Formation conditions of heavy rain

　　The formation process of rainstorm is quite complicated. Generally speaking, the main physical conditions for rainstorm are sufficient and continuous water vapor, strong and lasting upward movement of airflow and instability of atmospheric structure.

　　The water vapor source of heavy rain in China comes from the southerly South China Sea and the Bay of Bengal, and from the western Pacific Ocean. In addition, rivers and lakes are also one of the water vapor sources. How does a large amount of water vapor rise to the sky and become cold and condense into raindrops? There are three situations that can make water vapor rise.

　　Sunlight causes water vapor to rise into clouds and cause rain.Under the strong solar radiation, the water surface is heated and evaporated, turning into invisible water vapor and entering the lower atmosphere. The lower atmosphere also rapidly increased thermal expansion and became lighter, and the hot and light air full of water vapor soared into the blue sky like "taking an elevator".

　　Water vapor rises on the front to form clouds and cause rain.Cold air forms an invisible "inclined staircase", which makes water vapor slide up into the upper atmosphere. Cold and warm air masses meet to form a front. Warm and light air rich in water vapor will slide or be lifted above cold and dry air, causing water vapor to rise and form thick clouds.

　　Water vapor is lifted into clouds on windward slopes to cause rain.If the warm and humid airflow moving horizontally from the water vapor-rich area is blocked by mountains, hills or plateaus, it will be forced to "climb" up the hillside and enter a higher place, thus forming clouds and causing rain on the windward slope.

(Editor: Liu Jia)