A rain gauge is an instrument used by meteorologists and hydrologists to measure precipitation (e.g. rain, snow, hail or sleet) in a certain amount of time. It is usually measured in millimetres. Rain gauge is a meteorological instrument for determining the depth of precipitation (usually in mm) that occurs over a unit area (usually one metre square) and thus measuring rainfall amount. One millimetre of measured precipitation is the equivalent to one litre of rainfall per metre square.