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How is Rainfall Measured Out in the Ocean?

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How is Rainfall Measured Out in the Ocean?

Measuring rainfall in your own back yard is a simple task. All you need to do is put a rain gauge in the ground (or attach one to a fence post or porch railing). Then let it fill up with rain and check the measurement. Unfortunately, if you are a meteorologist, that method doesn’t work out over the open ocean. The rain gauge would be thrown off by waves and ocean spray.

Yet somehow, meteorologists still manage to know how much rain fell in a particular spot at sea. So how do they do it? By listening to the rain!

The Sound of Raindrops in the Ocean
When a raindrop hits the ocean — or any body of water — there are two different sounds that it makes:

  1. First is the splash that comes from the drop hitting the surface of the water, creating a sound wave that ripples outwards and downwards. Scientists can distinguish between five different raindrop sizes based on the sounds made by these splashes.
  2. The second sound is produced by bubbles that are formed when the raindrop hits the water. Interestingly, the sound that the bubbles make is generally a lot louder than the sound of the rain drop splashing. These bubbles have a lifecycle in sound which scientists have termed the screaming infant stage and the quiet adult stage. A bubble starts as a screaming infant bubble because as a newly-formed bubble, it is unstable. As it stabilizes, it “screams,” which is a resonating sound that meteorologists can measure. Once the bubble has reached an equilibrium, it becomes a quiet adult bubble, capable of absorbing sound.

The key portion of information here is the frequency of all those screaming infant bubbles. Larger bubbles scream at lower frequencies which means that scientists are able to measure raindrops based on the frequency of the sound made by screaming infant bubbles.

How Scientists Listen to the Rain
This is where the acoustic rain gauge comes in. It’s a specialized device that organizations like the National Oceanic and Atmospheric Administration use to record sound in the ocean. Acoustic rain gauges are typically attached to mooring lines below the ocean’s surface. When it starts raining, the gauge comes to life and records everything that it can hear.

Scientists then take these recordings and use the unique sounds of raindrops to determine the size of the drops, how many were falling, and how long the rainfall went on. With that data, scientists can calculate how much rain fell in a given area of the ocean.

The hard part about listening to rain falling on the ocean is that there are a lot of other sounds that get in the way. Waves and wind, for instance, the sounds of sea creatures, and even sounds caused by human activities. This means that meteorologists have to identify and filter out all of these extra noises so that they are listening to raindrops only.

Measuring the rain falling at sea is no easy feat but it is necessary to predict rainfall from storms that are moving towards land. Modern acoustic rain gauges can do their jobs with a high-degree of accuracy, sending their data to researchers in real time. The information gleaned from these recordings is part of the information used to create the weather forecasts we all live by.

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