How is rainfall measured?
Displayed by a linear measurement like the US Customary Inches or the Metric System.
Rainfall is collected by a collection container (rain gauge) that can vary in shapes from a straight sided cylinder to a funnel shape leading to a calibrated measuring device. If the rain gauge is a simple cylinder then simply measuring the amount of water in the cylinder with a ruler would tell you the amount of rainfall if there was no evaporation. Most often rain gauges are designed to be more accurate by collecting the rain in a funnel device that has a specified size larger than the collector. The collector has been calibrated with the measuring markers to match the desired unit of measure.
Another form of rain measure is electronic by weight. The principle is the same, but since rain water generally has a constant weight a certain amount of water accumulates in the collector until it flips a weight sensor allowing it to spill past. At this time an increment in rainfall by the calibrated amount is added to the total and the electronic sensor springs back to collect more rainfall and repeat the process.