A funnel gauge consists of a funnel connected to the top of a narrow cylindrical tube. The diameter of the mouth of the funnel is much larger than the diameter of the tube.
Rain falls into the funnel and collects in the tube.
Markings on the side of the tube indicate the amount of rainfall. This design makes the depth of the rain easy to measure. A small amount of water falling into the mouth of the funnel will fill the tube to a considerable and easily readable level. Networks of gauges can measure rainfall over large regions, such as river basins, states and provinces, and even countries. However, a network can produce an inaccurate result if the rainfall is unevenly distributed across a region.