Why are the walls of the Forbidden City blown off by the wind?

In winter in Beijing, the radiation on the Asian land cools rapidly, forming a dry and cold Siberian high and a cold and wet Aleutian low in the North Pacific. Both are very developed, especially the former is very powerful and almost controls the whole Asian continent. It is the center of continental anticyclone and the source of dry and cold polar continental air mass, and also the source of Beijing monsoon in winter.

Because the subtropical high and the continental high are linked together in winter, they are powerful and become the dominant climate in the Asian continent in winter and half a year. High-pressure dry and cold air flows outward. In winter, northwest wind prevails along the Pacific coast and northeast wind prevails along the Indian Ocean coast. This is the winter wind in East Asia and South Asia, with the characteristics of sunny, cold and dry winter high-pressure weather.

Winter wind is a dry land wind, which is generally not easy to rain, but it can form a blizzard when it blows to the northwest of Japan. The warm sea invading Jiangnan metamorphic polar continental air mass can form cloudy and warm weather in winter and spring or rainy weather. In winter, the Qinghai-Tibet Plateau blocks the westerly airflow and divides it into two branches, forming a circulation situation in the southern trough of the northern ridge.

Because of the existence of the high-pressure ridge in the northern plateau, it is very beneficial to the southward movement of the winter monsoon, which strengthens the influence of the winter monsoon and causes the winter climate in China to be cold. The low pressure trough in the south of the plateau has a great influence on the weather and climate in the south of China because of the large amount of warm and humid air flow in front of the trough. Due to the blocking of the plateau, the southern Xinjiang and Hexi areas on the north side of the plateau are dry and cold in winter; There is little cold air activity in India and Myanmar in the south, and it is dry and warm in winter. In winter, the Qinghai-Tibet Plateau is a cold source, which will strengthen the downdraft near the plateau, thus strengthening the winter monsoon circulation. ?

The Asian continent is controlled by thermal depression, with its center in northwest India, southern Iran and Arabian Peninsula, and integrated with equatorial depression. At this time, the Hawaiian high extends westward and pushes northward, with the strongest force and the largest range, thus forming the southeast monsoon blowing from the western edge of the North Pacific high to the eastern part of Asia.

This is the East Asian summer monsoon, which is a warm and humid air flow originating from tropical ocean air mass, and has the greatest influence on the Asian continent in summer. In South Asia, at this time, due to the northward shift of the planetary wind belt,

To sum up, as the equatorial depression moves to the northern hemisphere, the southeast trade winds in the southern hemisphere cross the equator and turn into southwest winds, which is the summer monsoon in South Asia.