Humid climate

According to W. P. Köppen, it is defined as a climate with enough rainfall for tree growth, and is distinguished from an arid climate. The boundary between the two is called the dry limit, and takes into account not only rainfall but also temperature. The amount of water that can actually be used is the amount of rainfall remaining after subtracting the water that escapes to the outside world through evaporation. Since evaporation is influenced by temperature, the rainfall r on the boundary line that separates wet and dry temperatures can be expressed by the following formula, using the annual mean temperature t (℃) and the annual precipitation r (mm).

Rainy all year round, no dryness …r＝20(t＋7)
Summer rainy season, winter dry season …………r=20(t+14)
Winter rainy season, summer dry season …………r=20t
In this way, instead of using a single formula for the entire world, an empirical formula is used to determine climatic limits by dividing the world into three regions based on differences in the seasonal distribution of rainfall. Furthermore, humid climates are divided into tropical, temperate, and cold (subarctic) zones based on temperature, but the cold zones, along with the dry climates, belong to the treeless climate zones due to their coldness, and are not included in the humid climates. With the exception of savanna climates with dry winters, tropical monsoon climates (tropical monsoon climates) with a weak dry season in winter, and Mediterranean climates (summer), humid climates are distributed over a wide range of the tropical, temperate, and cold zones.

[Yoshitaka Fukuoka]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

W・P・ケッペンによれば、樹木の成長に十分な雨量をもつ気候と定義され、乾燥気候と区別される。両者の境界を乾燥限界と称し、雨量のみならず気温も考慮している。実際に利用できる水の量は、雨量から蒸発などにより外界へ逃げる水を差し引いた残りである。蒸発は気温によって左右されるので、年平均気温t（℃）と年降水量r（ミリメートル）とから、湿潤・乾燥両気温を分ける限界線上の雨量rは次式により表される。

　　一年中多雨で乾燥なし
　　　…r＝20(t＋7)
　　夏雨期、冬乾期
　　　…………r＝20(t＋14)
　　冬雨期、夏乾期
　　　…………r＝20t
　このように、全世界について、単一な式を用いず、降雨量の季節的配分の違いにより3地域に分けて、気候的限界値を求める実験式を用いている。さらに、湿潤気候は気温により熱帯、温帯、冷帯（亜寒帯）に分けられるが、寒帯は、寒冷なために乾燥気候とともに無樹木気候帯に属し、湿潤気候には含まれない。冬乾燥のサバンナ気候、弱い乾期をもつ冬の熱帯季節風気候（熱帯モンスーン気候）と地中海性気候（夏）などを除いて、熱帯、温帯、冷帯の広い範囲にわたり湿潤気候が分布している。

［福岡義隆］

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