Seasons across Japan: different weather in the same month
日本の四季
Japan’s seasons unfold across a long island chain. Northern cold, southern subtropical conditions and the mountain divide produce different weather within the same month.
01
Regions and changing air masses
A long north–south range
JMA’s climatic regions.
During the 1991–2020 reference period, Sapporo’s January mean temperature was −3.2°C and Tokyo’s was 5.4°C. Average monthly precipitation was 108.4 and 59.7 millimetres respectively. Latitude, surrounding seas, winds and terrain all help explain the contrast. Continental cold strongly affects Hokkaido, while Tokyo’s Kantō plains often lie on the drier, sunnier side of winter mountain barriers.
Station
January mean temperature
January precipitation
Sapporo
−3.2°C
108.4 mm
Tokyo
5.4°C
59.7 mm
Two sides of the mountains
ContinentSea of JapanMountainsPacific side
Winter airflow gains moisture over the Sea of Japan, rises over mountains and sheds snow, then descends drier towards the Pacific side. Schematic, not to scale.
In winter, cold continental air gains moisture over the Sea of Japan. Snow falls frequently on that side and in mountain areas, while Pacific-side plains often have drier, sunnier weather. Local conditions also matter. JMA records more snowfall at Takada in Jōetsu than at Niigata city. A strong winter monsoon favours heavy mountain snow; particularly cold air aloft can also bring heavy snow to the lowlands.
The changing weather of spring and autumn
In spring, highs and lows move eastwards. Warm southerlies ahead of a low can give way to colder northerlies behind it, changing temperatures within days. In early autumn, the weakening Pacific high and cooler continental high can support an autumn rain front, sometimes intensified by a typhoon. Later, migratory highs favour clearer weather. These passing systems produce fluctuations within the season’s overall warming or cooling.
Regional rainy seasons
JMA’s summer pattern: the baiu front near Japan, the Pacific high to its south, and blue arrows marking upper-level westerlies.
Baiu is the cloudy, wet early-summer season that precedes hotter midsummer. It generally reaches Okinawa about a month before Honshū. The Okinawan name sūmanbōsu comes from the solar terms Shōman and Bōshu. A stationary front often stretches west to east near Japan, with the rainy season advancing and retreating northwards. The front usually becomes indistinct near Hokkaido, where JMA does not identify the same seasonal phenomenon. Onset and withdrawal take several days, so their dates are described as “around” a particular day. The rain replenishes agricultural water supplies for midsummer.
What brings midsummer?
Surface chart, 23 July 2010, 09:00; H: high pressure. JMA.Infrared image, 12:00 the same day, three hours after the chart. JMA.Yamase schematic: Pacific at right, Sea of Japan at left. General pattern, not a section of the dated case. JMA.
When the Pacific high extends west over Japan, hot, humid and sunny weather can persist. Strong daytime heating can nevertheless build cumulonimbus clouds and produce local afternoon thunderstorms. On the Pacific coast of Tōhoku, a different pattern sometimes intervenes: cool, damp easterlies from the Okhotsk high, called yamase, bring cloud and low temperatures. The Ōu Mountains impede this air, so the Sea of Japan side may remain sunnier and warmer. Prolonged yamase can reduce rice yields.
02
Observations and calendars
Observing a particular tree
Somei-yoshino (left) and Ezo-yamazakura (right) in JMA’s source.
Normal dates, not this year’s forecast; Okinawa and Amami mainly use Hikan-zakura
JMA records seasonal changes on designated trees, using common criteria for cherry flowering, maple colouring and yellow ginkgo leaves. On a cherry observation tree, flowering begins when at least five or six blossoms open; full bloom means roughly 80 per cent of buds have opened. Each record describes the designated tree at that observation site.
Why “new spring” can feel wintry
Japan adopted the solar calendar in 1873. The old lunisolar calendar organised months by lunar phases and used leap months to stay aligned with the solar year. Its first three months were called spring, while the 24 solar terms followed the Sun’s position. Transferring old festival dates directly to the Gregorian calendar could leave names such as “new spring” attached to cold weather. Calendar seasons, daily weather and plant observations mark time differently.
Several calendars describe the same day
Meteorological seasons group March–May as spring, June–August as summer, September–November as autumn and December–February as winter. Traditional solar-term divisions instead begin at Risshun, Rikka, Risshū and Rittō. Thus “the beginning of spring” can fall within meteorological winter, and “the beginning of autumn” within summer heat. These names mark different divisions of the year.
03
Climate in everyday life
From Kiso’s autumn frost to winter food
Sunki. Source: MAFF, にっぽん伝統食図鑑.
1
Blanch stems and leaves
2
Add earlier sunki or liquid with bacteria; keep warm.
3
Mature in cool conditions
Kiso’s sunki is made by salt-free lactic fermentation of local red-turnip stems and leaves. The agriculture ministry records a local view that two or three frosts before the late-October-to-November harvest improve their sweetness and suitability for pickling. Fermentation and maturation turn the harvested greens into a food that keeps for winter; cold weather alone does not make the pickle.
Check your understanding
Why compare January data from the same normal period?
It holds the month, measure and reference period constant, rather than mixing a cold spell or different months.
How can winter air change as it crosses the sea and mountains?
Cold air gains moisture over the Sea of Japan and can deposit snow as it rises; descending air beyond the mountains often brings drier Pacific-side conditions.
Why is Kiso’s sunki season more than a calendar date?
The calendar gives a window, but frost and harvest affect the crop and processing still requires fermentation and maturation.
Why can it still be hot after Risshū?
Risshū marks a traditional solar-term boundary, not the end of hot weather.
