Overview of Japan's climate

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.

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.

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.

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.