Low pressure - teikiatsu

Japanese: 低気圧 - ていきあつ

An area on a weather chart that is surrounded by closed isobars (contours) and has a relatively lower air pressure (altitude) than the surrounding area. The academic terms for this are cyclone, depression, and low, but cyclone also refers to a developed tropical cyclone that occurs in the Indian Ocean. A cyclone is not something that is lower than a certain air pressure (altitude). Some cyclones are accompanied by fronts, while others are not. Those that are accompanied by fronts include temperate cyclones that occur in temperate and polar zones. Those that are not accompanied by fronts include tropical cyclones that occur in the tropics, thermal cyclones that are formed by local heating, orographic cyclones that occur in the lee of mountains, and tornadoes. There are also cold cyclones called cut-off cyclones that occur due to blocking phenomena in the upper westerly winds. However, when people normally say cyclones, they are usually referring to temperate cyclones.

The strength of a low pressure system is expressed by its central pressure (or central altitude). The central pressure on a surface weather chart varies with the season, but in the mid-latitudes it ranges from about 960 to 1020 hectopascals.

Because the air pressure inside a low pressure area is lower than the surrounding area, air blows in from all sides. However, due to the Earth's rotation, the air cannot head straight toward the center of the low pressure area, so it swirls toward the center. Therefore, the winds of a low pressure area circulate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The air blowing in from all sides rises near the center and is carried to the upper troposphere, from where it blows out. The fact that the total weight of the air surrounding the low pressure area is relatively less than the surrounding area indicates that more air is flowing out at the upper levels than is flowing in at the lower levels.

When low pressure systems are classified based on the vertical structure of temperature, they can be divided into two types: cold low pressure systems, which are made up of air that is colder than the surrounding area at the bottom and become increasingly more pronounced at higher levels, and warm low pressure systems, which are made up of air that is warmer than the surrounding area at the bottom and become weaker as they move up the layers, so that they are no longer noticeable at high enough levels.

When a temperate cyclone is in the westerly wind belt and moves westward, the surface low pressure system tilts westward toward the upper atmosphere and overlaps with a trough or low pressure system at an upper level. In contrast, a tropical cyclone usually overlaps with a low pressure system at an upper level in a straight vertical direction. Thermal cyclones and orographic cyclones are phenomena occurring near the ground and do not overlap with a trough at an upper level.

[Yo Narumura]

Tropical Cyclones and Weather

The rising air currents around the center of a low pressure system create clouds and bring rain, so the weather is generally bad and the wind and rain are strong within the low pressure system. In general, the smaller the interval between the isobars (contours) surrounding the low pressure system, that is, the steeper the gradient of the pressure, the stronger the wind will blow in.

A temperate cyclone generally has a warm front ahead of it and a cold front behind it, so as the cyclone approaches, thin upper clouds appear due to the warm front, and as it gradually gets closer, the cloud cover increases with mid-level clouds and then lower-level clouds, and a steady rain begins to fall. The wind and rain are strongest when passing near the center. As the center of the cyclone moves eastward, it is influenced by the cold front, causing sudden rain and thunderstorms due to cumulus clouds, but after the cold front passes, the weather recovers rapidly and the temperature drops.

The course of low pressure systems that travel northeast from the East China Sea along the Pacific coast of Western and Eastern Japan is the most common course around Japan, bringing rain to a relatively wide area of the country. Low pressure systems that follow this course in January and February often bring snow to the Pacific coast, causing traffic disruptions. Also, when low pressure systems that develop over the Sea of Japan travel northeast, like the low pressure systems that bring the first spring winds, the southerly winds around Japan become stronger, causing the föhn phenomenon in areas along the Sea of Japan, making it more likely that large fires will break out.

The speed of low pressure systems varies depending on the season, but around Japan it typically travels at around 40 kilometers per hour in winter, spring, and autumn, and around 30 kilometers per hour in summer, although some can travel at speeds of as much as 100 kilometers per hour.

Rain from tropical cyclones occurs mainly because air rises along the front, so the faster the warm air rises and the more water vapor it contains, the more rain falls. Therefore, heavy rainfall associated with cyclones is more common in warm seasons.

[Yo Narumura]

Low pressure model

In 1922, the Norwegian School of Meteorologists, including J. A. B. Bjerknes, created a model in which a typical extratropical cyclone occurs on a front, which is the boundary between warm and cold air masses, and develops by converting the released potential energy into kinetic energy as the heavy cold air moves downward and the light warm air moves upward, eventually becoming a vortex of cold air. This model was slightly revised in later years as high-altitude observations became possible, but it remains the basis of weather chart analysis today.

[Yo Narumura]

The life cycle of a tropical cyclone

If the conditions are right, a tropical cyclone that occurs on a front will develop, reach its peak, and then weaken and disappear. The process is as follows (1) to (4).

(1) Tropical cyclones appear as small disturbances along a front, the boundary between cold and warm air. Such cyclones gradually increase in amplitude.

(2) As the low pressure develops and moves eastward, the cold front moves faster than the warm front, causing the warm area to gradually change into a fan shape. A temperate cyclone develops when the potential energy of the cold air trying to slip under the warm air is converted into kinetic energy, so the greater the temperature difference between the cold air and the warm air at the front, the more it develops.

(3) When a cold front catches up with a warm front, the intersection of the cold front and the warm front separates from the center of the low pressure system. A low pressure system in this state is called an occluded low pressure system, and a front in which the cold front and the warm front become one as a result of occlusion is called an occluded front. The point where the three fronts, cold, warm, and occluded, intersect is called an occlusion point.

(4) As the low pressure system weakens, a new low pressure system may form at the occlusion point and then become occluded again. The occluded low pressure system gradually loses strength, and the wind and rain weaken as well, until it turns into a weak vortex and disappears.

[Yo Narumura]

Schematic diagram of the three-dimensional structure of tropical cyclones and anticyclones
©Shogakukan ">

Schematic diagram of the three-dimensional structure of tropical cyclones and anticyclones

Low pressure model (Bjerknes model)
©Shogakukan ">

Low pressure model (Bjerknes model)

The life cycle of a tropical cyclone
©Shogakukan ">

The life cycle of a tropical cyclone

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

天気図上で閉じた等圧線（等高線）で囲まれた、周囲よりも気圧（高度）が相対的に低い領域。学術用語としてはcyclone, depression, lowがあてられるが、サイクロンはインド洋に発生する、発達した熱帯低気圧をさすことばでもある。一定の気圧（高度）より低いものを低気圧とよぶわけではない。前線を伴うものと伴わないものがあり、伴うものには、温帯および寒帯で発生する温帯低気圧がある。また、伴わないものには、熱帯で発生する熱帯低気圧、局所的に熱せられてできる熱的低気圧、山脈などの風下にできる地形性低気圧、竜巻(たつまき)などがある。このほか、切離低気圧とよばれる、上層の偏西風帯の中でブロッキング現象によってできる寒冷な低気圧もある。しかし、普通、低気圧というと温帯低気圧をさすことが多い。

　低気圧の強さは、中心の気圧（または中心の高度）で表す。地上天気図での中心気圧は、季節によって変わるが、中緯度の場合は、960から1020ヘクトパスカルぐらいの値をとる。

　低気圧内は、周囲より気圧が低いため、四方から空気が吹き込む。このときに空気は、地球の自転の影響でまっすぐに低気圧の中心に向かうことができずに、渦を巻いて中心に向かう。したがって、低気圧の風は、北半球では反時計回り、南半球では時計回りの循環となる。四方から吹き込んだ空気は、中心付近で上昇して対流圏上部に運ばれ、そこから外へ吹き出す。低気圧を包む空気の総重量が、周囲より相対的に少ないということは、下層で流入する空気量より多くの空気が上層で流出していることを示している。

　低気圧を気温の垂直構造から分けると、低気圧は、下層が周囲より冷たい空気からなり、上層ではますます顕著な低気圧となる寒冷低気圧と、下層が周囲より暖かい空気からなり、上層になるほど弱くなって十分な高さでは低気圧は認められなくなる温暖低気圧の二つになる。

　温帯低気圧は、偏西風帯にあって西へ進むときは、地上低気圧は上層に向かって西方へ傾いて上層の気圧の谷や低気圧と重なっている。これに対し熱帯低気圧は、通常、垂直方向にまっすぐ上層の低気圧と重なっている。また、熱的低気圧や地形性低気圧は、地上付近の現象で、上層の気圧の谷とも重なっていない。

［饒村　曜］

温帯低気圧と天気

低気圧の中心付近の上昇気流は、雲をつくり、雨を降らせるので、低気圧内では一般に天気が悪く、風雨が強い。また一般に、低気圧を取り巻く等圧線（等高線）の間隔が小さいときほど、すなわち気圧の傾きが急なときほど、吹き込む風は強いということができる。

　温帯低気圧は、一般に、進行する前方に温暖前線、後方に寒冷前線をもっているので、低気圧の接近に伴って温暖前線による上層の薄い雲が出て、しだいに近づくにつれ中層雲、さらに下層雲と雲量を増し、一様な降り方の雨が降りだす。中心付近を通過するときは、風雨がもっとも強くなる。低気圧の中心が東に去るにつれて寒冷前線の影響を受け、積雲系の雲によるにわか雨や雷雨がおこるが、寒冷前線の通過後は天気は急速に回復し、気温が下がる。

　東シナ海方面から西日本や東日本の太平洋側を北東に進む低気圧のコースは、日本付近でもっとも頻度の高いコースで、日本の比較的広い範囲で雨が降る。1、2月にこのコースをとる低気圧は、しばしば太平洋沿岸部で雪を降らせて交通障害をもたらす。また、春一番を吹かせる低気圧のように、日本海を発達した低気圧が北東へ進むときには、日本付近は南風が強くなり、日本海側の地方ではフェーン現象がおこり、大火災が発生しやすくなる。

　低気圧の速度は季節によって異なるが、日本付近では冬と春、秋は時速40キロメートル、夏は30キロメートルくらいが普通であるが、なかには100キロメートルくらいのものもある。

　温帯低気圧の雨は主として前線面に沿って空気が上昇するためにおこるので、暖気のはい上がる速度が大きいほど、暖気に含まれる水蒸気量が多いほど雨量が多くなる。したがって、低気圧に伴う大雨は暖候期に多い。

［饒村　曜］

低気圧モデル

1922年にノルウェーの気象学者J・A・B・ビャークネスらのノルウェー学派は、典型的な温帯低気圧は、寒暖両気団の境である前線上に発生し、重い寒気が下方に、軽い暖気が上方に移動する際に開放される位置エネルギーを運動エネルギーに変えながら発達して、最後には寒気の渦巻になるということをモデル化した。このモデルは後年、高層観測ができるようになって若干の修正がなされたが、現在でも天気図解析の基本となっている。

［饒村　曜］