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北纬40度是个神奇的纬线,这条线以北,年均降水低于400毫米,霜降时间长、气候寒冷,大片的戈壁草原适合于放牧,而这条线以南,降水逐渐丰富、气候逐渐温暖,肥沃的平原土地适合于农耕。在400毫米降水量等值线的南北,农耕民族和游牧民族有着相互的交流和融合,但更多的则是绵延千年的征战杀伐和对资源的舍命争夺。中国万里长城的走势,恰恰与分布在我国的400毫米降水量等值线重合,也恰
Latitude 40 degrees north latitude is a magical latitude, the north of this line, the average annual rainfall is less than 400 mm, frost a long time, the climate is cold, the vast Gobi grassland suitable for grazing, and this line to the south, gradually increasing precipitation, climate gradually Warm, fertile plains land suitable for farming. Between the north and the south of the 400-mm precipitation contour, there are mutual exchanges and integration between farming people and nomadic peoples, but more are the campaign to kill the monuments and the sacrifice of resources for life. The trend of the Great Wall in China precisely coincides with the contour of the precipitation of 400 mm distributed in China,