Climate classification - climatic classification

Japanese: 気候区分 - きこうくぶん（英語表記）climatic classification

There are climates all over the world with different characteristics. These diverse climates are classified according to certain criteria, and the resulting distribution areas of each climate type are determined, and a certain area is divided into several climatic regions (climatic zones). Climate classification refers to the classification of types, and is an important part of the basis for climate classification.

There are several ways to classify climates and therefore the climate of a region:

(1) A causal or deductive method that takes into account regional discontinuities in the distribution of climatic characteristics, the atmospheric general circulation (atmospheric circulation) that creates them, and air masses, etc.
(2) Empirical or inductive methods that focus on discontinuities that appear in the distribution of vegetation and soil, which are believed to reflect the characteristics of the climate, find values of climatic elements that are suitable for supporting and explaining the discontinuities, and use these as the criteria for classification. In principle, it would be reasonable to use discontinuities in the climate itself as in (1), but it is difficult to find discontinuities in a seamless and non-representational climate, and to quantify and express a climate that is a combination of various atmospheric conditions. By combining as many elements as possible, it is possible to obtain something close to the actual climate classification, but this is difficult to apply to fine-grained climate classifications on a small to medium scale. On the other hand, methods such as (2) classify through phenomena that are believed to reflect the overall climate, so they provide a sense that is close to the actual climate classification, but it is possible that influences other than climate may affect the distribution of the phenomena.

Below, we will select and list some of the most representative climate divisions in the world.

[Yoshiharu Mizukoshi and Yoshitaka Fukuoka]

Alisov climate classification

This was published in 1950 by BP Alisov (1891-1972) of the Soviet Union, and is a representative example of a genetic division. He focused on the seasonal movement of the fronts that form the boundaries between air masses and the air masses that form in relation to the global atmospheric circulation, and defined the world's climatic regions according to the differences in the main air masses that govern the weather in summer and winter. In other words, he established four air masses: equatorial, tropical, mid-latitude, and polar, and the areas that are governed by one of these air masses all year round are called the equatorial air mass zone, tropical air mass zone, mid-latitude air mass zone, and polar air mass zone, respectively. He also proposed seven main climatic zones: the equatorial monsoon zone is the zone where equatorial air masses alternate between summer and winter, the subtropical zone is the zone where tropical air masses alternate between mid-latitude air masses, and the subpolar zone is the zone where mid-latitude air masses alternate between polar air masses. Alisov further attempts to subdivide climatic zones by taking into account factors such as land surface conditions, characteristics of the atmospheric circulation, and topography.

Other examples of etiological classifications include the one by Hermann Flohn (1912-1997) of Germany, who focused on the characteristics of wind systems and precipitation linked to the general atmospheric circulation, incorporating seasonal changes (1950), and the classification of vegetation zones by Mikhail Ivanovich Budyko (1920-2001) of the Soviet Union, who used net radiation and a radiative aridity index (radiative aridity) (1956).

[Yoshiharu Mizukoshi and Yoshitaka Fukuoka]

Köppen climate classification

This is the most famous of the many climate classifications, and can be considered a representative empirical classification. The German climatologist Köppen believed that the distribution of vegetation, especially forest vegetation, reflects the overall climatic environment, and empirically determined climatic values that matched the boundaries of vegetation distribution and used them to set climatic boundaries. Köppen published the results of his work on the world's climate classification several times since 1884, but the one that is widely used today is based on the one he published in 1918. The classification method is outlined below:

First, we can broadly classify climates into treeless climates and treeless climates. Treeless climates are divided into three types, A (tropical), C (temperate), and D (cold), in ascending order of temperature, while treeless climates are divided into B (arid), which is caused by low rainfall, and E (cold), which is caused by low temperatures. Climate A is defined as a climate where the coldest month has an average temperature of 18°C or higher, climate C is defined as a climate where the coldest month has an average temperature below 18°C and above -3°C, and climate D is defined as a climate where the coldest month has an average temperature below -3°C and above 10°C. Climate B is defined as a climate where the annual precipitation is below the dry limit, and climate E is defined as a climate where the warmest month has an average temperature below 10°C. Here, the dry limit is expressed as p = 20(t + α), where α is 7 for places with heavy rainfall all year round, 14 for places with heavy rainfall in summer, and 0 for places with heavy rainfall in winter.

Next, the climates of A, C, and D are divided into three types according to the seasonal distribution of precipitation: f, which is rainy all year round with no significant dry periods; w, which is rainy in summer with a dry period in winter; and s, which is rainy in winter with a dry period in summer. However, As and Ds are rarely seen in reality, so seven types are defined. Am is set as an intermediate type between Af (tropical rainforest climate) and Aw (savanna climate), and is distributed quite widely in the tropics, but Köppen treats it as a subtype of Af. The B climate is divided into BS (steppe climate) with more precipitation than the annual precipitation (p) half of the dry limit value, that is, p = 10 (t + α), and BW (desert climate) with less precipitation. The E climate is classified as ET (tundra climate) when the warmest month average temperature is above 0°C, and EF (snow and ice climate) when it is below 0°C.

The above 11 are the main climate types according to the Köppen classification, but C and D climates are further divided into four types, a, b, c, and d, depending on the average temperature of the warmest month, and are further subdivided into Cfa, Dwa, etc. If Cf, Cw, Cs, Df, and Dw are each divided into four types, there are 20 types, but only 14 of these types actually occur on Earth.

The Köppen climate types can be easily determined if you have the data on the average annual and monthly values of temperature and precipitation, and the world climate classification using them matches the actual conditions quite well. This is why it has not lost its value even though it has been published for nearly 100 years. On the other hand, it has been pointed out that there is a discrepancy between the climate zones and the vegetation distribution zones in some areas, and there are regional differences in climate that cannot be expressed by the average values of temperature and precipitation, and many revisions have been proposed. One example of this is the idea of a method of expressing annual climate, which applies the Köppen classification to the climate values of any one year. For example, it has become clear that the Köppen climate zone Cfa (warm and humid climate) has never appeared in the Penghu Islands in the Taiwan Strait, and there are many areas where the climate zones have changed over a period of several years to several decades due to global warming in recent years.

[Yoshiharu Mizukoshi and Yoshitaka Fukuoka]

Thornthwaite climate classification

This method was announced in 1948 by American Charles Warren Thornthwaite (1899-1963), and is an attempt to classify the world's climates based on the water balance at the earth's surface. Basically, like Köppen's classification, it is empirical in that it takes into account the extent to which the climate is beneficial for plant growth, but it also has experimental and semi-theoretical aspects. Through experimental methods, Thornthwaite devised an index that can explain the distribution of vegetation primarily within the United States. The classification is outlined below:

First, the maximum possible amount of evapotranspiration (potential evapotranspiration) from the earth's surface is assumed to be proportional to temperature, and monthly potential evapotranspiration is calculated from the monthly average temperature. In practice, this can easily be determined using a nominal chart. This value is compared with the average precipitation for each month to calculate the monthly surplus or deficit of water on the earth's surface. In this case, it is assumed that a maximum of 100 mm of precipitation is stored underground. Next, if the annual totals for potential evapotranspiration, monthly water surplus, and deficit are n, s, and d, respectively, then from these three indices,
Wetting coefficient (Ih) = 100 (s/n)
Drying coefficient (Ia) = 100 (d/n)
Moisture index (Im) = Ih - 0.6Ia
From the above, the climate types were classified according to four indices: humidity index, potential evapotranspiration, dryness coefficient or humidity coefficient, and the concentration rate of potential evapotranspiration in the three summer months.

Thornthwaite's climate classification is an excellent method in that it emphasizes the surface water balance, which is the main characteristic of climate. However, due to the cumbersome procedures and the fact that the classification criteria were established based only on the vegetation distribution within the United States, although work has been done to create classification maps for some regions, no climate map that covers the entire world has yet been created. For more information on Thornthwaite's classification process, see the books by Takeshi Kawamura (1929-2007) or Kinji Tanaka (1934-).

[Yoshiharu Mizukoshi and Yoshitaka Fukuoka]

Climate classification by Tatsuo Kira

Tatsuo Kira (1919-2011) used the monthly average values of temperature and precipitation, which are well documented, to devise two indices, the warmth index and the humidity index, and by combining them, he created a world climate classification map based on an ecological perspective (1967. See figure ). The warmth index (also called the "warmth index") is calculated by taking the monthly average temperature as t°C, calculating (t-5) for months in which the temperature is 5°C or higher, and integrating this value throughout the year. In other words, it is expressed as WI = Σ(t-5) (WI is the warmth index). The humidity index is expressed as H = P/(WI + 20) when WI is less than 100, and H = 2P/(WI + 140) when WI is 100 or higher (P is the annual precipitation). This climate classification method has the advantage of being simple and easy to understand, and of corresponding well to the distribution of natural vegetation. However, this is a plant-climate classification, and the classification names are slightly different from those of Köppen et al.

[Yoshiharu Mizukoshi and Yoshitaka Fukuoka]

Japan's climate

Well-known classifications for Japan's regions include those by Fukui Eiichiro (1905-2000), based on monthly mean temperature and precipitation and their seasonal distribution, and Sekiguchi Takeshi (1917-1997), based on temperature, precipitation, sunshine rate, and annual total amount of excess moisture and seasonal change patterns. Since the 1960s, other research has been published, including that of Suzuki Hideo (1932-2011), who classified climates by their etiology, focusing on the frequency of appearance and seasonal changes of air masses that control the weather in various regions, or the characteristics of the weather brought by each air mass, Maejima Ikuo (1929- ), who classified climates by seasonal changes in precipitation phenomena, and Yoshino Masatoshi (1928- ), who classified climates on a small- and medium-scale.

[Yoshiharu Mizukoshi and Yoshitaka Fukuoka]

"Introduction to Climatology" by Fukui Eiichiro (1961, Asakura Publishing)" ▽ "Various Issues Concerning Climate Classification" by Maejima Ikuo (included in "Modern Climatology Essays" edited by Sekiguchi Takeshi, 1969, Tokyodo Publishing)" ▽ "Climate Expression Methods and Climate Classification" by Kawamura Takeshi et al. (included in "Physical Geography Survey Methods" edited by Odomegawa Shohei et al., 1973, Asakura Publishing)" ▽ "Climate and Life, Volumes 1 and 2, by M.I. Budyko, translated by Uchijima Zenbei and Iwakiri Satoshi (1973, University of Tokyo Press)" ▽ "Physical Geography Lectures 2: Climatology" by Yoshino Masatoshi (1978, Daimeido)" ▽ "Climate Expression and Climate Classification" by Mizukoshi Masaharu (included in "Science of Climate I" edited by Mizuyama Takayuki et al., 1982, Sozosha)" ▽ "Climate and Environment, Volumes 1 and 2, by M.I. Budyko, translated by Uchijima Zenbei (1983, Kokin Shoin)" ▽ "Geography of Culture, revised edition, by Tanaka Kinji (1994, Daimeido)" ▽ "Nature from an Ecological Perspective, by Kira Tatsuo (Kawade Bunko)" ▽ "The Structure of Climate, by Suzuki Hideo (Kodansha Academic Library)"

Thornthwaite Climate Classification (Table)
©Shogakukan ">

Thornthwaite Climate Classification (Table)

Eco-climate division map of the world (figure)
Created by Tatsuo Kira. Based on "Nature from an Ecological Perspective" (1979, Kawade Shobo Shinsha) ©Shogakukan

Eco-climate division map of the world (figure)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

世界各地にはそれぞれ特徴の異なる気候が存在する。これらの多様な気候を、ある基準に従って類型区分し、その結果得られた各気候型の分布地域を定め、ある範囲をいくつかの気候地域（気候区）に分けることを気候区分という。気候分類とはこのうち類型区分をさす語で、気候区分を行う基礎となる重要な部分をなす。

　気候を分類し、それによってある地域の気候区分を行うには、以下のような方法がある。

(1)気候特性やそれをつくりだす大気大循環（大気環流）、気団などの分布に認められる地域的不連続部を考慮した成因的または演繹(えんえき)的方法
(2)気候の特性をよく反映していると思われる植生や土壌の分布などに現れる不連続部に注目し、その不連続性を裏づけ説明するに適した気候要素の値をみつけて、これを区分の基準とする経験的または帰納的方法
　本来は(1)のように、気候自体にみられる不連続部を区分界にすることが合理的であろうが、縫い目がなくかつ具象的でない気候のどこに不連続部をみつけるか、またいろいろな大気条件の総合である気候を、どのように定量化して表現するかがむずかしいところである。できるだけ多くの要素を総合することにより、現実の気候区分に近いものが得られるが、中・小スケールのきめ細かい気候区分には適用しにくい。一方(2)のような方法は、気候の総合的な反映とみられる事象を介して区分を行うので、現実の気候区分に近い感覚のものが得られるが、気候以外の影響がその事象の分布に及んでいることも考えられる。

　次に世界の気候区分について、代表的なものをいくつか選んで記す。

［水越允治・福岡義隆］

アリソフの気候区分

ソ連のアリソフB. P. Alisov（1891―1972）が1950年に発表したもので、成因的区分の代表的なものである。世界的な規模の大気大循環との関連で形成される気団と、気団相互の境界にあたる前線の位置の季節的な移動に注目し、夏、冬の天気を支配する主要気団の違いによって世界の気候区域を定めた。すなわち、赤道気団、熱帯気団、中緯度気団、極気団の四つを設定し、一年中このなかの一つの気団に支配される地帯をそれぞれ、赤道気団地帯、熱帯気団地帯、中緯度気団地帯、極気団地帯とする。また夏と冬とで赤道気団と熱帯気団との交替がある地帯を赤道季節風地帯、熱帯気団と中緯度気団との交替がある地帯を亜熱帯地帯、中緯度気団と極気団との交替がある地帯を亜極地帯とし、あわせて七つの主要気候帯を提示した。アリソフはさらに地表面の状態、大気大循環の特徴、地形などの条件を入れて気候帯の細分を試みている。

　成因的区分ではほかに、ドイツのフローンHermann Flohn（1912―1997）が、大気大循環と結び付けた風系および降水量の特性に注目し、これに季節的な変化を取り入れて行ったもの（1950）や、ソ連のブディコMikhail Ivanovich Budyko（1920―2001）の純放射と放射乾燥指数（放射乾燥度）を用いた植生帯の分類（1956）などがある。

［水越允治・福岡義隆］

ケッペンの気候区分

多くの気候区分のなかでもっとも有名なもので、経験的区分の代表といえる。ドイツの気候学者ケッペンは植生とくに森林植生の分布が総合的な気候環境の反映であると考え、植生分布の境界にあうような気候値を経験的に求めて、気候区界の設定に利用した。ケッペンは1884年以来数回にわたって世界の気候区分についての成果を発表したが、現在広く利用されているのは1918年に発表したものを基礎としている。区分方法のあらましは次のとおりである。

　まず樹木気候と無樹木気候とに大別する。樹木気候は高温なほうからA（熱帯）、C（温帯）、D（冷帯）の3気候に、無樹木気候は少雨がその原因であるB（乾燥）気候と、低温が原因であるE（寒帯）気候にそれぞれ分けられる。A気候は最寒月平均気温18℃以上、C気候は最寒月平均気温18℃未満でかつ零下3℃以上、D気候は最寒月平均気温零下3℃未満で最暖月平均気温10℃以上の範囲と規定される。またB気候は年降水量が乾燥限界値以下の範囲、E気候は最暖月平均気温が10℃未満の範囲と定められる。ここで乾燥限界値とは、年降水量をpミリメートル、年平均気温をt℃とした場合、p＝20(t＋α)で表され、αは年中多雨の場所では7、夏に多雨の場所では14、冬に多雨の場所では0とする。

　次にA、C、Dの気候は降水量の季節配分の状態によって、年中多雨で著しい乾期がないf、夏に多雨で冬に乾期のあるw、冬に多雨で夏に乾期のあるsの三つの型に分けられる。ただしAsとDsとは現実にはほとんどみられないので、七つの型が定められることになる。なお、Af（熱帯雨林気候）とAw（サバンナ気候）の中間型としてAmが設定され、熱帯にかなり広く分布するが、ケッペンはAfの副次型として扱っている。B気候は年降水量（p）が乾燥限界値の2分の1、すなわちp＝10(t＋α)を境にして、それよりも降水量の多いBS（ステップ気候）と、降水量の少ないBW（砂漠気候）とに分けられる。またE気候は最暖月平均気温0℃以上をET（ツンドラ気候）、0℃未満をEF（氷雪気候）としている。

　以上の11がケッペンの分類による主要気候型であるが、さらにCおよびD気候については、最暖月平均気温の高低によって、a、b、c、dの四つの型に分け、Cfa、Dwaなどのように細分される。Cf、Cw、Cs、Df、Dwを各四つに分けると20種類になるが、実際に地球上に現れるのはこのうちの14種類である。

　ケッペンの気候型は、気温と降水量について、年および各月の平均値の資料があればたやすく判定でき、しかもそれを用いた世界の気候区分は現実の状況とかなりよく一致する。発表後100年近く経っていてもなおその価値を失わない理由もここにある。反面、一部地域での気候区界と植生分布界との不一致や、気温、降水量の平年値で表せない気候の地域差の存在などが指摘され、修正案も数多く提案されている。その一例として、ケッペン分類法を任意の1年分の気候値について適応させた年候（year climate）という表現法が考え出された。たとえば、台湾海峡にある澎湖島(ほうことう)のケッペン気候区Cfa（温暖湿潤気候）が一度も現れないことなどが明らかになったように、近年の地球温暖化で、数年から数十年単位で気候区が変化している地域が少なくない。

［水越允治・福岡義隆］

ソーンスウェートの気候区分

アメリカのソーンスウェートCharles Warren Thornthwaite（1899―1963）が1948年に発表した方法で、地表面における水収支を通して世界の気候区分を試みたものである。基本的には、植物の生育にとって気候がどの程度有効に作用しているかを考慮する点で、ケッペンの区分と同様経験的なものであるが、実験的・半理論的な面もある。ソーンスウェートは実験的方法を通して、主としてアメリカ合衆国内の植生の分布を説明できるような指標を考案した。その区分の概要は次のとおりである。

　まず、地表面からの最大可能蒸発散量（蒸発散位）は気温に比例するとし、毎月の平均気温から月別の蒸発散位を算出する。実際には計算図表で容易に求められる。この値を各月の平年の降水量と比較し、月別の地表面における水の過不足量を算出する。この際、地中には最大限、降水量にして100ミリメートル分までの水分が蓄えられるものとする。次に蒸発散位、月別の水分過剰量、不足量それぞれの年合計値をn、s、dとすると、この3指標から、
　　湿潤係数(Ih)＝100(s/n)
　　乾燥係数(Ia)＝100(d/n)
　　湿潤指数(Im)＝Ih－0.6Ia
が得られる。以上から、湿潤指数、蒸発散位、乾燥係数または湿潤係数、および蒸発散位の夏3か月間への集中率をあわせた四つの指標によって気候の類型区分を行ったのが表である。

　ソーンスウェートの気候区分は、気候の主要特性である地表面の水収支を重視した点で優れた方法である。ただ、手続がめんどうであることや、アメリカ合衆国内の植生分布に限った対応によって区分の基準を定めたことなどもあって、一部の地域について区分図作成の作業は行われているものの、まだ全世界をカバーする気候図はつくられていない。なおソーンスウェートの分類作業手続については、河村武（1929―2007）または田中欣治(きんじ)（1934―　）の著書に詳しい。

［水越允治・福岡義隆］

吉良竜夫の気候区分

吉良竜夫(きらたつお)（1919―2011）は資料の豊富な気温と降水量の月平均値を用い、温量指数と乾湿度指数の二つの指数を考案し、その組合せによって、生態学の視点にたった世界の気候区分図を作成した（1967。図参照）。温量指数（「暖かさの指数」ともいう）とは、月平均気温をt℃とすると、その値が5℃以上の月について、(t－5)を求め、この値を年間を通して積算したものである。すなわち、WI＝Σ(t－5)で表される（WIは温量指数）。また乾湿度指数は、WIが100未満の場合には、H＝P/(WI＋20)、WIが100以上の場合は、H＝2P/(WI＋140)で表される（Pは年降水量）。この気候区分方法は簡単で理解しやすく、自然植生の分布ともよく対応する利点をもっている。ただこれは植物気候区分であり、区分名はケッペンらと多少異なっている。

［水越允治・福岡義隆］

日本の気候区分

日本を対象地域にした区分には、月平均気温、降水量とその季節配分に基礎を置いた福井英一郎（1905―2000）のもの、気温、降水量、日照率、水分過剰量の年総量と季節変化型とをもとにした関口武（1917―1997）のものがよく知られている。1960年代以後には、各地の天気を支配する気団の出現頻度やその季節的変化、あるいは各気団のもたらす天気の特徴などに注目して成因的区分を行った鈴木秀夫（1932―2011）の研究、降水現象の季節推移による区分を行った前島郁雄(いくお)（1929―　）の研究、また中・小気候スケールの気候区分を行った吉野正敏(まさとし)（1928―　）の研究などが発表されている。

［水越允治・福岡義隆］

『福井英一郎著『気候学概論』（1961・朝倉書店）』▽『前島郁雄著「気候区分に関する諸問題」（関口武編『現代気候学論説』所収・1969・東京堂出版）』▽『河村武他著「気候表現法と気候区分」（尾留川正平他編『自然地理調査法』所収・1973・朝倉書店）』▽『M・I・ブディコ著、内嶋善兵衛・岩切敏訳『気候と生命』上下（1973・東京大学出版会）』▽『吉野正敏著『自然地理学講座2　気候学』（1978・大明堂）』▽『水越允治著「気候の表現と気候区分」（水山高幸他編『風土の科学Ⅰ』所収・1982・創造社）』▽『M・I・ブディコ著、内嶋善兵衛訳『気候と環境』上下（1983・古今書院）』▽『田中欣治著『教養の地理学』新訂（1994・大明堂）』▽『吉良竜夫著『生態学からみた自然』（河出文庫）』▽『鈴木秀夫著『風土の構造』（講談社学術文庫）』