Acoustic Rain Gauge (ARG) attached to a yellow mooring line. Photo by Dr. Barry Ma, Chinese Naval Academy, courtesy of Jeffrey A. Nystuen, Applied Physics Laboratory, University of Washington.

       Rain is one of the most important components of weather and climate. Improved understanding of the global pattern of rainfall is needed to improve weather and climate forecasting. Information on rainfall over the ocean helps meteorologists, oceanographers, and climatologists.

       Rainfall is difficult to measure over the ocean. The rain gauges commonly used on land collect water in a tube and do not work at sea, where they have to be mounted on surface buoys or ships that pitch and heave in response to ocean waves. Specialized rain gauges have been developed for use at sea, but they are easily damaged or stolen when mounted on buoys on the ocean surface.

       Acoustic Rain Gauges (ARGs) have been developed that can be placed safely beneath the ocean surface to record the sounds produced by rain. The unique characteristics of the sounds produced by different kinds of rainfall allow scientists to use these recordings to identify and measure raindrop size, rainfall rate, and other features of rainfall over the ocean. The sound of rain recorded by the acoustic rain gauge comes from an area of the sea surface above the instrument. This area increases as the acoustic rain gauge is placed deeper in the ocean. An average measurement of rain over an area is better than a measurement at a point because rainfall can vary dramatically over short distances.