A rain gauge records the amount of precipitation that has fallen.
The simplest rain gauge is a calibrated container with a funnel that directs rain into a measuring tube. The collected depth is usually reported in millimeters or inches. One millimeter of rainfall represents one liter of water spread over one square meter, assuming no loss or runoff from the measurement area.
Modern weather networks use tipping-bucket gauges, weighing gauges, or optical instruments. In a tipping-bucket design, a small container tips after collecting a fixed volume, producing an electronic record of rainfall rate and total accumulation.
Rain gauges must be positioned carefully. Buildings, trees, wind, splashing, evaporation, and snow can affect the reading. A rain gauge measures precipitation at its site, so one gauge cannot always represent rainfall across an entire city or storm.