A rain gauge (also known as an udometer, pluviometer, or an ombrometer) is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a set period of time.

This can be interesting: People began to record rainfall a long time a ago: Ancient Greeks and Indians kept the first rain statistics in history. Ancient scientists understood the importance of weather recording for a deeper study of the laws of nature and more precise weather forecasts.

More Info: en.m.wikipedia.org