A Rainfall Rate Sensor

An instrument designed to measure rainfall rate accurately with fine time resolution is described. Rain water passes from a collector into a reservoir formed between two concentric electrodes while it empties from the reservoir through a small nozzle at its base. Thus, the water depth in the reservoir is related to the inflow or rainfall rate and determines the capacitance between the electrodes. An oscillator having the electrodes as capacitance elements translates the depth changes into frequency variations. The equations relating rainfall rate to frequency are developed and their relevance to the design of the instrument for various specific applications is stressed. Results from laboratory tests as well as actual rainfall measurements are given.