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.
Precipitation
Precipitation in China generally follows the same pattern as temperatures, decreasing from the southeast to the northwest. The annual total of certain areas along the southeastern coast amounts to more than 80 inches (2,000 mm). The Yangtze valley receives about 40 to 45 inches (1,000 to 1,150 mm). Farther north, in the Huai River valley, the annual rainfall decreases to some 35 inches (880 mm). In the lower reaches of the Huang He, only 20 to 25 inches (500 to 650 mm) falls annually. The Northeast generally receives more precipitation than the North China Plain, with upwards of 40 or more inches falling in the Changbai Mountains.
The southeast monsoon loses much of its moisture by the time it reaches the northern part of the Loess Plateau, where the annual precipitation is reduced to between 12 and 20 inches (300 and 500 mm). Northwest of a line linking the Da Hinggan (Greater Khingan), Yin, Lang, Qilian, and Altun ranges, the annual precipitation is less than 10 inches (250 mm). Because these regions are far from the sea, high mountains prevent the southern monsoon from reaching them, and only grasslands are found there. In western Inner Mongolia, the Gansu Corridor, and the Tarim Basin, the annual precipitation drops to 4 inches (100 mm) or less. These are areas of true desert, where sometimes not a single drop of moisture is received for several years.
The Junggar Basin and the Ili River valley of northern Xinjiang are open to the influences of the westerlies, and precipitation is heavier there. Precipitation on the Plateau of Tibet, following the national pattern, decreases from southeast to northwest. More than 40 inches falls annually in the valleys in the southeastern part of the plateau, and the eastern edge receives 20 inches. However, in the enclosed Qaidam Basin in the north, the yearly total is only 4 to 10 inches.
The high variability of precipitation is another characteristic of China’s climate. Usually, variability increases as annual amounts decrease, a circumstance that is closely connected with the country’s high frequency of drought and flood. Spring rain is of immense significance to Chinese farmers, but spring is also the season with the highest variability. In South China the variability exceeds 40 percent, along the Yangtze it is about 45 percent, and in North China it is more than 50 percent. The variability of a vast area in North China exceeds 70 percent in some places; east of Beijing, for example, the rainfall variability in spring may even exceed 80 percent, as it also does in the central parts of the Yunnan-Guizhou Plateau.
Rain falls mostly in the summer months, when plants need water the most. This is an important asset for farmers, but summer rainfall is usually too intense. In July, when the frontal zone shifts northward, cyclones (circulation of winds around centres of low atmospheric pressure) are much more active in North China than in South China, and North China begins to receive heavier rainfall. More than half the area of the North China Plain records 0.8 inch (20 mm) of rainfall daily, and in some places it may reach as much as 1 inch (25 mm) or more daily. During that time, areas south of the Yangtze are covered by the tropical Pacific air mass, so that the weather becomes comparatively stable, the amount of rainfall usually decreases, and the average rainfall intensity is less than that of June. The intensity of August rainfall is in general less than that of July.
In the southeastern coastal regions, around Fuzhou and Shantou, the maximum daily rainfall may even approach 12 inches (300 mm). Such accumulations are directly related to the high frequency of typhoons (tropical cyclones) striking that part of the coast, usually during the period from May to November; July, August, and September are the three months when typhoons are the most frequent.
In May, typhoons usually strike the coast south of Shantou. Later in June they shift northward, arriving between Shantou and Wenzhou, and after July they invade areas north of Wenzhou. August has the highest frequency of typhoon invasions, when more than one-third of the typhoons reaching China arrive. After September the frequency of typhoons decreases, and the pattern again shifts southward. In October, typhoons usually land south of Wenzhou; the late typhoons arriving in November and December strike south of Shantou.
Plant and animal life
Flora
China’s great topological and climatic diversity has produced a vast array of natural vegetation types. The country’s total number of seed-plant species is approximately 30,000, representing some 2,700 genera; more than 200 of these genera are endemic to China. There are about 2,500 species of forest trees, including some 95 percent of the known broad-leaved types. Many of these trees are of economic importance, such as tung trees, camphor trees, varnish trees (Rhus verniciflua), star anise (which yields an oil used as a flavouring additive), and glossy privet (Ligustrum lucidum).