Tsuyu is Japan's rainy season. In simple terms, it is the period when cloudy and rainy days become more common as the country moves from late spring into summer.
The key point is that the start of the rainy season is not treated as a clean one-day switch. That is why the Japan Meteorological Agency uses wording like "around June 7" and "is considered to have started."
The timing also depends on the region. Okinawa and Amami usually enter the rainy season much earlier than most of Honshu. Kanto-Koshin is often discussed in early June, while northern Tohoku is later.
The seasonal rain pattern is closely tied to the baiu front, a front formed where warm, humid air meets cooler air to the north. When that front lingers near Japan and cloudy or rainy weather continues, a rainy-season pattern becomes easier to identify.
The practical takeaway is simple: tsuyu is best understood as a seasonal transition, not a single rainy day. For broader context in Japanese, see the related hub page at attrip.jp/tsuyu/.
