Japan's rainy season and autumn rains are seasons of prolonged rain created by the competition between air masses with different characteristics.
Basic Knowledge
The rainy season occurs from June to July when a stationary front (rainy-season front) forms at the boundary between the Okhotsk Sea air mass (cold and humid) and the Ogasawara air mass (warm and humid). The rainy-season front stretches from east to west and brings prolonged rain as it slowly moves north and south over Honshu.
The end of the rainy season occurs when the Ogasawara air mass becomes dominant and pushes the rainy-season front northward until it disappears, marking the beginning of full summer.
Autumn rains occur in September and October. As the summer Ogasawara air mass retreats and is replaced by cold air from the north, the autumn-rain front remains stationary and prolonged rain continues. If a typhoon passes near Japan during the same period, it can intensify the autumn-rain front and cause heavy-rain disasters.
Rainy-season front (stationary front at the boundary between the Okhotsk Sea and Ogasawara air masses)
Autumn-rain front (stationary front in autumn; September–October)
Beginning and end of the rainy season (seasonal turning points announced by the Japan Meteorological Agency for each region)
Rainy-season drought (a year with little rain during the rainy season)
Linear precipitation band (a phenomenon in which cumulonimbus clouds form one after another, line up in a band, and produce heavy rain)
Back-building phenomenon (a mechanism in which cumulonimbus clouds continue to form one after another in the same place)
Further Study (Background and Significance)
Localized torrential rain increases at the end of the rainy season, and in recent years disasters caused by linear precipitation bands have become more serious. During the July 2020 heavy-rain event (including southern Kumamoto Prefecture), linear precipitation bands remained stationary for a long time, and some locations recorded more than 400mm of rain in 24 hours.
Linear precipitation bands form through the back-building phenomenon, in which cumulonimbus clouds form one after another in the same place. Since 2021, the Japan Meteorological Agency has issued real-time information about the occurrence of linear precipitation bands.
The rainy season and autumn rains are essential for agriculture (rice planting and rice growth), but they are also periods of high disaster risk. Because the frequency of short-duration heavy rain is increasing due to climate change, disaster-prevention awareness is becoming increasingly important.
- Rainy season = June–July; Okhotsk vs. Ogasawara
- Rainy-season front = stationary front (long from east to west)
- Autumn rains = September–October; autumn-rain front
- Linear precipitation band = cumulonimbus clouds lined up in a band
- Back-building = cumulonimbus clouds forming one after another at the same location
- The end of the rainy season is a period when disasters often occur
- The Japan Meteorological Agency announces the beginning and end of the rainy season
Points of Caution (Easily Confused)
① The rainy-season front and autumn-rain front are stationary fronts. ② The end of the rainy season is around the time when the Ogasawara air mass becomes dominant. ③ Both linear precipitation bands and typhoons cause heavy rain, but their mechanisms differ. ④ A rainy-season drought means a rainy season with little rain; it does not mean that there is no rainy season.
Practice
- Which two air masses form the rainy-season front?
- What is the phenomenon in which cumulonimbus clouds line up in a band and produce heavy rain?
- State the months when autumn rains are likely to occur.