Hunan's largest river, the Xiangjiang, originates in the heart of the Nan Ling. Many smaller rivers that originate in the mountains along the eastern border flow westward to join the Xiangjiang in its northward courses. The north generally experiences more extreme weather conditions, both in summer and in winter than the south. In winter, occasional waves of cold front form a high-pressure zone centered over Mongolia sweep southward, damaging tea bushes and fruit trees in northern Hunan. The average minimum temperature in December and January is 6 degrees Centigrade.
Summer is usually long and humid, and temperatures are slightly higher in the north.
The average maximum temperature in July and August is 30 degrees Centigrade. The north has an average of 260 frost-free days a year, while the south has 300 days. Rainfall is ample, with the maximum precipitation occurring between spring and summer. The total annual rainfall of 1,602 millimeters decreases from south to north. Hunan lies in the path of cyclones that pass from west to east along the Yangtze Basin in summer, bringing with them at times long periods of heavy rain, resulting in extensive flooding of low-lying lands around the Dongting Lake.