Japan’s climatic conditions vary considerably from north to south, owing to the mountainous spine than runs through the islands and the archipelago’s deep latitudinal span. Main moderating influences on Japan’s weather are its mountains, surrounding oceans, proximity to the Asian mainland and the effects of monsoons, in which alternating wet/dry periods are caused by the seasonal reversal of prevailing winds.
Japan’s prevailing summer and winter conditions are the result of interactions between two huge air masses: the moist tropical air mass coming off the Pacific Ocean and the cold continental polar air mass coming mainly from Siberia.
The summer monsoon (April to early September) brings rains and warmer temperatures, often causing cyclones, typhoons and storms in later summer. The winter monsoon (late September to early March) brings cold air that picks up moisture in the Sea of Japan, depositing rain and heavy snow along Japan’s western side and bringing cold, dry winds to the eastern side.
Japan is wettest in the early summer and driest in the winter, with the exception being the Sea of Japan coast, which receives heavy snowfall in winter. East Asia experiences a summer rainy season commonly called the “plum rains”, or baiu in Japanese, bringing hot, humid weather, typhoons and torrential rain during June and July.
Japan has some of the world’s best and heaviest snowfall, and is naturally a haven for skiers and snowboarders. The main island of Honshu receives snow in many parts, but it is the northern-most island of Hokkaido that is world-renowned for heavy powder snow during its ski season of December to March.
As a general rule, the coldest month is January, and the warmest month is August. The transitional seasons of autumn and spring are generally shorter, and temperatures are generally lower, in the north compared to the south.
In Tokyo, average daily temperatures vary from a minimum of 2.5°C (36.5°F) to a maximum of 9.9°C (50°F) in January; to a minimum of 24.5°C (76.1°F) to a maximum of 31.1°C (88°F) in August.
In Sapporo, average daily temperatures vary from a minimum of -7°C (°F) to a maximum of -0.6°C (°F) in January; to a minimum of 19.1°C (°F) to a maximum of 26.4°C (°F) in August.