Air mass - Kidan

Japanese: 気団 - きだん

A mass of air that has roughly the same horizontal properties over a wide area. It usually extends several thousand kilometers horizontally and several kilometers vertically. Although there are various elements to the properties of the atmosphere, an air mass is considered to be a collection of air with roughly the same temperature, humidity, and vertical distribution. Here, roughly the same has a relative meaning, in that the horizontal changes are gradual compared to the sudden changes seen at the boundary between air masses.

The concept of air masses, along with the concept of fronts, was proposed in the 1920s by the Norwegian School, including J. A. B. Bjerknes, a Norwegian meteorologist who had naturalized in the United States, and brought about a groundbreaking advance in estimating the physical structure of the atmosphere at a time when weather phenomena were considered mainly from surface weather charts. After World War II, an international network of high-altitude observations was established, and atmospheric properties could be measured directly in three dimensions, and the classification of air masses has been changing from one that emphasizes geographical origins to one that emphasizes the physical properties of the atmosphere.

[Yo Narumura]

Formation of air masses (surface air masses and upper air masses)

Air masses are formed when air remains over a wide area of uniform surface for a long time, and heat and water vapor are exchanged between the surface and the air. For this reason, air masses tend to form in areas with a wide area of uniform surface, such as over continents or oceans, where the wind is weak and the air can fully absorb the characteristics of the surface. In other words, air masses tend to form in the following conditions: (1) over continents or oceans, (2) high or low latitudes, (3) areas of stationary high pressure or huge low pressure areas with small pressure gradients. Air masses formed in this way are strongly influenced by the characteristics of the surface of the earth; for example, air masses formed over continents are dry, air masses formed over oceans are humid, and air masses formed at low latitudes are hot. For this reason, they are also called surface air masses.

In contrast to surface air masses, air masses that are formed without the direct influence of the earth's surface are called upper air masses. When there is widespread downdrafts, such as above high pressure areas, the warm, extremely dry submerged air mass that is formed is an upper air mass. When the downdraft is strong, the submerged air mass may reach the ground or mix with the surface air mass. The air in the lower stratosphere can be considered to be almost homogeneous in the horizontal direction, and is called a stratospheric air mass, but stratospheric air masses are also upper air masses.

[Yo Narumura]

Classification of Air Masses

Surface air masses are classified according to the region where they are formed (their origin). Among polar air masses, there are maritime polar air masses that originate over the ocean. These are cool, slightly moist, and unstable, and in winter they are limited to a small area in the northeastern Pacific and Atlantic Oceans, but in summer they originate in a belt of oceans north of 50 degrees north latitude in the Atlantic and Pacific Oceans. On the other hand, there are polar air masses and continental polar air masses that originate over the continents. The former are very cold, dry, and stable, and have properties similar to maritime polar air masses in summer, but as the air layer becomes thinner, they lose these properties as soon as they move south. The latter are also cold, dry, and stable, and therefore stable and sunny at their origin, but when they leave their origin and go out onto the warm ocean, they change, and if there is land downwind, they can cause snow.

Tropical air masses are divided into maritime tropical air masses that originate over the ocean, equatorial air masses, and continental tropical air masses that originate over the continent. Maritime tropical air masses are hot and humid, and somewhat unstable near the surface, but dry and stable at higher altitudes. Air masses to the east of the Pacific and Atlantic highs are stable due to strong subsidence, but unstable in the west. Equatorial air masses originate over the ocean south of 20 degrees north latitude in both summer and winter, and the characteristics of the lower layers are similar to those of maritime tropical air masses, but the upper layers are hot and humid and quite unstable. Continental tropical air masses are hot, dry, and stable. Because there is little water vapor, there is a large diurnal change in temperature.

The main air masses mentioned above are classified by adding the monsoon air mass (symbol M). Monsoon air masses are air masses in tropical monsoon regions, and are extremely hot and humid. They are also defined as air masses that move across the equator from the winter hemisphere to the summer hemisphere. There are other classifications of air masses, such as classifications that focus on the large-scale flow of the atmosphere (general atmospheric circulation), and classifications that are based on the temperature and water vapor content of the atmosphere, regardless of the place of origin.

[Yo Narumura]

Transformation of air masses

When an air mass leaves its source, it comes into contact with a different earth surface and exchanges heat and water vapor, gradually changing its properties. An air mass that is still in its source or that does not change much in properties even after it leaves its source is called a fresh air mass, and an air mass that has changed along the way is called an altered air mass.

When the temperature of an air mass is higher than the temperature of the ground it is moving through, it is called a warm air mass, and when it is lower, it is called a cold air mass. In the case of a warm air mass, the wind blows in a steady direction, visibility is poor, and precipitation is ground rain or drizzle, whereas in the case of a cold air mass, gusts of wind blow, visibility is good, and precipitation is showers (sudden rain). (Note: In addition, when two air masses are adjacent to each other, the one with the higher temperature is sometimes called a warm air mass and the one with the lower temperature is sometimes called a cold air mass). Air masses are classified as stable or unstable depending on the stability of their stratification, and generally, cold air masses are stable air masses and warm air masses are unstable air masses. Also, air masses that are neither cold nor warm air masses but have lost the characteristics of their source are called neutral or unbiased air masses. Note that the latter are sometimes called air masses to distinguish between air masses at their source and air masses moving in.

The properties of an air mass are largely fixed once its origin and route are determined, but the transformation of an air mass is also related to the route as well as the ascending and descending air currents in the middle and lower layers of the air mass. When there is a descending air current in the middle layers, there is an action that tries to suppress the altered air in the lower layers from being carried upward by convection activity, so the air mass is highly conserved, but in the opposite case, the altered air in the lower layers is rapidly carried up into the sky, and the properties of the air mass change suddenly.

[Yo Narumura]

Speech bubble

When an air mass leaves its source and penetrates widely and rapidly into a distant area, it is called an "outbreak," and is also called a cold air outburst (polar air mass outburst) or a monsoon outburst. In the Northern Hemisphere, when cold air blows southward, a stable layer is likely to form due to subsidence and heating, so even if the lower layer is unstable and cumulonimbus clouds form, the tops are often suppressed by the stable layer and become stratocumulus clouds, but when it blows eastward, there is no stable layer and cumulonimbus clouds develop vigorously.

[Yo Narumura]

Air masses around Japan

The air masses that have the greatest impact on the weather in each region and season are usually named after the place where they originate. Air masses that appear near Japan include the Siberian air mass, the Ogasawara air mass, the Sea of Okhotsk air mass, the Yangtze River air mass, and the equatorial air mass. Also, when a strong high pressure system covers the area during the warm season, an upper air mass appears and dry, clear weather may continue.

[Yo Narumura]

Air Mass Analysis

This is a weather chart analysis that elucidates the characteristics of weather distribution and its changes based on the concept of air masses. It is also called air mass analysis. The movement of air masses is represented by fronts that are their boundaries, so air mass analysis and front analysis are inseparably linked.

[Yo Narumura]

Air Mass Climatology

A branch of climatology that describes climate based on the frequency of air masses and their weather characteristics. Since air masses usually have their own weather characteristics, the dominance and alternation of various air masses can explain daily weather changes, characteristics of large-scale weather distribution, seasonal change mechanisms, classification and distribution of climate zones, climate divisions, and general atmospheric circulation. For example, in Tokyo, the Siberian air mass is overwhelmingly more prevalent in winter and the Ogasawara air mass is predominant in summer, which explains why it is relatively dry in winter and humid in summer.

[Yo Narumura]

[Reference] | Atmospheric pressure distribution

Air masses near Japan and their origins
©Shogakukan ">

Air masses near Japan and their origins

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

広い地域にわたって水平方向にほぼ同じ性質をもった空気の塊のこと。通常水平方向の広がりは数千キロメートル、垂直方向には数キロメートルに達する。大気の性質には種々の要素があるが、気団は温度、湿度、およびそれらの垂直分布の状態がほぼ同じである空気の集まりと考えられている。ここでほぼ同じというのは、水平方向の変化が、気団の境目にみられるような急な変化に比べて緩やかであるという相対的な意味である。

　気団の概念は、1920年代に前線の概念とともに、アメリカに帰化したノルウェーの気象学者J・A・B・ビャークネスなどのノルウェー学派によって提唱され、おもに地上天気図だけから天気現象を考えた時代に、大気の構造を物理的に推定するという画期的な進歩をもたらした。第二次世界大戦以後は、国際的な高層観測網が整備されて、大気の性質が立体的に直接測定されるようになり、気団の分類も地理学的な発源地を強調するものから、大気の物理的特性を重視する方向へ変わりつつある。

［饒村　曜］

気団の形成（地表気団と上層気団）

気団は主として、広範囲で性質の一様な地表面上に空気が長時間滞留し、地表面と空気との間に熱や水蒸気の授受が行われることによって形成される。このため気団は、大陸上や海洋上といった地表が広い範囲で一様な場所で、空気が十分に地表面の特性を吸収できる風の弱い地方で発生しやすい。すなわち気団が発生しやすい条件とは、(1)大陸上か海洋上、(2)高緯度地方か低緯度地方、(3)停滞性の高気圧圏内や気圧傾度の小さい巨大な低圧部、などである。このようにしてできた気団は、大陸上で形成された場合は乾燥、海洋上で形成された場合は湿潤、また、低緯度で形成された場合は気温が高いというように、地表面の特性の影響を強く受けている。このために地表気団という呼称もある。

　地表気団に対して、地表面の影響を直接受けないで形成された気団を上層気団という。高気圧圏内の上空のように広範囲に下降気流があると、形成される暖かくてきわめて乾燥した沈降気団は上層気団である。沈降気団は、下降気流が強いときには地表に達したり地表気団と混合したりする。また、下部成層圏の空気は水平方向にはほぼ等質と考えることができ、これを成層圏気団というが、成層圏気団も上層気団である。

［饒村　曜］

気団の分類

地表気団は、形成された地域（発源地）によって分類されている。寒帯気団のうち、海洋上で発生するものに海洋性寒帯気団がある。これは冷涼でやや湿潤、不安定で、冬季における発生域は太平洋および大西洋北東部の狭い区域に限られるが、夏季には大西洋および太平洋の北緯50度以北の帯状の海域で発する。一方、大陸上で発生するものには、極気団と大陸性寒帯気団がある。前者は非常に寒冷で乾燥し安定していて、夏季には海洋性寒帯気団に似た性質を持つが、気層が薄く南下するとただちに特性を失う。後者はやはり寒冷で乾燥し安定しており、したがって発源地では安定で晴れているが、発源地を離れて暖かい海上に出ると変質し、さらに風下に陸地があると雪を降らす。

　熱帯気団も海洋上で発生する海洋性熱帯気団、赤道気団と大陸上で発生する大陸性熱帯気団がある。海洋性熱帯気団は、高温湿潤で、地表付近でやや不安定だが、高層では乾燥し安定である。また太平洋および大西洋高気圧の東側にある気団は、沈降が強いので安定であるが、西部では不安定である。赤道気団は、夏季、冬季ともに北緯20度以南の海洋上で発生し、下層の特徴は海洋性熱帯気団と似ているが、上層まで高温多湿でかなり不安定である。大陸性熱帯気団は高温で乾燥し、安定している。水蒸気が少ないので気温の日変化は大きい。

　以上が主だった気団だが、これにモンスーン気団（記号M）を付加して分類される。モンスーン気団は熱帯モンスーン地方の気団で、高温多湿の度がきわめて大きい。このモンスーン気団を、冬半球から夏半球へ赤道を越えて移動してくる気団と定義することもある。気団分類にはこのほか、大気の大規模な流れ（大気大循環）に着目した分類や、発源地にこだわらず、大気の気温と水蒸気の含有量を基準とした分類などがある。

［饒村　曜］

気団の変質

気団は発源地を離れると、発源地と異なる地表面に接し、熱と水蒸気の授受を行ってすこしずつその性質を変える。気団が発源地にあるか、あるいは発源地を離れたあとでも性質があまり変化しない気団を新鮮な気団とよび、途中で変化した気団を変質気団という。

　気団の温度が、移動しつつある地面の温度より高い場合を暖気団、低い場合を寒気団という。暖気団の場合、風の吹き方は一定で、視界は悪く、降水型は地雨または霧雨であるのに対し、寒気団の場合は、突風状の風が吹き、視界は良好、降水型は驟雨(しゅうう)（にわか雨）である（なお、これとは別に、二つの気団が隣接している場合に気温の高いほうを暖気団、低いほうを寒気団とよぶこともある）。気団はその成層の安定度により安定気団と不安定気団に分けられるが、一般に寒気団は安定気団、暖気団は不安定気団である。また寒気団でも暖気団でもないが、発源地の性質を失った気団を中立気団または不偏(ふへん)気団とよぶ。なお発源地における気団と、移動してくる気団とを区別して、後者を気塊とよぶこともある。

　気団は、発源地と経路が決まるとその性質はだいたい固まるが、気団の変質には、経路のほかに気団内の中層や下層の上昇気流、下降気流とも関係している。中層で下降気流になっていると、変質した下層の空気が対流活動によって上方へ運ばれるのを抑えようとする働きが生ずるため、気団の保存性は大きいが、逆の場合は、急速に下層の変質した空気が上空に運ばれて気団の性質は急激に変わってしまう。

［饒村　曜］

吹き出し

発源地を離れた気団が遠くの地域に急速に広く侵入することを「吹き出し」といい、寒気の吹き出し（寒帯気団の吹き出し）、季節風の吹き出しという言い方をする。北半球においては、寒気の吹き出しが南へ向かっての吹き出しの場合は、沈降昇温で安定層ができやすいため、下層が不安定で積雲が発生しても、その頂部は安定層で抑えられて層積雲になることが多いが、東へ向かっての吹き出しの場合には、安定層がないため積雲が盛んに発達する。

［饒村　曜］

日本付近の気団

各地域の各季節の天気にもっとも大きい影響を与える気団は、通常、発源地の地名をつけてよぶ。日本付近に現れる気団は、シベリア気団、小笠原(おがさわら)気団、オホーツク海気団、揚子江(ようすこう)気団、赤道気団があげられる。また、暖候期に強い高気圧で覆われると、上層気団が現れ、乾燥した晴天が続くことがある。

［饒村　曜］

気団分析

天気分布の特徴とその変化を、気団という概念に基づいて解明する天気図解析のことで、気団解析ともいう。気団の移動は、その境界にあたる前線によって表されるから、気団分析は前線解析と表裏一体をなしている。

［饒村　曜］

気団気候学

気団の出現頻度とその天気特性によって気候を記載する気候学のことである。気団には普通それに特有な天気が対応するから、日々の天気変化、大規模な天気分布の特徴、季節変化の仕組み、気候帯の分類と分布、気候区分、大気大循環の様相などを、種々の気団の卓越と交替によって説明することができる。たとえば、東京では冬季はシベリア気団が圧倒的に多く、夏は小笠原気団が多いことから、冬は比較的乾燥し、夏は湿度が大きいことがわかる。

［饒村　曜］

[参照項目] | 気圧配置

日本付近の気団とその発源地

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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