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 (2.5 centimeters) of rain falls into the funnel, it would fill 10 inches (25.4 centimeters) 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.
If the rainfall is so heavy that the water in the tube overflows, this extra rain flows into the space between the outside of the cylinder and the tube. After the rain in the tube is measured, it is poured out and the extra rain is placed in the tube and measured. The total rainfall equals the sum of these two measurements. A gauge is usually placed on the ground away from buildings and trees to ensure accuracy.
Some rain gauges can record the amount and the rate of rainfall. A tipping bucket rain gauge has a small bucket that tips and empties after it fills with rain. Each tip of the bucket activates an electrical switch that records the amount of rain. A weighing rain gauge collects water in a bucket that stands on a platform attached to a scale. As the bucket fills, the weight of the rain water pushes down the platform. This movement is recorded on a tape and processed by a computer.
Some rain gauges can be used to measure snowfall. But they do not provide very accurate measurements when used for this purpose.
