Meiyu

The East Asian rainy season, commonly called the plum rain (Chinese and Japanese: 梅雨; pinyin: méiyǔ; rōmaji: tsuyu/baiu; Korean: 장마; romaja: jangma; Russian: затяжные, zatyazhnie), is caused by precipitation along a persistent stationary front known as the Mei-Yu front for nearly two months during the late spring and early summer between eastern Russia, 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.

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. The rainy season usually lasts from May to June in Taiwan and Okinawa, from June to July (approximately 50 days) in Russian Primorsky Krai, Japan and Korea and from July to August in Eastern China (especially the Chang Jiang and Huai River regions).

The weather front forms when the moist air over the Pacific meets the cooler continental air mass. The front and the formation of frontal depressions along it brings precipitation to Primorsky Krai, Japan, Korea, eastern China, and Taiwan. As the front moves back and forth depending on the strength of cool and warm air masses, there is often prolonged precipitation and sometimes flooding in eastern China. However, in the years that it does not rain as much as usual, a drought might result. The rainy season ends when the warm air mass associated with the subtropical ridge is strong enough to push the front north and away.

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Wikipedia