Why does the south wind blow in summer?

Monsoon is a large-scale convection phenomenon with a period of one year, which is caused by land and sea distribution, atmospheric circulation, continental topography and other factors. Asia is the most famous monsoon region in the world, and its monsoon characteristics are mainly manifested in the existence of two main monsoon circulations, namely, the northeast monsoon prevailing in winter and the southwest monsoon prevailing in summer, and their transformation has an explosive mutation process with a short transition period. Generally speaking, 1 1 is the winter monsoon period, June-September is the summer monsoon period, and April-May and 10 are the transition periods between summer monsoon and winter monsoon. However, the seasonal differences in different regions are different, so the division of monsoon is not completely consistent.

Monsoon is a widely prevailing wind system, and the wind direction changes significantly with the seasons. Like the wind belt, it belongs to the planetary circulation system, and its formation is caused by the temperature difference between the ocean and the land in winter and summer. Summer monsoon blows from the ocean to the mainland, and winter monsoon blows from the mainland to the ocean.

Monsoon has a wide range of activities, affecting the area of 1/4 on the earth and the life of 1/2 people. Western Pacific, South Asia, East Asia, Africa and northern Australia are all areas with obvious monsoon activities, especially Indian monsoon and East Asian monsoon. There is also a small monsoon region along the Pacific coast of Central America, while there is no obvious monsoon region in Europe and North America, only some monsoon trends and phenomena appear.

In winter, the temperature of the mainland is lower than that of the adjacent ocean. There is a cold high pressure on the mainland and a corresponding low pressure on the ocean, and the airflow blows from the mainland to the ocean in a large range, forming a winter wind. Winter winds prevail in the northern hemisphere, especially on the east coast of Asia. The northbound monsoon extends from the mid-latitude to the equatorial region. This monsoon originated from Siberian cold high, and in the process of its southward eruption, it produced strong northerly winds and northeast winds in East and South Asia. In Africa and the Bay of Bengal, there is also an obvious northeast wind blowing near the equator. Although there are winter winds in the East Pacific and South America, they are not as obvious as those in Asia.

In summer, the ocean temperature is relatively low, the mainland temperature is high, and the ocean appears high pressure or the original high pressure is strengthened, and the mainland appears thermal low pressure; At this time, southwest and southeast monsoons prevail in the northern hemisphere, especially in the Indian Ocean and South Asia. Most of the southwest monsoon originated in the South Indian Ocean, crossed the equator on the east coast of Africa, reached South Asia and East Asia, and even reached central China and Japan. The other part of the southeast wind mainly comes from the northwest Pacific, and the southerly or southeast wind affects the eastern coast of China.

Summer monsoon generally goes through four stages: outbreak, activity, interruption and regression. The East Asian monsoon broke out the earliest, starting in early May, advancing from southeast to northwest, and stabilizing in late July. Usually retreat begins in mid-September, and the path is opposite to that when advancing. Under the counterattack of the northerly airflow, it gradually retreated from northwest to southeast.

The Xia Feng that affects China comes from three air currents: one is the Indian summer monsoon. When the Indian monsoon moves northward, the southwest monsoon can go deep into Chinese mainland; Second, the cross-equatorial airflow flowing through Southeast Asia and the South China Sea belongs to the low-altitude southwest airflow; Thirdly, the southeast monsoon from the west side of the subtropical high in the northwest Pacific sometimes turns to the southerly or southwest airflow.

Monsoon begins to appear in the northern part of the South China Sea at the beginning of May every year, with three sudden northward pushes and four static stages in the middle. At the end of May to June 5- 10, it reached the northern part of South China, at the end of June to early July, it reached the Yangtze River valley, and from mid-July to 20th, it reached the northern boundary-North China. In China, the winter wind is stronger than the summer monsoon, especially in the eastern coastal areas, where there is often a north-northwest wind of magnitude 8 or above, accompanied by a cold wave. The south China sea is dominated by the northeast wind, and the number of strong winds is less than that in the north.

The monsoon region has a unique climate, and most of the precipitation comes from the prevailing summer monsoon. In ancient China, the use of monsoon to carry out navigation activities made brilliant achievements. Zheng He set sail for the first time in summer and returned in autumn in Ming Dynasty. The other six times set sail during the northeast monsoon in winter and returned during the southwest monsoon. This fully shows that the ancients had a profound understanding of the law of wind activity.