East Asian Rainy Season: Understanding the Meiyu, Tsuyu, and Jangma
Every year, during the transition from late spring to early summer, a significant meteorological phenomenon sweeps across East Asia. Known variously as Meiyu, Tsuyu, or Jangma, this rainy season brings persistent precipitation to China, Taiwan, Korea, and Japan. Driven by a stationary weather front, this period is characterized by heavy rainfall that can shape the environment, agriculture, and daily life across the region.
Plum rain falling on two clear umbrellas, looking up toward the Tokyo Metropolitan Government Building, in June 2009
Key Facts
- Cause: A persistent stationary front known as the Meiyu front.
- Mechanism: The meeting of moist Pacific air and cooler continental air masses.
- Regional Names: Meiyu (China), Tsuyu (Japan), and Jangma (Korea).
- Duration: Typically lasts between 50 days and two months depending on the location.
- Environmental Impact: High humidity can lead to mold, rot, flooding, and mudslides.
The Science of the Rainy Season
The rainy season is driven by the interaction of two major air masses. As moist air from the Pacific meets cooler air from the continent, a stationary front—a boundary between these two air masses that does not move significantly—is formed. This front, often referred to as the Meiyu front, creates a zone of disturbed weather that stretches from the East China coast, through Taiwan and Okinawa, and eventually northward into South Korea and Japan.
The timing and intensity of the rainfall depend on the strength of these air masses. The season typically concludes when the subtropical ridge (a large area of high pressure) becomes strong enough to push the weather front north, away from the region. While this period usually brings much-needed moisture, extreme fluctuations can occur; in some years, the front may move too quickly, leading to unexpected droughts instead of heavy rains.
Regional Variations and Etymology
While the meteorological cause is similar across the region, the cultural names and specific timings vary significantly by country.
China: The Plum Rain (Meiyu)
In China, the season is called Meiyu (梅雨), which literally translates to "plum rain." This name stems from a traditional belief that the rain coincides with the ripening and falling of plums south of the Yangtze River. The phenomenon is even immortalized in classical literature, such as the 760 AD poem by Du Fu, which describes the fine, persistent rain and dense fog of the season.
Japan: Tsuyu
In Japan, the season is known as Tsuyu (梅雨). The Japanese term uses the same Kanji as the Chinese name, a linguistic example of jukujikun, where characters are chosen for their meaning rather than their individual sounds. In most of Japan, including Honshu, Kyushu, and Shikoku, the season lasts from early June to mid-July. However, in the southernmost region of Okinawa, it arrives earlier, typically from May to June. Notably, the northernmost region of Hokkaido does not experience a pronounced rainy season because the front often weakens before reaching it.
Korea: Jangma
South Korea refers to this period as Jangma (장마), meaning "long rain." The term evolved from the 1500s expression "Dyangmah," combining the Hanja character for "long" with a native Korean word for "rain." The Korean rainy season is caused by the meeting of the North Pacific anticyclone and the Asiatic continental high pressure, forming the jangmajeonseon (weather front). This typically occurs between June and mid-July.
Summary of Regional Rainy Seasons
| Region | Local Name | Typical Timing | Key Characteristics |
|---|---|---|---|
| China | Meiyu | Mid-June to early July | Associated with the ripening of plums. |
| Japan | Tsuyu | Early June to mid-July | Earlier in Okinawa; absent in Hokkaido. |
| Korea | Jangma | June to mid-July | Driven by the jangmajeonseon front. |
| Taiwan/Okinawa | N/A | May to June | Earlier onset compared to mainland regions. |
Environmental and Social Effects
The impact of the rainy season extends beyond simple wet weather. The sustained high humidity is a significant factor in the growth of mold and rot, affecting both food supplies and textiles. From a geological perspective, the heavy rainfall can trigger mudslides and flooding.
Extreme weather events have been recorded during these periods. In 1982, Nagasaki, Japan, recorded a massive 153 mm of rain in a single hour. On a larger scale, Miyazaki Prefecture in Japan recorded a staggering 8,670 mm of total rainfall during the 2003 rainy season.
Frequently Asked Questions
Why is it called "plum rain" in China?
The term refers to the traditional observation that the rainfall occurs during the fourth and fifth lunar months, which is the same time that plums ripen and fall in the regions south of the Yangtze River.
Does Hokkaido experience a rainy season?
No, Hokkaido does not have a pronounced rainy season because the weather front usually weakens or attenuates before it can reach the northernmost part of Japan.
What causes the rainy season to end?
The season ends when the subtropical ridge strengthens sufficiently to push the stationary weather front northward, away from the affected regions.
Can the rainy season cause droughts?
Yes. In years where the weather systems do not behave as usual, the expected rainfall may not occur, resulting in drought conditions instead of the typical wet season.
How does the timing of the season vary in Japan?
In most of Japan, the season runs from early June to mid-July. However, in Okinawa, the season begins much earlier, typically spanning from early May to mid-June.