The instrument used for measuring the among of rainfall is called a rain gauge. The rain gauge consists of a cylinder with a funnel placed on top of it. A ruler is attached to the cylinder so that the amount of rain that falls into the rain gauge can be measured.
The rain falls into the funnel at the same rate in which it falls upon the ground. The rain is then measured with a ruler. The amount of rain which was measured in the cylinder is equivalent to the amount of rainfall that fell if the rain would have remained in one place. The amount of rain collected in the rain gauge over a given period is expressed in millimeters or centimeters. The weather observers in the United States express the amount of rain that has fallen in inches.