Why is the weather changeable in summer?

When it comes to summer, the first thing people think of is heat. Summer is the highest temperature period in a year, with dry and hot inland areas and humid and muggy coastal areas. But summer weather is by no means a hot word. Summer is the most violent and complicated period of the year, and the rainfall in most parts of China is mainly concentrated in this period. In the past 30 years, the annual precipitation in Beijing is 570 mm, and the precipitation in summer alone is 423 mm, accounting for 74% of the annual precipitation, especially at the end of July and the beginning of August, which is often the concentrated period of heavy rain and heavy rain. In addition, lightning, hail, thunderstorms, floods, droughts, typhoons and other disastrous weather also occurred at this time.

Hail-one of the important reasons why the disastrous weather in summer makes the summer weather so changeable is water vapor, and abundant water vapor is the basic substance of various weather changes. Speaking of water vapor, we want to introduce a weather system to you, that is, subtropical high. The subtropical high is a high-pressure system with an average latitude of 35 degrees, which is roughly arranged along the latitude circle. The position of subtropical high has obvious seasonal variation. In the northern hemisphere, summer is north and winter is south. The airflow rotates clockwise outward from the high pressure center, forming a southerly airflow in the western part of the high pressure, and continuously transporting warm and humid air from the ocean to Chinese mainland, thus providing water vapor for rainfall. When the warm and humid airflow meets the cold air from the north to the south, it will form a wide range of rainy weather. Because the position of this high pressure changes with the seasons, the rain belt in the summer of China will advance from south to north in turn. In autumn, the subtropical high moves south, and the rain belt also moves south. This is why the rainy season in the south of China starts early, ends late and lasts for a long time, while the rainy season in the north starts late and ends early and lasts for a short time.

The warm and humid airflow south of the edge of subtropical high can bring a lot of water vapor to China, but the interior of subtropical high is often hot, dry and sunny due to the influence of descending airflow. Some abnormal subtropical high is the chief culprit of disastrous weather in China.