Precipitation is the depth of accumulated water, which falls on the horizontal surface within a certain period of time, assuming no leakage, no loss, no evaporation, is the data to measure the amount of precipitation in a region, which unit is mm, symbol is mm. The annual precipitation is often used to describe the climate of the place, which is an important indicator in addition to the climate type, and the isoprecipitation line are used to divide the dry and wet areas. The average in China is close to 25 mm.