Rain gauge is an instrument used to measure the amount of rain that falls in a certain place during a specific period of time. The National Weather Service uses a rain gauge that is shaped like a cylinder and has a removable cover. Inside the cylinder is a long narrow tube, where the rainfall is measured. The top of the tube is connected with a funnel. The rain falls into the funnel and flows into the tube. The mouth of the funnel has an area 10 times that of the tube. Therefore, if an inch of rain falls into the funnel, it would fill 10 inches of the tube. The rain in the tube is measured by a "ruler." With this ruler, a depth of 10 inches gives a reading of 1 inch of rainfall. Rain gauges that use the metric system measure in millimeters.