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The month of September is a transition month in China, a bridge from summer to fall. The Siberian High is beginning to grow in size and becoming increasingly cold in the rapidly dwindling sunshine in the northern latitudes of Siberia. As this high pressure system drops south and expands, northern China sees its first relief from the heat and humidity of summer since May or June. Cold fronts begin to extend further and further south. Temperatures above 29°C (85°F) are still possible across northern China, but by the end of the month frosts and freezes are common on the Manchurian Plain and across the northern plateaus. Daytime temperatures range from 20 to 30°C (68 to 86°F) and there is less rain. Although it can be quite warm during the day, nights can be bitterly cold and bring frost.
China's northern interior is the first to feel the change of seasons. The Siberian High grows increasingly cold as sunshine dwindles quickly above the Arctic Circle. As this polar air mass drops into northern China, it brings significant drops in temperature, gusty winds, and frosts and freezes at night. Average temperatures, though still blistering hot, drop an average 5 to 6°C (9 to°F) lower than in August. The descending cold fronts displace warm, moist tropical air, triggering heavy thunderstorms, even some severe weather across central and eastern China. The daily air mass showers and thunderstorms become less frequent and tend to produce less rainfall. The tropical moisture is suppressed to the south and rainfall diminishes to less than half the summer monthly averages. Foliage begins to turn to show its autumnal brilliance during the month and in far northern areas will actually near the peak by months end.
September is also the peak of the typhoon season. The tropical disturbances that form over the western Pacific, the Philippine Sea, and the South China Sea are at their peak frequency in September. Many of these developing weather systems become tropical storms or typhoons and recurve north over the Philippine Sea to die over the cool waters of the Sea of Japan or become absorbed in extratropical storm systems. Others however, manage to make their way westward, eventually striking the Chinese mainland.
As temperatures begin to accelerate downwards over the North China Plain during September, Beijing enjoys clear skies and cool temperatures in its first real break from the heat and humidity since June. The Fragrant Hill (Western Hill) is usually covered with red leaves. Bouts of warm and humid weather are still possible, but frontal systems increasingly bring warm, noticeably drier Siberian air into northern China. Cold fronts bring showers and heavy thunderstorms across the plain, but the afternoon thunderstorms caused by the daytime heating so prevalent during the summer diminish in frequency. The big rain maker and the cause for most concern is the tropical storms or typhoons that increasingly threaten the region with flooding rains, especially when the moist winds are forced upward over the coastal hills.
Rainfall diminishes in all but southernmost China, as autumn brings pleasantly mild days and the first hints of cooler air at night. Along the South China Sea, the daily sea breeze circulation weakens and diminishes the daily thunderstorm activity that develops in the summer just inland along the sea breeze front. Even though daytime high temperatures range from 20 to 25°C (68 to 75°F), tropical moisture is usually still in control or easily within reach to make the days miserably wet and cold, with perpetual rain or drizzle. On Taiwan, the brief transition months of October and November can bring generally clear conditions and cool, moderate temperatures.
Southern and southeastern China are the first to feel the heat of summer and often the last to see it go. Average temperatures begin to drop off from their August levels but it is still very warm. This region is increasingly vulnerable to tropical disturbances, storms, and typhoons threatening heavy flooding rains, damaging winds, and tides. The first polar fronts to reach southern China tend to be weak, bringing little more than drier air.
Tibet and Qinghai Province can undergo a rapid transition from summer to winter in the early fall. Weather conditions rapidly oscillate from day-to-day. The heat of summer can be felt one day and a strong cold trough can bring mountain snows the next.
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