That heat is one of the primary sources of energy driving atmospheric circulation. Understanding the patterns of distribution and intensity of rainfall helps us figure out the Earth s complicated climate system. Rain falling over the oceans is, however, very difficult to measure because gauges used on land obviously won t work and there are such vast areas to be monitored. Tropical Rainfall Measuring Mission is, for example, using a satellite and a variety of other instruments to study rain, lightning and storms over the tropical oceans. Nystuen s acoustic rain gauges are part of the field programs being conducted in conjunction with the project.
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 that are trapped underwater by the splash. Different sizes of raindrops generate distinctive sound signals. Small drops, the size typical in what people call drizzle, generate small, uniform-sized bubbles that resonate at a higher frequency than other drops and are surprisingly loud underwater. Medium-sized drops, on the other hand, do not generate bubbles and are actually quieter underwater than the small drops. The largest raindrops, which often occur at the beginning of a heavy downpour, create bubbles of many sizes.