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There are a couple of major climatic forces determining the weather of Japan. Prevailing winds sweep across Eurasia from the west, drift over the Japanese islands, and then head east through the Pacific Ocean. Furthermore, large cyclonic airflows (masses of rapidly circulating air) that form along the western equatorial Pacific spin northeast through Japan and the surrounding areas. In winter, cold masses of air from Siberia determine the weather in Japan and the areas surrounding it. Continual cold winds blow from the northwest through the Sea of Japan, picking up moisture that later result in snowfall on the western side of the mountains on Honshu Island. As the cold air drops its moisture, it passes over high mountain peaks and cascades down into the eastern slopes, bringing cold, relatively dry weather to valleys and coastal plains and cities. In springtime, this mass of Siberian wind becomes warmer and less dense, allowing atmospheric currents over the Pacific to steer warmer air into northeast Asia. Therefore, in June and July, the majority of southern Japan is covered by warm, moist-laden air. The resulting late spring rains then give way to a comparatively dry summer that is sufficiently hot and humid, despite the islands' northernly situation, to permit widespread rice farming. Following the hot months of summer is a rainy and unpredictable autumn that accompany typhoons, or intense tropical windstorms. The cyclonic storms are formed above the western Pacific Ocean and travel clockwise in large sweeping arcs, at first traversing west toward the Philippines and southern China, and later in the season curving northward. Cold weather pushes these storms towards the east over Japan during early autumn, breathing life into the Siberian air mass and ushering in the new yearly weather cycle. Due to this annual weather cycle, the civilization of Japan has been affected profoundly. It has provided the islands more than sufficient levels of precipitation, annually ranging irregularly from about 100 centimeters in the northeast to levels exceeding 200 in the southwest, giving the country an overall average of 180. The moisture allows the islands to have unusually lush forest cover; however the precipitous slopes combined with heavy rainfall also gives the islands one of the world's highest rates of natural erosion, made worse by human activity as well as tremors caused by earthquakes and volcanoes in the region. These conditions have allowed Japan to possess plentiful sedimentary basins, but on the other hand, they have resulted in mountainsides which are extremely susceptible to landslides and erosion, leaving the area generally unsuitable for agricultural industry.   The mountainous regions of the island chain, coupled with their great length from north to south, results in a diverse climate that has contributed to regional differences. In most cases, sunny winters along the Pacific seaboard have made an agreeable environment for inhabitants. Conversely, the cold and snow surrounding the Sea of Japan has made habitation difficult and unpopular. Additionally, despite yearly precipitation being abundant in that region, the bulk of it comes as snow and rushes to the sea as spring runoff, leaving little moisture for cultivation. In the north part of Honshu, and mainly near the Sea of Japan, the summer weather patterns have also made farming difficult and less prevalent. The area is subject to the yamase effect, when cool northern air occasionally lowers temperatures drastically and damages agricultural output. The outcome of this effect has been particularly substantial on rice farming given that Japanese rice needs a mean summer temperature of 20° centigrade or higher to grow well. A decline of 2°-3° can cause a 30-50 percent drop in rice yield, and the yamase effect is capable of going beyond that level. Nonetheless, the yamase effect does not reach very far south, where the majority of precipitation falls as rain mostly in spring, summer, and autumn, when most useful for cultivation. Even the fall typhoons, which deposit the majority of their moisture along the southern seaboard, are valuable as they stimulate the beginning of winter crops, which have been grown in southern Japan for hundreds of years. To summarize, for the past couple millennia, the climate in general and precipitation patterns in particular have promoted the Japanese to group their settlements on the southern coast, especially along the sheltered Inland Sea, moving into the northeast. In these regions, the limitations imposed by topography on production have been tightened by climate, with the result that agricultural yield has been more modest and less reliable, making the risk of crop failure and hardship commensurately greater.