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The East Asian rainy season (梅雨), pronounced "meiyu" in Chinese and "tsuyu" or (less commonly) "baiu" in Japanese, refers to the frontal precipitation caused by the meiyu or baiu front, a persistent east-west zone of disturbed weather during spring which is quasi-stationary and stretches from the east China coast, across Taiwan, and eastward into the Pacific south of Japan.[1] The rainy season usually lasts from June to July in Japan and from July to August in the Chang Jiang and Huai He region of China.

The mei-yu front forms when the moist air over the Pacific meets the cooler continental air mass. The front and the formation of frontal depresisons along it bring rain to Japan, Korea and eastern China. 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. In June, Tokyo experiences an average of 176 mm of rain[2], more than any month other than September. (Even though more rain falls in September, those rain totals are caused by typhoons, which are nowhere nearly as regular and predictable as the rains in June.) The islands of Okinawa, Japan are the first to receive rain, though all islands are eventually affected. However, the rain front normally does not bring much rain to Hokkaidō as it weakens on its way north.

When the Pacific warm air mass weakens in autumn, the front comes back from the north and it rains for several weeks in Japan. The autumnal rain is called "Akisame" (秋雨).

In some years, the rainy season's start and finish are under debate. For example, in 2005, the subtropical ridge moved quickly northward in late June/early July. The Mei-yu front skipped the Chang Jiang region and there was no rainy season there. Then, the ridge retreated southward and there was significant rainfall in the region. This gave rise to the question of whether this was the summer-type rainfall pattern that is common after the rainy season or the second mei-yu rainy season. Some meteorologists even argued that the rainy period in late June was not a true rainy season.[3][4][5][6][7]

• East Asian monsoon

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