Wet and dry reading

...It corresponds well to the distribution of vegetation, with W = 180 or more being tropical and subtropical forests, W = 85 to 180 being warm temperate broadleaf forests (sclerophyllous forests in winter rain climates, evergreen broadleaf forests in summer rain climates), W = 45 to 85 being cool temperate deciduous broadleaf forests, W = 15 to 45 being subarctic coniferous forests, and W = 15 or less being tundra. Indices corresponding to the distribution of vegetation are K = P / ( W + 20) (when W < 100) and K = 2 P / ( W + 140) (when W > 100), which are called wet and dry indices ( P (mm) is the annual precipitation).
[Precipitation effectiveness]
The ratio of monthly precipitation (in inches) P to monthly evaporation (in inches) E ( P - E ratio) multiplied by 10 over 12 months is called the precipitation efficiency or P - E index, which was devised by C. W. Thornthwaite for his climate classification (old classification).

*Some of the terminology that refers to "wet and dry readings" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…植生分布との対応がよく，W＝180以上が熱帯林および亜熱帯林，W＝85～180が暖温帯広葉樹林(冬雨気候では硬葉樹林，夏雨気候では照葉樹林)，W＝45～85が冷温帯落葉広葉樹林，W＝15～45が亜寒帯針葉樹林，W＝15以下がツンドラになる。なお植生分布に対応する示数としてK＝P/(W＋20)(W＜100の場合)，K＝2P/(W＋140)(W＞100の場合)を乾湿示数と名付けている(P(mm)は年降水量)。
［降水効率precipitation effectiveness］
　月降水量P(インチ)を月蒸発量E(インチ)で割った比(P‐E比)を12ヵ月合計してこれを10倍したものを降水効率またはP‐E示数といい，C.W.ソーンスウェートが気候分類(旧分類)のために考案したもの。…

※「乾湿示数」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報