Rainfall is measured in either millimeters or inches. The specific unit of measurement used depends on the county. While rainfall is measured in inches in the United States, most countries in the world adopt the metric system and use millimeters or centimeters instead of inches.
The rate of rainfall can be heavy, moderate or light. Heavy rainfall refers to a measurement of more than 0.30 inches per hour, while light rainfall refers to a measurement of less than 0.10 inches per hour. Moderate rainfall falls in-between.
To measure rainfall, meteorologists rely on an instrument known as a rain gauge. There are different types of rain gauges that vary based on the mechanism of operation. These include tipping bucket gauges, weighing gauges and graduated cylinders. Regardless of the type, rain gauges come with a funnel or a storage container that collects the rainfall for measurement.