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Overview of Japan's climate

Seasonal variation of meteorological elements in Sapporo, Tokyo, Fukuoka, and Naha

Figure 1 Seasonal variation of meteorological elements in Sapporo, Tokyo, Fukuoka, and Naha
The green, red and blue lines indicate monthly averages of daily mean, maximum and minimum temperatures, respectively. The blue and brown bars show monthly precipitation amounts and monthly sunshine durations, respectively.

Winter (December-January-February)

Weather chart at 00UTC, 10 December 2012

Figure 2 Weather chart for 00UTC on 10 December 2012
The Siberian High developed over the Eurasian Continent and the Aleutian Low developed over the northern North Pacific. Cold air flowed southeastward across Japan, bringing heavy snowfall to its Sea of Japan side.

In winter (December-January-February), the Siberian High develops over the Eurasian Continent and the Aleutian Low develops over the northern North Pacific. Prevailing northwesterly winds cause the advection of cold air from Siberia to Japan and bring heavy snowfall to Japan's Sea of Japan side (upstream of mountainous land) and sunny weather to its Pacific side (downstream of mountainous land). Temperatures as low as -20°C are frequently observed in inland areas of Hokkaido, while Okinawa and Amami have mild winters due to their subtropical location.

Spring (March-April-May)

Weather chart at 00UTC, 4 May 2006

Figure 3 Weather chart for 00UTC on 4 May 2006
An anticyclonic system covered Japan and brought sunny conditions to northern and eastern Japan.

In spring (March-April-May), migratory cyclones and anticyclones that alternately move eastward prevail across Japan. Temperature increases (decreases) in front (back) of cyclonic systems due to warm southerly (cold northerly) flow. Temperature rises gradually with large short-term variations. Sunshine duration is long in the second half of spring due to the predominance of anticyclonic systems. The rainy season (known as the Baiu) begins in early May in Okinawa and in mid-May in Amami.

Summer (June-July-August)

Weather chart at 00UTC, 16 August 2007

Figure 4 Weather chart for 00UTC on 16 August 2007
The North Pacific high extended westward, bringing very hot and sunny conditions to eastern and western Japan. Japan's record high temperature (at that time) of 40.9°C was observed at Kumagaya in Kanto/Koshin district and Tajimi in Tokai district..

Early summer is the rainy season, known as the Baiu, in Japan. Its precipitation is caused by a stationary front, called the Baiu front, which forms where a warm maritime tropical air mass meets a cool polar maritime air mass. In the second half of summer, the North Pacific High extends northwestward around Japan, bringing hot and sunny conditions to the country. Western Japan sometimes experiences temperatures of 35°C or above. On the other hand, the Okhotsk High sometimes appears over the Sea of Okhotsk and causes cool and moist easterly winds (known as Yamase), which bring cloudy and rainy conditions to the Pacific side of northern and eastern Japan. The number of tropical cyclones approaching Okinawa/Amami peaks in August.

Autumn (September-October-November)

Weather chart at 00UTC, 21 September 2011

Figure 5 Weather chart for 00UTC on 21 September 2011
The strong typhoon Roke and the autumnal rain front caused torrential rains in and around eastern Japan.

In autumn (September-October-November), temperatures fall gradually. Monthly precipitation amounts are large in September due to the active autumnal rain front and tropical cyclones. In October, the frequent passage of anticyclonic systems brings sunny conditions and refreshing air to Japan. The frequency of cold northwesterly flows across Japan and precipitation (rainfall or snowfall) on the Sea of Japan side of the country show an increasing tendency in November.