Why does it rain?

We have seen drizzle and downpour. We have seen some rain for a short time, and some rain for several days. Sometimes, the sky is covered with thick clouds, sometimes it rains cats and dogs, and sometimes it clears up after rain. From the night of July 9, 1998 to the early morning of the next day, a torrential rain fell in a certain place in Danfeng County, Shangluo District, Shaanxi Province, China, which lasted for 6-7 hours and the rainfall exceeded 13 mm, equivalent to the sum of the annual rainfall in some areas in southern China. So, why does it rain in the sky? Where did the rain come from?

rain is water drops falling from the air to the ground. Water floating in the sky has gas, liquid and solid, and they will be transformed into each other. Gaseous water is called water vapor. When air rich in water vapor is cooled, many of the water vapor will become liquid or solid. Because as the temperature drops, the ability of air to contain water vapor will drop sharply. For example, when a mass of air drops from 3℃ to 1℃, its ability to contain water vapor will drop by more than two thirds. Therefore, when the air contains a lot of water vapor and is cooled, the part of water vapor that cannot be accommodated by the air will condense or coagulate with some fine dust particles as the core, resulting in small water droplets or small ice crystals. They are suspended in the air and form clouds. This kind of small water droplets or ice crystals will move up and down in the air and collide with each other. In this process, its volume will increase. When the updraft can't support them, they fall to the surface. If they fall to the surface in the form of liquid water, it is called rainfall; If it falls to the surface in the form of solid water, it becomes snow or hail.

according to the different cooling processes, we divide rainfall into frontal rain, convective rain and orographic precipitation.

let's talk about frontal rain first. When cold and warm air meet, they will form a transition zone with a certain inclination angle to the ground, which is called front. Warm air is above because it is lighter, and cold air is below because it is heavier. Warm air will move upward along the front. If the warm air contains a lot of water vapor, after reaching a certain height, the water vapor will condense because of the temperature drop, and the rain formed in this way is called frontal rain. At the turn of spring and summer every year, the front hovers in the middle and lower reaches of the Yangtze River in China, which causes long-term rainfall in this area. At this time, the plums are ripe, so people call the rain at this time plum rain. Zhao Shixiu's poem "The Guest" in the Song Dynasty said: "It rains at home in Huangmei season, and frogs are everywhere in the grass pond." This is a vivid portrayal of the scene of plum rain. Every autumn, over the vast areas of our country, there is often cold air pushing the front to move to the warm air side, and the warm air is forced to rise. If the warm air is humid at this time, it will also condense due to cooling, forming autumn rain. Because the whole area was occupied by cold air after the front, the temperature dropped, so there was a phenomenon of "an autumn rain and a cold".

let's talk about convective rain. In the summer afternoon, people often experience such weather: at first, the sun shines brightly, and people feel very stuffy. Later, dark clouds appeared in the sky, and the sky gradually darkened. When the ground is covered with thick black clouds, suddenly there will be a cool breeze blowing. This wind has a high speed, and sometimes you can see the scene of flying sand and stones. The temperature drops sharply, and sometimes the temperature drop can reach about 1℃. Pedestrians on the road hurried on; Vendors are busy collecting stalls; Housewives are busy taking back the dried clothes. After a while, it rained cats and dogs from the sky, sometimes accompanied by lightning and thunder. The rain usually doesn't last long. After the rain stops, the sky clears up and the air is fresh. This is commonly known as convective rain, also known as thunderstorm. Why did this rain form?

In the summer afternoon, the ground is strongly heated, and the temperature near the surface layer rises. Due to the heterogeneity of the surface, the air temperature in some places is higher than that in the surrounding areas. However, the air with higher temperature is very unstable, and it will move upward when it meets updraft or terrain uplift. Because the air density in the upper air is lower than that in the near ground, the rising air block will expand and do work for the outside world, thus lowering its own temperature. If the rising air mass is rich in water vapor, it will condense into a cloud, usually forming an isolated, dispersed and flat-bottomed cloud. When this cloud develops to a certain stage, its thickness increases, often in an anvil shape, and the airflow in the cloud moves strongly up and down. When downward flow brought high-altitude cold air to the surface, it formed a cool gale. Then, a heavy rain wrapped the earth in rain and fog. This is a common afternoon thunderstorm in summer.

finally, let's talk about orographic precipitation. On the windward side of the mountain slope, we can see another kind of rainfall. If the airflow contains a lot of water vapor, it will rise all the way along the hillside and gradually cool and condense, thus forming clouds and causing rain. This kind of rain is called orographic precipitation. There is a place in northeast India called Kilapangqi, which is one of the most rainy places in the world. The average annual precipitation in Beijing, Shanghai and Guangzhou is 644 mm, 1124 mm and 1694 mm respectively. However, the average annual precipitation in Kilapengqi is more than 11, mm, much more than that in the above cities. If the annual precipitation in Kilapangqi is evenly distributed to every day of the year, the daily rainfall will exceed 3 mm, which is heavy rain. Why is it raining so much in Kilaupunqi?

First of all, it's because Kila Pengqi is influenced by the continuous southwest airflow. This southwest airflow comes from the vast Indian Ocean with a lot of water vapor. Secondly, Kilapangqi is located on the windward hillside of southwest airflow, and the airflow is blocked by the hillside during the movement, so it rises along the hillside, the temperature drops, and a large amount of water vapor condenses, forming abundant rainfall.

After talking about all kinds of rainy and rainy areas, where in the world is the rainfall particularly scarce? In the Atacama Desert in Chile, South America, there has been no rain for several years, which is a particularly scarce area of precipitation. This is because this area is controlled by the subtropical high pressure zone, the airflow sinks and the wind direction is parallel to the coast, so the water vapor content in the air is low. Coupled with the strong Peruvian cold current, the temperature of the near-surface atmosphere is lowered, which makes the atmosphere very stable and difficult to form convection, so the precipitation is very scarce.

The place with the most precipitation in China is Huoshaoliao in Taiwan Province Province. According to statistics from 196 to 1944, the average annual precipitation reached 6557.8 mm, of which 848 mm was reached in 1942. The precipitation in Tarim basin and Qaidam basin in northwest China is scarce, and the average annual precipitation is less than 5 mm. Located in Qiemo, Tarim Basin, the average annual precipitation is only 18.6 mm; Another place, Ruoqiang, is only 15.6 mm.