East Asian rainy season

The East Asian rainy season, also called the plum rain, is caused by precipitation along a persistent stationary front known as the Meiyu front for nearly two months during the late spring and early summer in East Asia between China, Taiwan, Korea and Japan. The wet season ends during the summer when the subtropical ridge becomes strong enough to push this front north of the region. These weather systems can produce heavy rainfall and flooding.

In China, the term "plum rain" (梅雨; meiyu) refers to the rainfall during the fourth and fifth lunar months. It originates from the traditional belief that when plums ripen and begin to fall in the regions south of the Yangtze River during this period, moisture evaporating from the plants transforms into rain.

The term appears in the following poem, dated to 760, by Du Fu:

梅雨
南京犀浦道，四月熟黃梅。
湛湛長江去，冥冥細雨來。
茅茨疏易溼，雲霧密難開。
竟日蛟龍喜，盤渦與岸迴。

Plum rain
On the Xipu road from the Southern Capital the fourth month ripens the yellow p[r]unus.
The long river goes off surging, and, darkening, a fine rain comes.
Roof-thatch, loosely bound, is easily soaked, clouds and fog are dense and will not lift.
All day long the dragons delight, whirlpools turning with the bank.

In Japanese, the rainy season is called by the native term tsuyu (梅雨); the spelling is borrowed from the Chinese term, and is an example of jukujikun (a kanji compound whose components are selected for meaning and do not individually represent a sound).

The season is instead commonly called Jangma (장마) in Korea, which means "long rain". The term was originally spelled Dyangmah (댱맣) in 1500s, which was the mix of the hanja character 長 ("long") and the old native Korean word 맣 (mah, "rain"). The word Dyangmah eventually transformed from Jyangma (쟝마) in 1700s to the current form of Jangma after the 1900s.

An east–west zone of disturbed weather during spring along this front stretches from the east China coast, initially across Taiwan and Okinawa, later, when it has shifted to the north, eastward into the southern peninsula of South Korea and Japan. In Taiwan and Okinawa, the rainy season usually lasts from May to June. In Russian Primorsky Krai, Japan, and Korea, it lasts from June to July (approximately 50 days). In eastern China (especially the Yangtze and Huai River regions), it lasts from mid June to early July.