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Tsuyu, also called baiu, is a seasonal interval with more rain and less sunshine than the periods around it. A stationary front often lies near Japan and shifts north and south. Timing differs by region and year, and rain is not continuous every day.
The Japan Meteorological Agency defines the baiu front as a stationary front appearing from near Japan towards continental China during the transition to high summer. It commonly oscillates north and south while advancing gradually from Okinawa towards Tōhoku. The term does not pin it to one latitude for the whole season.
In early summer, relatively cool northern air and warm, moist air around the Pacific high meet near Japan. Converging flows support ascent and cloud formation in the frontal zone. Southwesterly flow between the Asian monsoon trough and the Pacific high can also keep delivering moisture from the East China Sea.
As the high-pressure systems change their extent and position, the zone oscillates and generally advances northwards. Expansion of the Pacific high often brings the transition to high summer. Contact between two air masses alone does not guarantee a downpour.
Cloudy spells, intermittent rain and heavy rainfall can all occur. One sunny day does not prove the season is over, nor does one downpour establish its beginning. A seasonal judgment uses a weather sequence; immediate hazards require current forecasts and warnings.
During the heavy rain of 11–14 July 2012, strong southwesterlies continually carried low-level moisture from the East China Sea into Kyūshū. The Meteorological Research Institute found cumulonimbus clouds repeatedly forming on the western, upwind side. Individual clouds could move while new ones replaced them, allowing successive cloud groups to keep raining over similar areas.
Some heavy rain lay roughly 100–200 kilometres south of the analysed front. A line on the map is therefore not an exact boundary of rainfall. This named event also does not make stationary rainbands the only form taken by the rainy season.
JMA announces onset and ending information using recent weather and roughly a week’s outlook, then reviews the season afterwards. The change typically takes about five days, which is why dates are given as occurring around a particular day rather than as a nationwide overnight switch.
JMA primarily considers changes in sunshine, also using rainfall and circulation when needed to judge whether seasonal conditions persist. Provisional announcements combine observations with an outlook of roughly a week or more; later confirmation uses the weather that actually occurred, so a date can be revised.
Continuing cloudy or rainy conditions can count as tsuyu even without a baiu front drawn on the map. A few sunny days likewise need to be read within the longer sequence.
Rain near a front also depends on incoming warm, moist air. Nagano’s meteorological office explains that continuing inflow from the south, lifted by terrain, can develop rain clouds and prolonged heavy rain in mountainous areas. The front’s position, airflow and terrain matter together; one line on a weather map does not imply equal rainfall everywhere.
JMA calls a temporary run of sunny days within the rainy season tsuyu no nakayasumi, a mid-season break. A few fine days alone therefore do not establish the season’s end. Japan’s tsuyu also belongs to a broader East Asian rainy-season setting, with different names and regional timing in China and the Korean Peninsula; one Japanese region’s dates do not describe all East Asia. JMA also uses tsuyu no modori for unsettled weather returning after the season has ended, a different situation from a break within the season.
JMA keeps separate regional records. Okinawa and Amami usually enter the season earlier; much of Honshū, Shikoku and Kyūshū experiences it in June and July, with a different progression in Tōhoku. A climatic normal averages a specified period. It cannot promise the weather of a particular journey.
A stationary front can reach Hokkaidō and bring cloudy spells or heavy rain. JMA explains that a distinct rainy season is generally not recognized there because the pattern does not occur for a sustained interval at a regular time every year. This is a climatic distinction, not a guarantee of dry weather.
Rainy-season water supports rice-field irrigation and summer supplies. A season with unusually little rain, karatsuyu, can contribute to water shortages, while intense or prolonged rain can cause floods and landslides. The season’s value for water storage and the hazards of particular rain events are both part of its effects.
Traditional terms such as samidare refer to the older calendar. Modern weather records identify regional rainfall, sunshine and seasonal transitions by year. Interpreting rain in an old poem or diary is therefore a different task from consulting a modern onset table.
Before 1964, local offices issued their own rainy-season announcements. JMA began national notices in 1964 and designated them weather information in 1986. Observation, outlook and retrospective assessment produce this record, rather than fixed Gregorian dates for the older-calendar word samidare.
Rainfall totals also use different windows. JMA’s rainy-season rainfall comparison uses June–July, or May–June for Okinawa and Amami. A provisional total for the announced rainy-season interval instead runs from onset to the day before its end, so the interval can vary between years.
Damp weather affects drying laundry, ventilation and outdoor plans. Seasonal timing helps with preparation, but rainfall, thunderstorms and changing river conditions need current information. A published onset date cannot replace the forecast for the next few days.
No. It describes a relatively rainy, less sunny seasonal interval.
No. Timing is regional.
Onset and ending are transitions rather than instant switches.
Yes. The absence of a regular tsuyu does not exclude heavy rain.
No. Check the year-specific information and forecasts.
The frontal zone involves converging flows and ascent, while moisture supply and terrain influence clouds. Heavy rain can fall away from the analysed line.
No. Moist inflow kept generating new clouds upwind, replacing moving clouds and sustaining rain over similar areas.
It includes a forecast; later assessment uses the actual sequence of sunshine, rain and circulation to judge a sustained transition.
The fixed window supports year-to-year comparison; the announced interval changes its dates and length, so it measures a different period.