What does the weather forecast mean by xxx millimeters of rainfall?

Rain falls from the sky to the ground without evaporation, infiltration and loss. The depth of water layer accumulated on the water surface is called rainfall (in millimeters), which can directly indicate the amount of rainfall. Daily rainfall is the amount of rain per unit area (1 m2) within 24 hours of that day. At present, the commonly used instruments for measuring rainfall are rainfall cylinder and measuring cup. The diameter of the rain tube is generally 20 cm, and it is equipped with a funnel and a bottle. The measuring cup has a diameter of 4 cm and is used together with the rainfall tube. When measuring, pour the rainwater in the rainfall cylinder into the measuring cup, and you can know the rainfall of the day according to the scale on the cup.

According to the regulations of China Meteorological Bureau, the rainfall within 24 hours is called daily rainfall, and any daily rainfall below10mm is called light rain,10.0-24.9mm is moderate rain, 25.0-49.9mm is heavy rain, 50.0-99.9mm is rainstorm, and100.0-250.

Due to China's vast territory, a few areas have other provisions according to the specific conditions of this province. For example, in rainy Guangdong, if the daily rainfall exceeds 80 mm, it is called rainstorm; In Yan 'an, Shaanxi, where there is little rain, it is called rainstorm when the daily rainfall reaches more than 30 mm.