Which instrument measures rainfall in millimeters?

• A. Rain Gauge
• B. Barometer
• C. Gnat Life Cycle Duration
• D. Volume

Answer: (A)

Rainfall is measured with a rain gauge, an instrument that collects rain and shows how deep the precipitation is, usually read in millimeters. Reading rainfall in millimeters makes sense because it describes depth: how high the water would rise if it spread evenly over one square meter. A barometer gauges atmospheric pressure, not precipitation. The gnats life cycle duration has no bearing on measuring rain, and volume is a general measure of amount, not a specific tool for capturing rainfall. The rain gauge is the tool designed for this purpose.