Baiu (rainy season) (梅雨)

Baiu (called also "tsuyu" and written also as "黴雨" [meaning 'rain of molds']) is a rainy period from May to July every year which is typical meteorological phenomenon seen in a wide area covering Japan excluding Hokkaido and Ogasawara Islands (Bonin Islands), the southern part of Korean Peninsula, coastal area of People's Republic of China including south and center China and Taiwan. If the season of baiu begins, it is called "tsuyuiri (or 'nyubai')" and if it ends and the summer begins it is called "tsuyuake (or 'shutsubai')." In most districts, meteorological agency announces tsuyuiri and tsuyuake.

It is a kind of rainy season. There are many countries in the world which have rainy season, but baiu is characterized by the fact that not so heavy rain lasts for a long period of time. Because of this fact, it is said that special caution against molds and food intoxication is necessary.

With respect to the origin of the word "baiu," there is a view that as fruits of Japanese apricot (written as "梅") mature in this season, it is called baiu (written as "梅雨"). There is also a view that, because it is highly humid and moldy in this season it have been called "黴雨" (read as 'baiu') and it was changed to "梅雨" which has same sound "baiu." There is still another view that, because it rains almost every day (written as "'毎'日") in this season, the character "梅" is used.

The view that since it rains double (written as "倍") compared to other seasons, it is called "倍雨" (pronounciated as 'baiu') is a far-fetched explanaiton.

In mainland China and Taiwan, it is called "梅雨" (pronounced as "mei-yu") and, in Korea, "장마" (pronounced as "chan-ma" and written as "長霖" in Chinese characters). In China, characters "霉雨" which has same sound was used in the past and it sometimes used even today. "霉" has the same meaning as "黴" and therefore "霉雨" has the same meaning as "黴雨" in Japan. In China, "黄梅雨" (fan-mei-yu) which means "rain in the period in which fruit of Japanese apricot matures and their color changes to yellow" is often used.

Cause and mechanism

In the season of baiu, the following four air masses exist in the East Asia.

Yangzi jiang air mass
It exist in the area covering the northern part of China, Mongol and Manchuria. It is a warm and dry continental air mass.

Okhotsk Sea air mass
It exists over the Okhotsk Sea. It is a cold and wet maritime air mass.

Tropical monsoon air mass
It exists in the area over Indochina, South China Sea and sea around Nansei Islands. It is a warm and wet maritime air mass. It is influenced strongly by the maritime air mass of the Indian Ocean.

Ogasawara air mass
It exists over the western part of the North Pacific Ocean. It is a maritime air mass.

When the season changes from spring to summer in East Asia, those air masses with different nature struggle with each other. In the Mainland China area, the Yangzi jiang air mass in the north and the tropical monsoon air mass in the south struggle with each other. In Japanese Islands area, the Okhotsk Sea air mass and the Ogasawara air mass struggle with each other. Between air masses in the north or between air masses in the south, no collision occurs and no front is formed because they have similar nature with each other.

In the part where northern and southern air masses collide with each other, a baiu front is formed for several thousands kilometers from east to west and it moves to the north gradually over several months. It rains continuously in the vicinity of this front and the period for continual rain lasts for one to two months in each district. This is baiu.

Initial stage of baiu front

During winter, a wide range from Siberia to China is covered by cold and dry Siberia air mass. The Siberia air mass comes down the south often accompanied by cold wave and brings a heavy snow to Japan on the Japan Sea side, but in Tibetan Plateau, high mountains prevent the air mass from further going down to the south. Subtropical jet stream flows in the sky of between India and Philippines to the south of Tibetan Plateau.

If the winter ends and the spring comes closer, Siberia air mass loses its strength and moves up to the north. Instead of it, warm and dry Yangzi jiang air mass begins to be spawned and increases its strength. If spring comes, the Yangzi jiang air mass releases a migratory anticyclone to Japanese islands and Korean Peninsula in its east. The migratory anticyclone is carried by the westerlies to the east and causes changeable spring weather together with the low-pressure system generated between high-pressure systems.

If spring comes close to its end, the tropical monsoon air mass in the vicinity of the South China Sea increases its strength and comes up to the north. Then, collision between the Yangzi jiang air mass and the tropical monsoon air mass begins. By looking at surface weather chart, we can find that high-pressure system spawned by the Yangzi jiang air mass and the high-pressure system spawned by the tropical monsoon air mass are struggling with each other on the South China Sea and the front has been formed between them. This is the baiu front in its initial stage.

Every year, a belt of cloud that is the forerunner of formation of baiu front (technically, it is sometimes called as "quasi-steady cloud belt") is spawned.

Baiu front becoming clear

At the beginning of May, the baiu front begins to influence Nansei Islands. Around the middle of May, the baiu front appears clearly on the weather chart and becomes stationary in South China and the vicinity of Nansei Islands.

On the other hand, around May in which the early summer begins, the subtropical jet stream moves also to the north and reaches Tibetan Plateau. However, as Tibetan Plateau is higher than the subtropical jet stream, the latter is divided by the former into two streams, northern and southern streams.

From divided subtropical jet streams, the northern split flow joins together with the polar jet stream in the vicinity of Sakhalin. This air current joins further together with the southern split flow around the Kamchatka. If the air in the sky becomes stationary because of the impact of this joining of streams, a descending air current is spawned and high-pressure system is created on the Okhotsk Sea below it. This high-pressure system is called Okhotsk anticyclone and cold and wet air mass which is the mother of this anticyclone is called Okhotsk air mass.

Around the same time, also on the sea in the middle of Pacific Ocean, an anticyclone increases its strength and enlarges its area to the west. This high-pressure system is called Pacific anticyclone and warm and wet air mass which is the mother of this anticyclone is called Ogasawara air mass.

Around the end of May or beginning of June, Kyushu and Shikoku regions begin to be influenced by the baiu front. Around this time, in the eastern part of the baiu front, the Okhotsk air mass and the Ogasawara air mass frequently collide with each other. On the other hand, in North China, Korean Peninsula and eastern part of Japan, weather as in spring, in which high-pressure systems and low-pressure systems come alternately.

Baiu front moving up to the north

In mid-June, baiu front which continues to go up to the north becomes stationary in China in the vicinity of Nanling and extends its force to the area near Honshu (the main island of Japan).

Then, the baiu front moves up to the north to the basins of the Yangtze River and the Huaihe River. In the end of June, South China and the Nansei Islands are excluded from the sphere of influence of the baiu front.

Then, the baiu front moves up to the north to North China and the north-eastern district and, with the middle and end of August as the border, it begins to move down to the south. After that, it changes to so-called "akisame" (autumn rain) and the name of front changes also to autumn rain front. However, the air masses which compose air in the south and north of the front are the same.

Nature of baiu front

In any place where two air masses which are different in their natures collide with each other, the state of air becomes unstable and a front is spawned. Because air masses which compose the baiu front has similar strength with each other, it becomes a stationary front that moves slowly between the south and north.

If two high-pressure systems struggle with each other and the strength of both systems is balanced, the baiu front hardly moves. If the balance between two high-pressure systems is lost or a low-pressure system approaches, however, the front may change temporarily to a warm front or a cold front. Baiu is deemed to have ended when activities of the baiu front are weakened by expansion of strength of the Pacific anticyclone or the baiu front has been forced to move the north of each area and no rain is expected because of the influence of the front.

Generally, it is said that there is no baiu in Hokkaido. This is partly because the baiu front becomes stationary mainly on Honshu and also because precipitation because of baiu is seldom observed in Hokkaido since the strength of the front is lost in the end phase of baiu and the speed of moving up to the north is high.

With respect to the Tohoku region, no announcement of tsuyuake is given in certain years and, because of this, the summer in the Tohoku region is in fact shorter than in Hokkaido. Therefore, the length of meteorological summer is extremely different between the southern part of Hokkaido (Hakodate and so on) which has comparatively warm climate and Shimokita and Tsugaru regions of Aomori Prefecture which are on the northern end of Honshu.

Ogasawara Islands (Bonin Islands) have no baiu because the Pacific anticyclone is dominant since the early summer and the baiu front cannot come close and the region is covered by the real summer air. As the period in which the region is covered by the real summer air is long, however, the region tends to have typhoons.

As both of two air masses which compose the southern part of the baiu front are air masses which have an ocean as its base (martitime air mass), they absorb a large amount of moisture from the ocean and contain wet air. On the other hand, air masses which compose the north and south of the front consist normally of weak rain cloud of nimbostratus because the difference in temperature between them is small. Therefore, it causes gentle rain which is not strong to last for a long time period. If cold air flows into the sky, or parent cloud for cumulonimbus called cloud cluster advances toward the east from the vicinity of Mainland China, however, a large amount of moisture is condensed to form cloud in the vicinity of the baiu front which cloud becomes strong rain cloud often accompanied by cumulonimbus.

The baiu front is meteorologically a front which brings monsoon (monsoon front). Monsoons in the South Asia including India and Southeast Asia are caused by air current of monsoon originated in the Indian Ocean. This air current extends further to the east and influences tropical monsoon air mass on the South China Sea and has relationship. If we look at the air in the sky in the vicinity of the baiu front, the polar front which forms a border between the winter air and spring or autumn air and the subtropical front which forms the border between the summer air and the spring or autumn air exist close to each other.


Extreme value for each region

In Japan, the local meteorological observatory in each region and the Japan Meteorological Agency declare tsuyuiri and tsuyuake every year. When it is deemed that tsuyuiri or tsuyuake occurs, provisional declaration is given and, after examining around September every year the progression of weather from May to August in a comprehensive manner, the judgment becomes final. On this occasion, the date of tsuyuiri or tsuyuake may be amended or an expression "not identified" may be given. In general, baiu comes earlier in the southern region and, on an average, it is from the middle of May to the end of June in Okinawa and from the end of June to the end of July in the Tohoku region and the Hokuriku region.

There is no significant difference in the length of the period of baiu and it is around 40 to 50 days, but the amount of annual precipitation during the period is larger toward west and south.

If the baiu front stays stationary until risshu (the first day of autumn) comes, no declaration of tsuyuake is made. Even in such case, "tsuyuiri" occurs as usual in the next year. However, it is not considered that "One year passed without tsuyuake and another tsuyuake occurs redundantly."

Announcement of the end of the period of baiu is called tsuyuake sengen (declaration of tsuyuake), but baiu does not end because of the declaration by the Japan Meteorological Agency. Usually, tsuyuake occurs in the direction from south to north, but, in certain cases, it does not occur in such order. If the front remains in certain region, tsuyuake occurs earlier in the area that is located to the north of such region. In the past, there were cases in which tsuyuake occurred in the Hokuriku region earlier than in the Chugoku region in which tsuyuiri occurred earlier and tsuyuake in the Kanto region was delayed largely compared to western Japan.

Immediately before tsuyuake, the amount of rainfall increases and it tends to cause intense rainfall. On the other hand, with respect to the period after tsuyuake until the beginning of August, it is said "tsuyuake toka" (literally, ten days after tsuyuake) and the weather is stabilized and sometimes boiling hot days come.

History and significance of forcasting of Tsuyu

It is said that the Japan Meteorological Agency began to provide information of tsuyuiri and tsuyuake around 1955 and it was communicated to information mediums as "oshirase" (notice). However, the Japan Meteorological Agency had a policy not to carry out this provision of information actively. Declaration as weather information was begun in 1986. It is said that the reason was to make people to have interest in disaster caused by heavy rainfall.

Mainland China and Taiwan

Baiu is seen in Central and South China as well as Taiwan. In the Mainlnad China, local meteorological observatory in each city and in Taiwan, Central Weather Bureau announce tsuyuiri and tsuyuake. According to a research on the average value of the date of tsuyuiri and tsuyuake in each city for 1971 through 2000, tsuyuiri in the downstream basin of the Yangtze River is June 14 and tsuyuake July 10 and, in the basin of the Huaihe River, Tsuyuiri is June 18 and tsuyuake July 11.

As rough indication, in Taiwan and South China, a long rain by the baiu front begins aroung the middle of May and ends around the end of June. As time advances, the area of long rain is shifted gradually to the north. The season of long rain comes to Center China (middle and downstream basin of the Yangtze River) between the middle of June and the beginning of July and to a part of North China between the end of June and the end of July. The long rain lasts for approximately a month in each region.

Korean Peninsula

With respect to the southern part of Korean Peninsula, the period of long rain begins in a part of the Republic of Korea between the end of June and the end of July. The long rain lasts for approximately a month. Korea Meteorological Administration announces tsuyuiri and tsuyuake of each region and carries out also forecast of tsuyuiri and tsuyuake in advance.

General characteristics

Around May or June before tsuyuiri, weather that is similar to baiu is seen sometimes and it is called "hashiri tsuyu," "tsuyu no hashiri" or "mukae tsuyu."

It occurs rather often that, immediately after tsuyuiri, comparatively gentle rain continues. In the middle of baiu, there is sometimes a period in which weather gets better. Such period is called "tsuyu no nakayasumi" (literally, "break of tsuyu").

During the period of tsuyu, in particular in the case of long rain, as the hours of sunlight is short, the up and down of air temperature (difference between the highest and lowest air temperature in a day) is small and it is sometimes felt chilly. Such chill is called "tsuyu samu" or "tsuyubie." On the other hand, sunny spells during the period of baiu is called "tsuyubare" or "tsuyu no harema," and during such periods, the air temperature gets high and humidity is also high. Therefore, on the day of tsuyubare, the discomfort index gets high and it is felt unbearable and it tends to cause heat attack.

In the end stage of baiu, the amount of rainfall tends to increase and localized torrential rain occurs sometimes. This tendency is stronger in the south and west, and especially in Kyushu, at the rate of once in ten-odd years, the amount of rainfall equal to that for a year rains in a week.
(Typical examples are seen in the area along mountain range that runs through Kyushu from north to south in Kumamoto, Miyazaki and Kagoshima Prefectures.)
To the contrary, in eastern Japan such as Kanto and Tohoku regions, the amount of rainfall is bigger in the period of akisame (autumnal rain) that in the period of baiu.

Rain in the last stage of baiu is called "arazuyu" or "abarezuyu." In addition in the last stage of baiu, it often rains accompanies by thunder and it is called "okurizuyu." Sometimes even after tsuyuake, it continues to rain or, after becoming sunny once, it rains again. It is called "kaerizuyu" (it is written as '返り梅雨' instead of '帰り梅雨') or "modorizuyu." Such expressions are not used so often in the recent years.

In the year in which tsuyuake is delayed, there is a tendency a cold summer occurs and tends to cause cold weather damage.

Karatsuyu (literally, empty baiu)

Sometimes, there is a period of baiu in which hardly any rainfall is seen. Such baiu is called "karatsuyu." If karatsuyu occurs, in many cases, water to be used in the summer cannot be secured resulting in drought. In the northern Kyushu and Setouchi regions in which the amount of rainfall in autumn and winter is small, if there is no considerable amount of rainfall by typhoon and so on after karatsuyu, such drought continues for more than a year sometimes.

Negative and positive baiu

Baiu in which not so strong rainfall continues for a long period of time is expressed as negative baiu and is baiu in which, if it rains, a large amount of rainfall occurs in a short time and, if not rains, sunny weather comes is expressed as positive baiu. Negative baiu is called also "onnazuyu" (literally, 'female baiu'), and positive baiu "otokozuyu" (literally, 'male baiu'), but they are seldom used in recent year as such expressions are deemed as discrimination by sex.

As a general tendency, in the case of negative baiu, strength of Okhotsk anticyclon tends to be large and, in the case of positive baiu, strength of Pacific anticyclone tends to be large; however, this is not necessarily appropriate as it also depends on the conditions such as passage of the westerlies, and Arctic oscillation or southern oscillation (ENSO, El Nino/La Nina).

Relation with typhoon

Typhoon and tropical cyclone suck up air from the environment in the space near ground surface on one hand, and on the other hand, in the upper troposphere several thousands to ten thousands meters high, makes sucked air wet and discharges it in large amount into the environment. Therefore, if typhoon or tropical cyclone comes close to or lands in the vicinity of bail front, the baiu front which has been supplied with plenty of moisture vapor becomes active resulting in torrential rain. In certain cases, the baiu front comes up to north together with a typhoon or tropical cyclone which has lost its strength and the baiu ends all of a sudden.

Pattern of torrential rain by baiu

It is said that, if you look into cases of heavy rain and torrential rain during the period of baiu, there are certain patterns. The pattern in which low-pressure system advances to the east in the vicinity of the baiu front which is stationary in the south Japan Sea tends to cause torrential rain on the Japan Sea side. Wet air flows into the low-pressure system from southwest and clashes to mountains and causes torrential rain locally.

The pattern in which baiu front stays stationary for a long time period or baiu front is stationary in the vicinity of Pacific side and low-pressure system is stationary on the west side tends to cause torrential rain on the Pacific Ocean side. Wet air flows in from south or southeast and clashes to mountains and causes torrential rain locally.

Also, sometimes, torrential rain is caused by a pattern in which unique moisture vapor called moist tongue flows in under above-mentioned conditions or by developed cumulonimbus such as tapering cloud becomes stationary or flows into the same region one after another.

Relation with ocean fluctuation

From a statistical point of view, if El Nino in which the temperature of sea water increases in the middle and east Pacific Ocean and decreases in the west Pacific Ocean in the vicinity of the equator occurs, there is a tendency tsuyuiri and tsuyuake in Japan delay and the amount of rainfall stays as usual and the hours of sunlight increases. If La Nina in which the temperature of sea water decreases in the middle and east Pacific Ocean and increases in the west Pacific Ocean in the vicinity of the equator occurs, there is a tendency tsuyuiri and tsuyuake in certain part of Japan come earlier than usual except Okinawa where tsuyuiri is delayed and the amount of rainfall increases excluding certain part and the hours of sunlight decreases.


Continual rainfall mainly from the end of March to the beginning of April is called "Natanetsuyu." As rain falls in the season in which flowers of oilseed rape bloom. It is rare that rainfall continues for several days or localized torrential rain is observed as in baiu, but there are many cloudy or rainy days and vague weather continues sometimes for many days. This phenomenon occurs because of the fact that a front tends to become stationary along the southern shore of Honshu as expansion of continental anticyclone which was dominant during winter in the vicinity of Honshu or the path of the migratory anticyclone leans to the north, and on the other hand, cold and wet northeast airstream flows along southern edge of the expansion of anticyclone from the north. On such occasion, it is also one of the characteristics that small low-pressure systems tends to be spawned frequently. Therefore this expression is usually used for the Pacific coastal area in west and east Japan and this phenomenon cannot be seen in north Japan. In recent years, partly because of tendency of unusually warm winter and influences of global warming, natanetsuyu tends to be carried forward to the winter and fluctuation of weather is worried about. As an example, in February 1990, cloudy days continues centering on the latter half of the month and total hours of sunlight was only 81 hours symbolizing quite unusually warm winter. In March 1985, in regions on Pacific coast from the west up to the Kanto region, cold rainy days continued throughout the month. In Tokyo in the same month and year, there was no clear day (the worst record excluding June and July which fall under tsuyu season) and it was commented in the Weather Chart Diary published by the Japan Weather Association as "dark March." In 1988, 1991, 1992, 1995, and 1999 also, cloudy or rainy days continued rather long in March, hours of sunlight in the month was short excluding north Japan. Therefore, March in certain years around the end of the twentieth century were labeled unpleasantly as "no sun above field mustard," "month unfavorable for outdoor amusement," "rain apparel such as umbrellas indispensable for cherry blossom viewing party," "commencement day ceremony and closing exercises on rainy day" and so on. Other than those years, in 2002 and 2006, the south coast front became stationary and unclear sky which tended to rain mainly in the morning and night continued and certain weather-related casters commented "the forerunner of natanetsuyu."

Autumnal long rain (susuki tsuyu)

Refer to the section for autumnal rain.

Hashiri tsuyu

It means the condition in which rain continues like forerunner of the baiu front mainly from the end of May. As unohana (deutzia) blossoms bloom just in this period, it is called also "unohanakutashi," which hints the rain that spoils blossoms. "Hashiri" means "forerunner" and "hashiri tsuyu" is also understood as a rain continues in advance of baiu. It is also called "tsuyu no hashiri" sometimes. It is the baiu season for Nansei Islands such as Okinawa and it is often seen when the baiu front in the vicinity of Nansei Islands goes up to the north temporarily along the south coast of Honshu. If Okhotsk anticyclon appears in the first half of May, as it tends to be influenced by northeast airstream, on the Pacific side from the Kanto region to the north, low temperature and cloudy or rainy weather continues for a long period of time. The case like May storm in which cold front extending behind developed low-pressure system passing the Japan Sea or north Japan, after passing through Honshu and reaching the Pacific side, becomes a stationary front along the north side of the dominant high-pressure system on the southern sea and unsettled weather continues mainly on the Pacific coastal area in east Japan is a similar phenomenon.

Sazanka tsuyu

Continual rain mainly around the end of November and the beginning of December is called "sazanka tsuyu." This name is given because it is rainfall in the period in which sazanka (Camellia sasanqua) blossoms bloom.

Ezo tsuyu

Generally, it is said that there is no baiu in the Hokkaido region. In certain years, however, in the same period as Honshu, unsettled chilly weather caused mainly by force of Okhotsk anticyclone coming down to the south continues for around two weeks. Japan Meteorological Agency does not acknowledge this as a baiu. Weather in this period in Hokkaido is commonly called "rirabie" (literally, "chill with lilac").

Disasters brought by baiu front

Meteorological records of baiu
Largest hourly amount of rainfall
153 mm

Mt. Nagaura-dake, Nagasaki Prefecture (July 23, 1982; second largest in recorded history for all observation points in Japan)
129 mm
Toi, Shizuoka Prefecture (July 4, 2003; 16th largest)
127.5 mm
Nagasaki, Nagasaki Prefecture (July 23, 1982; 17th largest)
127 mm
Makurazaki, Kagoshima Prefecture (June 25, 2000; 18th largest)
Among top twenty in recorded history for all observation points in Japan, only four points have been recorded for baiu. As reasons for this, it is mentioned that, in the period of baiu, localized torrential rain accompanied by cold does not occur often and that typhoon does not come often.

Largest yearly amount of rainfall
8670 mm
Ebino, Miyazaki Prefecture (1993; top in recorded history for all observation points in Japan)
In this year, tsuyuiri occurred on May 17 and tsuyuake was "not identified." During the period of baiu, it rained 122 mm in May, 2,242 mm in June, 2,299 mm in July and 1,717 mm in August.
(The amount of rainfall for the period from tsuyuiri until the end of August exceeded annual precipitation.)
(Annual precipitation in an average year is 4,582.2 mm.)

Matters relating to baiu

As it falls on May by old calendar (lunar-solar calendar), it is also called samidare (literally, "rain in May"). "Samidareshiki" means that certain matter continues for a long time period like rainfall in the baiu season.

Darkness in the night in the period in which samidare falls is called satsukiyami (literally, darkness in May). Sunny spells during baiu is called "satsukibare" (literally, sunny spells in May), but, in recent years, it is more popular that this expression is read as "gogatsubare" and means fine weather in the beginning of May by new calendar (solar calendar). Japan Meteorological Agency, which is a specialist for weather, determined to call fine weather in May as "satsukibare" and sunny spells during baiu season as "tsuyuno aimano hare" (sunny spells during baiu).

Ame (rain) (Lyric writer: Hakushu KITAHARA; composer: Ryutaro HIROTA)
Amefuri (rainy weather) (Lyric writer: Hakushu KITAHARA; composer: Shinpei NAKAYAMA)
Amefuri otsukisan (moon in the rain) (Lyric writer: Ujo NOGUCHI; composer: Shinpei NAKAYAMA)
Amefuri kumanoko (child bare in the rain)
Teruteru bozu (a little hand-made doll made of white paper or cloth hanging outside of the window by a string) (Lyric writer: Rokuro ASAHARA; composer: Shinpei NAKAYAMA)
Katatsumuri (snail) (song for school music class)
Samidare (Lyric writer and composer: Eiichi OTAKI)

Tsuyuaoi (hollyhock)
Ajisai (Japanese hydrangea)
Typical flower that blooms in the season of baiu
It appears very often when baiu is expressed.

Living organism
It is humid during baiu and molds grow most actively in this period.

Food intoxication
Both humidity and air temperature are high during baiu, and the number of cases of food intoxication increases.

Sediment disaster
Damages caused by mudflows and landslides.

Floods from rivers and inland waters.

Haiku (a Japanese poem in seventeen syllables having a 5-7-5 syllabic form and traditionally containing a reference to the seasons)
By gathering water from samidare, Mogami-gawa River flows very quickly (Basho MATSUO)