Northwest winds in winter, southeast winds in summer; rain and heat arriving together created the rice civilization, and the 400-mm rainfall line divides farm from pasture. Understand the monsoon and you understand China.
Along the same latitudes: North Africa is the Sahara, Arabia is desert — yet Jiangnan in China is a land of fish and rice. The difference is one word: monsoon. In summer, warm moist air from the oceans pours onto the continent and brings rain precisely during the season of high heat. “Rain and heat together” is the greatest gift Chinese agriculture ever received.
In winter the Siberian high rules: cold, dry northwest winds blow from the continent toward the sea — so Chinese winters are dry in the north, cold everywhere, and short of rain. In summer the warm moist flows of the Pacific and Indian Oceans come ashore, and the southeast and southwest monsoons carry rain inland; precipitation falls away from the southeast toward the northwest.
The monsoon keeps no schedule: early means flood, late means drought, and its strength decides whether the south drowns while the north parches or the reverse. In two thousand years of Chinese history, the “Heaven” farmers depended on was mostly the temper of the monsoon.
The eastern monsoon zone — wherever the monsoon arrives is humid, densely farmed and thickly peopled: about half the land holding over ninety percent of the people. The northwest arid and semi-arid zone — beyond the monsoon’s reach, land of steppe and desert, where herding rules. The Qinghai–Tibet alpine zone — altitude strips away the heat; a high, cold snow world.
The logic of the three zones is “can you farm here?” The monsoon gives water, the plains give soil, the low latitudes give warmth. Where all three overlap, civilization is densest.
The 400-millimeter annual precipitation line runs diagonally from the northeast to the southwest: farmland to its southeast, pasture to its northwest. It roughly coincides with the Great Wall — no coincidence, for the Wall was the farming civilization’s border wall built along this rainfall line.
It has another astonishing identity: it tracks the Hu Line of population distribution closely. Rain drew the agriculture; agriculture drew the people — an invisible isohyet deciding where most of China’s hearth-smoke rises.
China’s climate records read like an extreme-sports manual: the Turpan Basin regularly tops 40°C in summer, with Lake Ayding 154 meters below sea level; Mohe once recorded −52.3°C; Huoshaoliao in northeast Taiwan averages over 6,000 mm of rain a year, while parts of the Tarim Basin edge receive less than 50 mm.
Piled together, these extremes are just another name for climatic diversity. They let China run all five farming types at once — rice, wheat, herding, orchards and fisheries — and make “vegetables north, grain south” a fixed annual logistics of state.