Blue warns how serious the rainstorm is.

The blue rainstorm warning means that the rainfall will reach more than 50 mm within 12 hours, or it has reached more than 50 mm and the rainfall may continue.

According to China's meteorological regulations, heavy rainfall with 24-hour precipitation exceeding 50 mm is called "rainstorm". According to its precipitation intensity, it is divided into three grades, namely, 24-hour precipitation of 50-99.9 mm is called "rainstorm", 100-249.9 mm is called "heavy rainstorm", and more than 250 mm is called "extraordinary rainstorm". However, due to the different characteristics of precipitation and topography, the standards of rainstorm and flood are different in different places.

In business practice, rainstorm can be divided into local rainstorm, regional rainstorm, large-scale rainstorm and extremely heavy rainstorm according to its occurrence and influence range. The local rainstorm lasts only a few hours or dozens of hours, generally affecting tens to thousands of square kilometers, causing less harm. However, when the rainfall intensity is particularly high, it will also cause serious casualties and property losses.

Rainstorm formation:

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

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