1. Rainfall is calculated by rain gauge and rain cup. The rain gauge is a cylindrical open cylinder, and its opening area is 3 14 square centimeter (20 cm in diameter) in China. In order to prevent precipitation from evaporating, the middle and upper parts are leaking, and the water storage bottle is placed at the lower part.

2. For convenient observation, the above caliber is equipped with a special measuring cup. The diameter of the rain cup is 4cm, so the length of precipitation per millimeter on the rain cup is 25 mm. In order to continuously record liquid precipitation, hydrometeorological departments often use siphon rain gauge or tipping bucket telemetering rain gauge. Their recording paper not only records the total precipitation, but also can judge the precipitation or precipitation intensity in different periods.

3. The accuracy is 0.25mm or 0.0 1 inch, which is expressed by the depth of rainwater collected on the plane. It is sometimes expressed in liters per square meter (1l m-2 =1mm). In meteorological statistics, rainfall can also be called rainfall, that is, the accumulated precipitation in a certain period of time, in which if the precipitation is less than 0. 1 mm, it is regarded as rain trace.