Polar regions are dry partly because their cold air cannot hold much water vapor. In addition, relatively little evaporation occurs in those regions, and the wind blows away from the poles. Measuring rainfall People measure rainfall with a variety of devices, including simple funnel gauges and sophisticated radar systems. 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. 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. A radar system can provide a more complete picture of rainfall.