At the most general level, two major climatic forces determine Japan’s weather. Prevailing westerly winds move across Eurasia, sweep over the Japanese islands, and continue eastward across the Pacific Ocean. In addition, great cyclonic airflows (masses of rapidly circulating air) that arise over the western equatorial Pacific move in a wheel-like fashion northeastward across Japan and nearby regions. During winter months heavy masses of cold air from Siberia dominate the weather around Japan. Persistent cold winds skim across the Sea of Japan from the northwest, picking up moisture that they deposit as several feet of snow on the western side of the mountain ranges on Honshu Island. As the cold air drops its moisture, it flows over high ridges and down eastern slopes to bring cold, relatively dry weather to valleys and coastal plains and cities.

In spring the Siberian air mass warms and loses density, enabling atmosphere currents over the Pacific to steer warmer air into northeast Asia. This warm, moisture-laden air covers most of southern Japan during June and July. The resulting late spring rains then give way to a drier summer that is sufficiently hot and muggy, despite the island chain’s northerly latitude, to allow widespread rice cultivation.

Summer heat is followed by the highly unpredictable autumn rains that accompany the violent tropical windstorm known as typhoons. These cyclonic storms originate over the western Pacific and travel in great clockwise arcs, initially heading west toward the Philippines and southern China, curving northward later in the season. Cold weather drives these storms eastward across Japan through early autumn revitalizing Siberian air mass and ushering in a new annual weather cycle

This yearly cycle has played a key role in shaping Japanese civilization. It has assured the islands ample precipitation, ranging irregularly from more than 200 centimeters annually in parts of the southwest to about 100 in the northeast and averaging 180 for the country as a whole. The moisture enables the islands to support uncommonly lush forest cover, but the combination of precipitous slopes and heavy rainfall also gives the islands one of the world’s highest rates of natural erosion, intensified by both human activity and the natural shocks of earthquakes and volcanism. These factors have given Japan its wealth of sedimentary basins, but they have also made mountainsides extremely susceptible to erosion and landslides and hence generally unsuitable for agricultural manipulation..

The island chain’s mountainous backbone and great length from north to south produce climatic diversity that has contributed to regional differences. Generally sunny winters along the Pacific seaboard have made habitation there relatively pleasant. Along the Sea or Japan, on the other hand, cold, snowy winters have discouraged settlement. Furthermore, although annual precipitation is high in that region, much of it comes as snow and rushes to the sea as spring runoff, leaving little moisture for farming.

Summer weather patterns in northern Honshu, and especially along the Sea of Japan, have also discouraged agriculture. The area is subject to the yamase effect, when cool air from the north sometimes lowers temperatures sharply and damages farm production. The impact of this effect has been especially great on rice cultivation because, if it is to grow well, the rice grown in Japan requires a mean summer temperature of 20°centigrade or higher. A drop of 2°—3°can lead to a 30—50 percent drop in rice yield, and the yamase effect is capable of exceeding that level. This yamase effect does not, however, extend very far south, where most precipitation comes in the form of rain and the bulk of it in spring, summer, and fall, when most useful for cultivation. Even the autumn typhoons, which deposit most of their moisture along the southern seaboard, are beneficial because they promote the start of the winter crops that for centuries have been grown in southern Japan.

In short, for the past two millennia, the climate in general and patterns of precipitation in particular have encouraged the Japanese to cluster their settlements along the southern coast, most densely along the sheltered Inland Sea, moving into the northeast. There the limits that topography imposed on production have been tightened by climate, with the result that agricultural output has been more modest and less reliable, making the risk of crop failure and hardship commensurately greater.

在最一般的层面上，两个主要的气候因素决定了日本的气候。盛行西风吹过欧亚大陆时，席卷日本的岛屿，继续向东穿过太平洋。另外，赤道西太平洋上空产生的大量气旋（大量快速循环空气），大量的气旋在日本和邻近地区以轮子状向东北方向移动。在冬季，来自西伯利亚的大量冷空气主宰着日本周围的天气。持续的寒风从西北方向掠过整个日本海，在本州岛的山脉西侧汲取湿度，并形成几英尺厚的积雪。在冷空气所带来的降水时，雨水会流过高山脊和东部斜坡，为山谷、沿海平原和城市带来寒冷且相对干燥的天气。 春季，西伯利亚气团升温，密度降低，使太平洋上空的气流能够将暖空气引入东北亚。整个六月和七月，温暖、潮湿的空气覆盖了日本南部的大部分地区。由此产生的后期春季降雨结束，开始进入干燥的夏季，虽然岛屿的北部纬度较高，但其夏季仍然非常炎热和潮湿，因而可以大面积地种植水稻。 炎热的夏季结束后，随之而来的是捉摸不定的秋季降雨，同时还伴有强烈的热带风暴，也就是台风。这些气旋性风暴起源于西太平洋，并以顺时针方向的弧形前进，最初向西前进，抵达菲律宾和中国南部，并在秋季晚些时候向北蜿蜒前进。寒冷的天气使这些风暴东移至日本通过初秋重现活力的西伯利亚西伯利亚气团，，进入新一轮的年度天气循环。 这个年度循环对日本文明的塑造起到了关键作用。它确保了岛屿充足的降水量，从年度降水超过200厘米的西南部分地区到年度降水约有100厘米的东北部地区，整个国家平均降水为180厘米。充足的水分使岛屿能够使日本能够达到罕见的茂密森林覆盖面积，但陡峭的山坡加上强降雨使得该岛屿成为世界上自然侵蚀率最高的地区之一，而且人类活动，以及地震和火山活动的自然灾害也加剧了这种情况的发生。这些因素使得日本地区形成了许多的沉积盆地，但这些因素也促使山坡极易受到侵蚀和滑坡，因此一般不适合发展农业。 该岛的山区主干线，从北到南延伸很长，从而形成了多样化的气候条件，不同地区之间差异很大。一般来说，太平洋沿岸地区冬季阳光明媚，使得居住在那里气候会更加宜人。另一方面，沿着日本沿岸地区，冬季寒冷多雪，阻碍了人们在这里定居。此外，虽然该地区的年降水量很高，但其中大部分来自降雪，并像春季径流一样涌向大海，因此农田几乎无法获得水分。 在本州北部，尤其是日本海沿岸地区，这些地区夏季天气模式也制约了农业的发展。该地区受到山脊的影响，北部的冷空气有时会促使温度急剧下降，破坏农业生产。这种效应严重影响了水稻的种植。在日本，如果要让水稻良好生长的话，夏季的平均温度需要达到20摄氏度或者更高。如果温度下降2到3度，那么水稻的产量会下降30-50％，而且山脊效应会带来更为严重的影响。但是，这种山脊效应并没有延伸到遥远的南方，南方地区大部分降水来自雨水，雨水集中在春季，夏季，秋季，这会对种植产生非常有利的影响。即使是秋季刮台风，台风中的大部分水分都会沿着南部海岸沉积下来，这是非常有利的，因为几个世纪以来，这些雨水为在日本南部种植的冬季作物打下了一个好的基础。 总之，在过去的两千年里，常规的气候，特别是降水模式促使了日本人定居在南部海岸，人口最密集的地区沿着受保护的内陆海到东北部地区。地形对生产造成的限制，因气候条件而变得更为紧张，结果农业产量变得更少，且更不可靠。因而，相应地增加了作物歉收的风险。