1. How the Rainy Season Start Date Differs by About 5 Weeks Between South and North Japan
The idea that "the rainy season begins in June" feels different depending on where you live. The Japan Meteorological Agency’s normal values, which are averages from 1991 to 2020, show that the rainy season starts around May 10 in Okinawa, May 30 in southern Kyushu, June 4 in northern Kyushu, June 7 in the Kanto-Koshin region, and June 15 in northern Tohoku[2]. Calculating the difference from Okinawa to northern Tohoku gives 36 days, or about 5 weeks. However, the speed of this movement is not constant. If you subtract the dates, the gap from Okinawa to southern Kyushu is 20 days, while the gap from southern Kyushu to northern Tohoku is 16 days. The dates for northern Kyushu (June 4) and Kanto-Koshin (June 7) are only 3 days apart. This means the rainy season begins almost at the same time from Kyushu to Kanto. These normal values are just a guide, and the actual start date shifts from year to year. Still, the order of these dates clearly shows that the rainy season is a phenomenon that moves from south to north across the Japanese archipelago. This "moving north" has a stopping point: Hokkaido.
2. Why the Japan Meteorological Agency Makes No Rainy Season Announcement for Hokkaido
According to the Japan Meteorological Agency’s explanation for children, the rainy season front usually moves north toward midsummer, but it often becomes unclear around Hokkaido[3]. Because of this, there is no official announcement of the start or end of the rainy season for Hokkaido by the agency[3]. Some years do have periods with many rainy or cloudy days, but the difference between years is large, and it does not appear as a clear "rainy season" every year[3]. The important point here is that the presence of a rainy season is not decided solely by whether there is a lot of rain. It is linked to whether a band of rain called a "front" appears clearly. So, what two things does a front separate?
3. How the Rainy Season Front Forms Between Warm, Moist Air and Other Air
A front is the boundary where air masses with different properties collide. The Matsuyama Local Meteorological Observatory explains that the rainy season front forms between the warm, moist air associated with the Pacific High and the dry, warm air from the Chinese continent or the cold, moist air around the Sea of Okhotsk[4]. The same explanation introduces June as the time when the rainy season begins and precipitation becomes the highest of the year, though this focuses on the Shikoku region. As the Pacific High strengthens toward summer, it pushes this front north. There is an explanation that in the late rainy season, the Okhotsk High weakens, allowing the Pacific High to push the front north, so the rainy season ends and summer begins in southern regions first as the front passes[5]. It enters from the south and ends from the south. The order of dates in the first section and the movement of the front being pushed north are two views of the same phenomenon. There is no source that narrows down "why June" to a single cause. It is reasonable to read that the position and timing of the front moving from south to north happen to fall in June for the middle of the archipelago. One of the forces pushing the front is the air from the Sea of Okhotsk. Recent research is re-evaluating this role.
4. Why the Sea of Okhotsk High Is Not the Only Cause of the Rainy Season
The Sea of Okhotsk is described as the coldest sea in the world at the latitude of 45 degrees North. Two reasons are given: it is covered by sea ice, and strong tidal currents bring cold water to the surface[6]. In explanations of the rainy season, the Okhotsk High that appears over this sea is often highlighted. For example, in the Japan Meteorological Agency’s one-month forecast released on June 18, 2026, it was expected that the rainy season front would have difficulty moving north in June because the Okhotsk High was strong, and that the Pacific High would become dominant in July, making the front active[5]. This is a forecast, not actual results. It is one example showing how the position of the front affects the weather. However, a study by Mie University and Hokkaido University published in the academic journal *Journal of Climate* in December 2020 used numerical simulations to show that the cold Sea of Okhotsk strengthens the Pacific High and increases rainy season precipitation, but the rainy season occurs even without the Okhotsk High. The study concluded that for the rainy season, the Sea of Okhotsk plays a secondary role, not a main one[6]. This is one study based on simulations. It is a separate question whether this completely overturns the established theory. Still, it clearly shows an example where the standard explanation has room for revision.
5. How the Calendar's "Nyubai" Differs from the Agency's Rainy Season Start
The calendar has a day called "nyubai" (entering the rainy season). In the National Astronomical Observatory’s calendar essentials, it is one of the zatsusetu (miscellaneous seasonal markers), and in 2026, it is June 11 at 6:14 (solar longitude 80 degrees)[7]. This is different from the actual start of the rainy season[7]. Since the normal start date for Kanto-Koshin is around June 7, the 2026 nyubai is calculated to be 4 days later. So, how is the agency’s start date determined? The Japan Meteorological Agency describes the rainy season as a seasonal phenomenon where rain increases and sunshine decreases compared to the periods before and after, during the transition from spring to summer[1]. It is not a sudden change on a specific day, but a phenomenon that changes gradually. Because of this, there is an average "transition period" of about 5 days for the start and end of the rainy season, and the announced "around [date]" indicates roughly the middle of this period[1]. Separate from the initial flash report, a confirmed value is determined after the season passes, based on the actual progression of the weather[1]. So, when did the word "rainy season" itself start being used?
6. Why "Samidare" Uses the Old Calendar's May for the Rainy Season Rains
In Japan, the word "rainy season" (tsuyu) entered from China after the Edo period. Before that, the long rains of this period were called "samidare" (May rain)[8][9]. The "May" in "samidare" refers to the 5th month of the old lunar calendar. Samidare was the long rain that continued around the old 5th month, which corresponds to the time of the rainy season[9]. It is surprising that although the name includes "May," it does not refer to the rain in the current May. There are various theories about the origin of the writing for "rainy season." One theory is that it is the long rain when plums ripen. Another theory is that it is a time when mold spreads due to the dampness, so it was called "baiu" (mold rain), and the character for "plum" (same sound) was used instead of "mold"[8]. No single theory has been decided. We can trace how Japanese names for the rain band moving from south to north have changed. So, when does this rain band come to your region?
7. How to Calculate Days by Comparing Your Region's Normal and Confirmed Rainy Season Dates
The Japan Meteorological Agency’s website lists the normal values and past confirmed values for the start and end of the rainy season. Open the page for your region and line up the normal dates with the confirmed values from recent years. Use subtraction to find out how many days early or late some years are. In years where the start date differed greatly from normal, compare how the movement of the front was different using agency data. Looking at the numbers lets you feel whether the range between early and late years is larger than you thought. It is also an interesting way to check if the date announced in the flash report differs from the confirmed value decided later, even within the same year[1].