extends upward. If we consider a specific altitude in the sky, the total weight of the atmosphere above that surface will increase in the area where the air column extends upward. This increase forms a high pressure system in that area on that surface. The rise and fall of the Tibetan High is thought to be closely related to Japan's rainy season, summer drought, cold damage, and the formation of the rainy season in Southeast Asia.

(6) Local high pressure In inland basins, cold air formed by radiation cooling accumulates at night, making the air pressure in the basin higher than the surrounding area and creating a small local high pressure area. Conversely, during the day, the air in the basin becomes particularly warm, and this often turns into a local low pressure area.

(7) Thunderstorm high: A localized high pressure system formed by the weight of cold air under a thundercloud. The cool breeze felt before and after the passage of a thundercloud comes from this high pressure system. A protrusion called a "thunderstorm nose" that appears on the pressure curve at the point where a thundercloud has passed indicates the passage of a thunderstorm high.

[Atsushi Kurashima and Takashi Aoki]

"Lectures on Marine Meteorology" by Fukuchi Akira (1994, Seizando Bookstore)" ▽ "New Edition Lectures on Earth Science Education 14: The Atmosphere and Its Movement" by Maruyama Taketo, Mizuno Ryo, and Muramatsu Teruo (1995, Tokai University Press)" ▽ "The ABCs of Marine Meteorology" by Fukutani Tsuneo (1997, Seizando Bookstore)" ▽ "Weather Class for a Million People" by Shiraki Masanori (2007, Seizando Bookstore)"

Mobile high pressure and weather distribution (Figure A)
©Shogakukan ">

Mobile high pressure and weather distribution (Figure A)

Subtropical high pressure (Figure B)
©Shogakukan ">

Subtropical high pressure (Figure B)

Siberian High (Figure C)
©Shogakukan ">

Siberian High (Figure C)

Blocking high pressure (Figure D)
©Shogakukan ">

Blocking high pressure (Figure D)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

同一高度面で、周囲に比べて気圧の高い区域を高気圧、低い区域を低気圧という。この場合の気圧の高低は相対的なものであるから、何ヘクトパスカル以上が高気圧、以下が低気圧という定めはない。周囲の状況により、中心気圧が1020ヘクトパスカルの低気圧もあれば、1010ヘクトパスカルの高気圧もありうる。これは、たとえば同じ800メートルの高さが関東平野では山になり、アルプス山中では谷や盆地になるのに似ている。

　天気図上では高気圧は何本もの閉じたほぼ楕円(だえん)形の等圧線に囲まれており、内側の等圧線ほど示度が高くなっている。高気圧域内の空気は、その等圧線を横切って中心部から周囲に向かい時計回りに渦を巻きながら吹き出す。このため中心部の地表付近では空気量の減少がおこり、それを補うように下降気流が生じる。この下降気流により、高気圧域内では雲は形成されにくく、すでにできていた雲も消散して、一般に晴天が卓越する。高気圧の上空では下降気流を補うように周囲から空気が吹き込むなど、空気量の増大がおこっている。この空気量の増大は、冷たく重い空気層が域内で形成されたり、域内に移動してくることによっても行われる。そして、そのような空気量の増大が、下層の空気量の減少を上回ると、域内の下層の気圧が高くなって高気圧は発達し、逆ならば衰弱する。通常、高気圧という場合は、単に形式的な気圧分布をさすのではなく、前述のような風系と、その維持機構や天気分布などの全体像をさすことが多い。

　低気圧に比べると高気圧域内では気圧傾度が緩く、とくに中心近くでは風が弱く風向は不定で、高気圧としての風系の特徴（時計回りの渦巻状の吹き出し）は、周辺部の風の分布によく現れている。

［倉嶋　厚・青木　孝］

種類

高気圧は構造や動きなどによって、次のように、さまざまに分類される。

(1)温暖高気圧と寒冷高気圧　高気圧域内の気温が周囲よりも高い高気圧を温暖高気圧、周囲よりも低いものを寒冷高気圧という。

(2)背の高い高気圧と背の低い高気圧　上空へいくほど高気圧の形が明瞭(めいりょう)になり、対流圏の中部より上空（5～10キロメートルまたはそれ以上）でも高気圧であるような場合、これを「背の高い高気圧」という。逆に、上空へいくほど高気圧の形が不明瞭になり、ある高さ（通常約3キロメートル）以上になると低気圧や気圧の谷になってしまう高気圧を「背の低い高気圧」という。一般に温暖高気圧は「背の高い高気圧」、寒冷高気圧は「背の低い高気圧」である。温暖高気圧では、地上気圧の増大をおこす機構は対流圏上部にあり、温暖部分は下降気流によって形成される。一方、寒冷高気圧においては、地上気圧の増大は、地表から3キロメートルぐらいの厚さの寒冷気団の重さによって維持されている、とみなすことができる。

(3)移動性高気圧と停滞性高気圧　ほぼ同じ方向に比較的規則正しく動くものを移動性高気圧といい、これに対し、ほぼ同じ地域を数日から数週間にわたって覆うものを停滞性高気圧または定常高気圧という。移動性高気圧は、上空の偏西風波動とともに中緯度帯を東進するものが多く、その東半分の上空には偏西風波動の気圧の谷が存在し、したがって背の低い寒冷高気圧型の構造をもっている。一方、西半分の上空には偏西風波動の気圧の尾根があり、背の高い温暖高気圧型の構造となっている。移動性高気圧は先行および後続の低気圧とともに、図Aにモデル的に示すような天気分布をつくりだすことが多い。この図からわかるように、移動性高気圧が、ある地点に対して南偏して通るときは（A地点の場合）、その地点では晴天期間が長いのに対し、北偏して通るときは（B地点の場合）、その地点の晴天は長続きせず、すぐに曇天または雨天域に入ってしまう。移動性高気圧の前半分（とくに北東部分）では、寒気団が流入しているうえに、晴天で夜間の放射冷却が強まるため、春や秋には、農作物に晩霜(おそじも)や早霜(はやじも)の害がおこりやすい。また移動性高気圧の後面の薄曇りの部分では、「日がさ」「月がさ」が現れたり、春の風景に特有といわれる「おぼろ月」になったりする。そして、図Aのような天気分布が全体として東へ移動するので、天気変化に周期性が現れる。

　停滞性高気圧の代表としては亜熱帯高気圧（小笠原高気圧(おがさわらこうきあつ)など）をあげることができる（図B）。これは亜熱帯と赤道方面との間の大気大循環によって形成される背の高い温暖高気圧である。シベリア高気圧で代表される大陸高気圧もまた停滞性高気圧である（図C）。これは海陸分布や大地形（チベット・ヒマラヤ山塊、ロッキー山脈など）の影響によって上空の偏西風帯に現れる停滞性の波長の長い偏西風波動と、寒候期の大陸の地面の放射冷却によってつくられる寒気団とによって、大陸上に形成される大きな高気圧で、構造的には、その東半分が背の低い寒冷高気圧、西半分が背の高い、相対的に温暖な高気圧になっている。停滞性の高気圧圏内では、大気は比較的長期間、大陸または海洋に接しているため、性質が広範囲にわたってほぼ一様になり、気団が形成される。また、停滞性の高気圧圏内では、風が弱い晴天の日が続き、地表面付近の大気が滞留し、地表面近くに気温の逆転層ができて拡散現象が弱まるため、汚染源のある地域では、大気汚染が深刻化することがある。

(4)ブロッキング高気圧　上空の偏西風波動が停滞性となり、その振幅が大きくなって、波が不安定化すると、北に偏西風の帯から切り離された温暖な切離高気圧、南に寒冷な切離低気圧が形成される（図D）。このような場合には、上空の切離高気圧の下にある地上の高気圧も、著しく停滞性となり、後続する移動性の高気圧や低気圧の動きを止めてしまう。その結果、晴天の所では、それが長続きして、ときには干魃(かんばつ)になり、逆に雨天の所は長雨となり、さらに南北の気団が大規模に入り乱れるため、異常寒波や異常熱波がおこることが多い。このような現象をブロッキングといい、上空の切離高気圧や、その下の地上高気圧をブロッキング高気圧とよぶ。ブロッキング高気圧は背の高い温暖型の停滞性高気圧である。梅雨期によく現れるオホーツク海高気圧は、その上空が、振幅を増大して停滞性となった偏西風波動の気圧の尾根または切離高気圧となっていることが多く、ブロッキング高気圧の性格が強い。

(5)チベット高気圧　チベット高原上の対流圏上部（8～15キロメートル）に夏季に現れる高気圧を、とくにチベット高気圧という。これはチベット・ヒマラヤ山塊が、夏季、強い日射により熱せられて、上空の空気を暖めることによって形成される。熱せられた気柱は上方に伸張する。そして上空の特定の高度面について考えると、気柱が上方に伸張した区域では、その面より上の大気の全重量が増大することになる。そのときの増加分が、その面のその区域に高気圧を形成するのである。チベット高気圧の消長は、日本の梅雨、夏の干魃、冷害、東南アジアの雨期の形成などに深い関係があると考えられている。

(6)局地高気圧　内陸部の盆地では、夜間、放射冷却によって形成された寒気がたまるため、盆地内の気圧が周囲より高くなり、小さな局地的高気圧ができる。日中は逆に、盆地内の空気がとくに暖められるので、局地的低気圧に変わることが多い。

(7)雷雨高気圧　雷雲の下に形成される寒気の重さによってできる局地的高気圧。雷雲の通過前後に感じる涼風は、この高気圧から吹き出してくる。雷雲の通過した地点の気圧曲線に現れる「雷雨の鼻」とよばれる突起は、雷雨高気圧の通過を示すものである。

［倉嶋　厚・青木　孝］

『福地章著『海洋気象講座』（1994・成山堂書店）』▽『丸山健人・水野量・村松照男著『新版地学教育講座14　大気とその運動』（1995・東海大学出版会）』▽『福谷恒男著『海洋気象のABC』（1997・成山堂書店）』▽『白木正規著『百万人の天気教室』（2007・成山堂書店）』