Monsoon winds, caused by differences in the heat-absorbing capacity of the continent and the ocean, dominate the climate. Alternating seasonal air-mass movements and accompanying winds are moist in summer and dry in winter. The advance and retreat of the monsoons account in large degree for the timing of the rainy season and the amount of rainfall throughout the country. Tremendous differences in latitude, longitude, and altitude give rise to sharp variations in precipitation and temperature within China. Although most of the country lies in the temperate belt, its climatic patterns are complex.
China's northernmost point lies along the Heilong Jiang in Heilongjiang Province in the cold-temperate zone; its southernmost point, Hainan Island, has a tropical climate (see table 4, Appendix A). Temperature differences in winter are great, but in summer the diversity is considerably less. For example, the northern portions of Heilongjiang Province experience an average January mean temperature of below 0°C, and the reading may drop to minus 30°C; the average July mean in the same area may exceed 20°C. By contrast, the central and southern parts of Guangdong Province experience an average January temperature of above 10°C, while the July mean is about 28°C.
Precipitation varies regionally even more than temperature. China south of the Qin Ling experiences abundant rainfall, most of it coming with the summer monsoons. To the north and west of the range, however, rainfall is uncertain. The farther north and west one moves, the scantier and more uncertain it becomes. The northwest has the lowest annual rainfall in the country and no precipitation at all in its desert areas.
Data as of July 1987