China

The air masses

The vast and topographically varied landmass of China lies in Asia, the world’s largest continent, and faces the Pacific, the world’s largest ocean, along an extensive shoreline. The country’s climate is thus heavily influenced by the seasonal movement of large air masses between the Pacific and the Chinese mainland. The polar continental air mass, originating to the north in Siberia, dominates a large part of China during the winter; likewise, the tropical Pacific air mass exerts its influence during the summer. The sharply varied climatic conditions prevailing in summer and in winter are a direct result of the interaction of these two air masses, which are entirely different in nature.

The Siberian air mass, which is quite stable, is extremely cold and dry and often has marked layers of temperature inversion. After crossing the Mongolian Plateau, the air mass spreads southward and begins to invade North China, where it undergoes a series of rapid changes; its temperature rises slightly, and its stability decreases. During the day the air there may be quite warm, but at night or in shaded places the cold is often unbearable. In general, the diurnal (daily) range of temperature is more than 18 °F (10 °C); in extreme cases it may exceed 45 °F (25 °C). Because North China is affected by this air mass most of the time, it is dry, with clear weather and an abundance of sunshine during the winter months.

The prevailing winter wind blows from November through March, but it changes direction as it moves to the south. In northern and northeastern China its direction is from the northwest, in eastern China it comes from the north, and on the southeastern coasts it is from the northeast. The height of the winter wind belt usually does not exceed 13,000 feet (4,000 metres). As it moves to the south, the height decreases; in Nanjing it is about 6,500 feet (2,000 metres), and in South China it is less than 5,000 feet (1,500 metres). The Qin Mountains become an effective barrier to the advance of the cold waves to the south, particularly in the western section, where the average elevation of the mountains is mainly between 6,500 and 9,000 feet (2,000 and 2,700 metres).

In China the tropical Pacific air mass is the chief source of summer rainfall. When it predominates, it may cover the eastern half of China and penetrate deep into the border areas of the Mongolian Plateau and onto the eastern edge of the Plateau of Tibet. In summer the Siberian air mass retreats to the western end of Mongolia, although it occasionally penetrates southward and sometimes may reach the Huai River valley, which constitutes a summertime battleground between the tropical Pacific and Siberian air masses.

The movement of the two air masses is of immense significance to the climate of central and North China. In summer, when the tropical air mass predominates, the frontal zone between the two shifts northward; as a result, North China receives heavier rainfall. When the southeastern monsoon slackens, however, the frontal zone moves southward, and central China receives more rainfall, which can cause flooding. The activity of the tropical Pacific air mass in winter is confined to the southeast coastal areas; during that season, therefore, it frequently drizzles in the hilly areas south of the Nan Mountains, and morning fog is common.

Besides these two air masses, three other air masses also influence China’s climate: the equatorial continental air mass (a highly unstable southwest monsoon), the polar maritime air mass, and the equatorial maritime air mass. Furthermore, because China is so vast and has such complex topography, the interaction between the air masses and relief produces a wide range of climatic conditions.