6? 1 How is the rainstorm formed?

The formation process of rainstorm is quite complicated. Generally speaking, the main physical conditions of rainstorm are abundant and continuous water vapor, strong and lasting upward movement of airflow and instability of atmospheric structure. The favorable combination of various scales of weather system and underlying surface, especially topography, can produce heavy rain. The main weather systems that cause large-scale rainstorm in China are fronts, cyclones, shear lines, eddies, troughs, typhoons, easterly waves and intertropical convergence zone. In addition, thermal thunderstorms will also cause short-term and small-scale heavy rains in arid and semi-arid areas. Rainstorms often fall from cumulonimbus clouds. The condition for the formation of cumulonimbus clouds is that the atmosphere contains enough water vapor, and there is a strong upward movement to transport the water vapor rapidly upward. The water droplets in the cloud are constantly affected by the upward movement until the upward airflow can't support them, and they drop sharply to the ground. Cumulonimbus clouds are usually very large. Cumulonimbus clouds are precipitation units in rainstorm areas. Although the horizontal range of each unit is only 1 ~ 20km, they are arranged in a rain belt with a width of 100 ~ 200km. Cumulonimbus clouds are like mountains, like mountains When they develop strongly, they extend from the height of 0.4 ~ 1 km above the ground to the height of 10 km. The higher the altitude, the lower the temperature, often reaching MINUS 10 degrees Celsius or even lower, and the water droplets at the top of the clouds will freeze. On the ground, people can see wisps of white bands at the top of the cloud with naked eyes, which is the result of ice crystals and snowflakes dancing in the sky. It is rainy summer on the ground, but snowy winter in the sky. In China, the water vapor of rainstorm comes from the South China Sea or the Bay of Bengal. Second, from the East China Sea or the Yellow Sea. Sometimes, in a rainstorm, water vapor comes from the east and south, or mainly from the south in the early stage and from the east in the later stage. The spread of "southeast wind, rain ancestor" in the Central Plains is an objective reflection of the precipitation law. Like running water, the motion of the atmosphere often produces fluctuations or eddies. When two air streams from different directions or different temperatures and humidity meet, fluctuations or vortices will occur. The largest is several meters in Qian Qian, and the smallest is only a few kilometers. In these fluctuating areas, with the operation of air flow, upward movement often occurs, and horizontal water vapor quickly concentrates in the same area, forming a rainstorm center. In addition, topography also has an influence on the formation of heavy rain and rainfall. For example, due to the existence of mountains, the airflow is forced to rise on the windward slope, which increases the vertical movement and the rainstorm; On the leeward slope of the mountain range, the airflow sinks and the rainfall is greatly reduced. On some leeward slopes, the rainfall is only110 of windward slopes. In early August, at 1963, there was a wet air flow from the South China Sea to North China, which happened to intersect with Taihang Mountain. Due to the uplift of the mountain range, there is a rare rainstorm in the east of Taihang Mountain. The narrow tube action of the valley can also strengthen the rainstorm. 1In August, 975, a torrential rain occurred in Henan Province. Its center, Zhuang Lin, is surrounded by mountains in the south, north and west, and gradually forms a trumpet-shaped terrain in the east. Due to this terrain, the airflow rises faster and the rainfall suddenly increases. From August 5 to 7, the precipitation reached 1600 mm or more, and Zhumadian, which is located in the plain area, is less than 40 kilometers southeast of Zhuanglin. In addition, when the rainstorm occurs, the lower air is generally warm and wet, while the upper air is dry and cold, which makes the atmosphere in an extremely unstable state, which is conducive to the release of energy in the atmosphere and the full development of cumulonimbus clouds.