The Acoustic Rain Gauge hears a slapping sound as the drop strikes the water surface, and then picks up a ringing sound as air bubbles are trapped beneath the surface. The bubble makes the most important sounds that Nystuen uses to determine raindrop size. The slapping sounds are more unique, relying on the size, shape and speed of the raindrops, and are most relevant for larger drops, he said. By placing hydrophones on deep ocean moorings, the team can listen to the ocean and determine when it is raining, how hard it is raining and even what type of rain is falling. This is because different size raindrops produce different sounds underwater.
Raindrops produce two types of sound underwater.
One is the slapping sound as the drop strikes the water surface, and the second is a distinct ring from air bubbles trapped underwater by the splash. For most raindrops, the sound of the bubble is, by far, the louder sound. Small raindrops are especially good at trapping air bubbles underwater and are surprisingly loud. Nystuen has used the Acoustic Rain Gauges in conjunction with other scientific experiments, which have taken place in the North Atlantic Ocean, the South China Sea and the central tropical Pacific Ocean. The results of the collaborative studies are promising.