JAPANESE CULTURE, EXPLAINED

Akisame: the wet transition from summer to autumn

秋雨

Akisame is autumn rain associated with the change from summer. A stationary front can bring prolonged wet weather, while typhoons are separate systems that may affect rainfall during the same season.

How the autumn rain front develops

As summer’s high weakens

Warm, moist air rises along a sloping boundary above cooler blue air, with cloud and rain. — A general frontal cross-section, not to scale or a depiction of the 2005 event.

As summer ends, the Pacific High weakens and continental high pressure extends southward. A front may stall between contrasting air masses. Warm air rises over colder air, expanding and cooling as it climbs; water vapour condenses into cloud, with rain developing under suitable conditions. A lingering front can keep the same area wet. A distant typhoon may supply warm, moist air to that front, so distance from the storm’s centre alone does not explain the rainfall.

A different transition from baiu

The baiu front generally shifts north as late spring gives way to midsummer. Autumn rain belongs to the transition in the other direction: JMA’s Tōhoku account describes the front moving south, followed by clearer spells under migratory highs. Both involve fronts, but their seasonal setting and broad progression differ. Individual fronts can still wander or stall rather than moving steadily each day.

Rainfall in observations and an event

Comparing Tokyo’s summer and autumn rainfall

Use the same place, reference period and interval length. Tokyo’s 1991–2020 monthly normals total 478.7 mm for June–August and 556.0 mm for September–November. Autumn is wetter in this comparison. These are all precipitation over two three-month seasons, not exact baiu and akisame periods; typhoons and other weather contribute too. The totals do not measure rain caused solely by the autumn front.

Period	Three-month precipitation normal
Summer: June–August	478.7 mm
Autumn: September–November	556.0 mm

2005: a distant typhoon feeds rain in Kantō

4 September 2005, 21:00 JST: Typhoon No. 14 and the front; the orange arrow indicates warm, moist air. JMA.

Kantō radar rainfall intensity at the same time, paired with the weather chart. JMA.

At 21:00 on 4 September 2005, Typhoon 14 lay east of Okinawa, while its warm, moist airflow fed a front near Kantō and enhanced cumulonimbus development. JMA records local rainfall above 100 mm in an hour in Tokyo and Saitama. A weather chart shows the larger pressure systems; the radar image at the same time locates the rain. Together they connect a distant moisture supply with rainfall near the front.

A weather term, not a fixed date

A seasonal name, not a fixed calendar

JMA places akisame broadly between the latter half of August and October, with regional differences. Shūrin emphasizes prolonged autumn rain; a rainy spell may include overcast intervals. These terms describe a seasonal pattern rather than a common start and end date for every part of Japan.

Check your understanding

How could a typhoon near Okinawa contribute to intense rain in Kantō?

Its circulation can feed warm, moist air into a distant front, where ascent and cooling help develop cloud and rain.

Can Tokyo’s 556.0 mm autumn total all be assigned to the autumn rain front?

No. It is the normal for all September–November precipitation, including other weather systems, not a total classified by cause.

How do the fronts’ broad seasonal progressions differ?

Baiu generally progresses northward and the autumn front southward, although daily positions can vary or stall.

Sources and images