In our previous article, we talked about places in Tokyo to enjoy hydrangeas - the flower of the rainy season in Japan. Today, we are going to look at the rainy season in depth and what you should keep in mind.
When is exactly the rainy season in Japan?
Rainy season is called “tsuyu or梅雨“ in Japan. The Japan Meteorological Agency (JMA) defines it as “the phenomenon where rainy or cloudy days appear frequently from late spring to summer or its period” and announces the start and the end of the period every year. The rainy season starts and ends in different timings depending on the region in Japan. For Kanto Region where Tokyo is located, the rainy season usually starts around June 7th and ends around July 19th, according to the official website of JMA. This year, JMA declared that Kanto Region had entered in the rainy season one day later than the average, June 8th.
What causes the rainy season?
What brings rain during the rainy season in Japan? It is the stationary front (“bai-u zensen” or 梅雨前線) developing over Japan from May to July. This seasonal rain front is where two different air masses – the high atmospheric pressure system over the Okhotsk Sea (Okhotsk High) and the high-pressure system over the Pacific Ocean (North Pacific High) – collide, creating upward current of air that makes rain clouds form and bringing heavy rain.
What is the weather like during the rainy season?
In Kanto Region, you will experience more rainy and cloudy days, and occasional downpours during the rainy season. When the cold and wet Okhotsk High from north is strong, the temperature drops. On the other hand, when the warm and wet North Pacific High is strong, the temperature rises. Toward the end of the rainy season, as the region is gradually covered with the North Pacific High, there will be more sunny and hot days.
What is a typhoon?
You may also have heard the term “typhoon” when watching weather news. The terms “typhoon” and “hurricane” are the regional names that mean the same weather phenomenon – a strong tropical cyclone.
Typhoons are generated in the ocean with high water temperature in the south of Japan and move westward. From summer to autumn, many typhoons turn north, approach Japan and sometimes make landfall.
▼30-year average numbers of typhoons generated, of which those approached* Japan, and those made landfalls (Source: Japan Meteorological Agency)
What do I need to keep in mind during the rainy season?
When the above-mentioned seasonal rain front is active, and especially when a typhoon is approaching Japan, it can cause heavy rainfall. Heavy rainfall could cause small to large-scale disasters including flooding and landslide. The humidity and sudden rise or drop of temperature could cause health problems as well.
<Rain-related disasters>
JMA website provides multilingual information on disaster mitigation ranging from weather forecast, weather warnings/advisories, tropical cyclone information to real-time risk map of inundation/flooding. If you are a resident in Japan, it is also important that you know the hazard map usually available on the local government’s website that demarcates areas most likely to flood, especially when major rivers overflow.
<Health Problems>
High temperatures and humidity promote the growth of bacteria that cause food poisoning. Clean your hands and work surface or table before cooking or eating. Separate raw meat, chicken and other poultry, seafood, and eggs from ready-to-eat foods. And cook thoroughly to kill the bacteria.
Change of temperature can cause physical problems, too. Especially when the temperature rises suddenly during the rainy season, the risk of heatstroke increases as the body is not yet adapted to the high temperature and humidity. Stay cool (wearing appropriate clothing and using air-conditioning as needed), stay hydrated (drinking plenty of water), and stay informed. The Ministry of the Environment provides heat illness prevention information.
Rainfall during the rainy season is said to be about one-fourth to one-tenth of the annual precipitation in Japan. The occasional torrential rains especially towards the end of the rainy season can cause various kinds of flood damage, but the rainfall of the rainy season, together with the snowfall in winter, is also an important water resource for Japan.
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