In general, Japan’s climate is characterized as monsoonal (i.e., governed by seasonal winds). The main influences are the country’s latitudinal extent, the surrounding oceans, and its proximity to the neighbouring Asian landmass. There are numerous local climatic variations, the result of relief features. In winter, the high pressure zone over eastern Siberia and the low pressure zone over the western Pacific result in an eastward flow of cold air (the winter monsoon) from late September to late March that picks up moisture over the Sea of Japan. The winter monsoon deposits its moisture as rain or snow on the side of Japan facing the Sea of Japan and brings dry, windy weather to the Pacific side. The pressure systems are reversed during the summer, and air movements from the east and south (the summer monsoon) from mid-April to early September bring warmer temperatures and rain. Cyclonic storms and frequent and destructive typhoons occur during late summer and early fall, especially in the southwest.
The warm waters of the Kuroshio (Japan Current), which corresponds to the Gulf Stream of the Atlantic, flow northward along Japan’s Pacific coast as far as latitude 35° N. The Tsushima Current branches westward from the Kuroshio off southern Kyushu and washes the coasts of Honshu and Hokkaido along the Sea of Japan; it is this current that lends moisture to the winter monsoon. The counterpart of the Labrador Current, the cold Oya (Kuril) Current, flows southeastward from the Bering Sea along the east coast of Hokkaido and northeastern Honshu. Its waters meet those of the Kuroshio, causing dense sea fogs in summer, especially off Hokkaido.
The physical feature that most affects climate is the mountainous backbone of the islands. The ranges interrupt the monsoonal winds and cause the gloomy weather and heavy snows of winter along the Sea of Japan coast and the bright and windy winter weather along the Pacific. Temperatures and annual precipitation are about the same on both coasts, but they drop noticeably in the mountainous interior.
Temperatures are generally warmer in the south than in the north, and the transitional seasons of spring and fall are shorter in the north. At Asahikawa, in central Hokkaido, the mean temperature in January, the coldest month, is 16 °F (−9 °C), and the mean temperature in August, the hottest month, is 70 °F (21 °C), with an annual average temperature of 43 °F (6 °C). At Tokyo, the mean temperature for January is 39 °F (4 °C), the mean for August 81 °F (27 °C), and the annual average 59 °F (15 °C). Inland from Tokyo, Matsumoto is cooler, with an annual average temperature of 52 °F (11 °C), whereas an annual average of 57 °F (14 °C) occurs on the Sea of Japan coast at Kanazawa. The warmest temperatures occur on Kyushu and the southern islands; at Kagoshima, the mean temperature for January is 45 °F (7 °C), the mean for August is 81 °F (27 °C), and the average is 63 °F (17 °C).
Precipitation in the form of rain and snow is plentiful throughout the islands. Maximum precipitation falls in the early summer, and the minimum occurs in winter—except on the Sea of Japan coast, which receives the country’s highest snowfall. The summer rainy season occurs through June and July; it is known as the baiu (“plum rain”) because it begins when the plums ripen. Torrential rains accompany the typhoons.
Precipitation patterns vary with topography, but most of the country receives more than 40 inches (1,020 mm) annually, mainly as rain during the summer. The smallest amount of precipitation occurs on eastern Hokkaido, where only 37 inches (940 mm) fall annually at Obihiro, whereas the mountainous interior of the Kii Peninsula of central Honshu receives more than 160 inches (4,060 mm) annually. Varying amounts of snow fall on Japan. From November to April snow blankets Hokkaido, northern and interior Honshu, and the northwest coast. "Japan" © Emmanuel BUCHOT, Encarta, Wikipedia
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