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Temperatures generally decrease from south to north. The mean annual temperature is above 68 °F (20 °C) in the Pearl River valley. It decreases to between 59 and 68 °F (15 and 20 °C) in the middle and lower reaches of the Yangtze, to about 50 °F (10 °C) in North China and the southern part of Xinjiang, and to 41 °F (5 °C) in the southern area of the Northeast, the northern part of Xinjiang, and places near the Great Wall. It drops below 32 °F (0 °C; i.e., freezing) in the northern part of Heilongjiang. The annual range of temperature between the extreme south and north is about 86 °F (48 °C). With few exceptions, January is the coldest month and July is the hottest.
South of the Qin Mountains–Huai River line, the mean January temperature increases progressively, rising from freezing to 72 °F (22 °C) on the southern coast of Hainan Island. Snow rarely falls, and the rivers do not freeze. North of this line, the temperature drops from freezing to −18 °F (−28 °C) in the northern part of Heilongjiang.
In April the mean temperature is above freezing for the whole of China, with the exception of extreme northern Heilongjiang. During that time the mean temperature for the Northeast Plain is between 36 and 46 °F (2 and 8 °C), and for the extensive plain between Beijing and Shanghai it is between 54 and 59 °F (12 and 15 °C). South of the Nan Mountains the mean temperature is considerably higher than 68 °F (20 °C). Along the coast of southern Guangdong, willows start to bud in late January, but in Beijing the budding of willows comes as late as early April.
In summer the temperature range between North and South China is quite small. In July the difference in temperature between Guangzhou and Beijing is only about 5 °F (3 °C), and the isotherms in July are roughly parallel to the coastline. In July the isotherm of 82 °F (28 °C) marks an extensive area. The hottest places in China are found along the valleys of the middle and lower Yangtze. The mean July temperature of Nanchang and Changsha is well above 84 °F (29 °C), and in many years it exceeds 86 °F (30 °C).
In North China autumn is generally cooler than spring. The mean October temperature in Beijing is about 55 °F (13 °C), and in April it is about 57 °F (14 °C). In South China the reverse is true. The mean October temperature in Guangzhou is 75 °F (24 °C), but in April it is only about 70 °F (21 °C).
The middle and lower reaches of the Huang He are where China’s civilization and agriculture first developed. There the seasonal rhythm is well marked, and the duration of each season is evenly spaced. In other parts of China, however, the duration as well as the starting and closing dates of each season vary among different regions. Summer is nonexistent in northern Heilongjiang, while there is no winter in southern Guangdong. At Kunming, in the Yunnan uplands, the climate is mild throughout the year, with only brief summer and winter periods.
In general, south of the Qin Mountains–Huai River line the mean daily temperature seldom falls below freezing, so that farming can be practiced year-round. In the Yangtze valley two crops are usually grown annually, but north of the Great Wall only one crop per year is possible.