Drizzle Recording
 
Drizzle

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  Drizzle is surprisingly lound underwater because the small raindrops generate bubbles as they hit the water. These similar-sized bubbles oscillate when they're young, producing a high frequency bell-like tone. (Image by Robert Simmon, NASA GSFC. Recording from Jeffrey A. Nystuen, University of Washington Applied Physics Laboratory)

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  Hard Rain Recording
 
Hard Rain

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  A heavy downpour produces more noise than any other typer of rain. Both the sound of raindrops hitting the surface and the bubbles they form are very loud across a wide specturm of frequencies. (Image by Robert Simmon, NASA GSFC. Recording from Jeffrey A. Nystuen, University of Washington Applied Physics Laboratory)

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  Large Drop Animation
 

next How do Raindrops Make Sound Underwater?

  For large (2.0-3.5 mm diameter) and very large (greater than 3.5 mm) raindrops, the splash becomes energetic enough that a wide range of bubble sizes are trapped underwater during the splash, producing a loud sound that includes relatively low frequencies (1 - 10 kHz) from the larger bubbles. For very large raindrops, the splat of the impact is also very loud with the sound spread over a wide frequency range (1-50 kHz). (Animation by Susan Gonnelli, NASA Television)

large drop recording

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  Large Drop Recording
 
Large Drops

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  In this recording individual large raindrops two distinct sound: the slap of the raindrop on the water surface and the bell tone of the bubble. (Image by Robert Simmon, NASA GSFC. Recording from Jeffrey A. Nystuen, University of Washington Applied Physics Laboratory)

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  Medium Drop Animation
 

next How do Raindrops Make Sound Underwater?

  Drops of medium size (1.2-2.0 mm diameter), do not trap bubbles underwater and, consequently, are relatively quiet—much quieter than small raindrops. The only acoustic signal from these drops is a weak impact sound spread over a wide frequency band. (Animation by Susan Gonnelli, NASA Television)

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  Listening to Raindrops

 

  Page 2

A hydrophone sits beneath the water surface and records the sounds of a summer thunderstorm passing overhead. The hissing of small raindrops hitting the water mingles with the gurgles and plops of larger drops. Occasionally, we hear the bass tones of thunder. These underwater sounds of a rain storm are being carefully recorded by hydrophones (underwater microphones) in remote stretches of ocean and analyzed by scientists to supplement global rainfall measurements.

Different sizes of raindrops produce dramatically different sounds as they hit water, primarily because some sizes of drops generate bubbles and others do not. Because the sound of rain underwater is loud and distinctive, we can use it to detect and measure raindrop sizes and amounts of rainfall over the ocean. Data from remote ocean areas is currently sparse, and this new recording technique will add new data and contribute to a global picture of rainfall. Scientists need these measurements to support climate studies of the distribution and intensity of global rainfall patterns.
 

   
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Rain is one of the most important components of climate. Knowledge of its distribution and intensity is important not only to farmers and flood control planners, but also to meteorologists, oceanographers, and scientists who study climate (climatologists). Of particular interest to climatologists is the release of latent heat each time a raindrop forms. It takes energy to evaporate liquid water and this energy is stored in water vapor as “latent heat.” The formation of raindrops releases latent heat, which is one of the primary sources of energy that drives atmospheric circulation. Thus, scientists need to improve their understanding of the global distribution and intensity of rainfall to improve weather and climate forecasts. Furthermore, layers of relatively fresh water due to rain at the ocean surface are now thought to significantly affect oceanic circulation (Anderson et al. 1996), another important component of global climate. Unfortunately, rainfall is very difficult to measure, especially over the ocean where few people live and where rain gauges commonly used on land don't work. But we know that rain falling onto a tin roof makes a lot of noise, and so does rain falling onto water. In fact, rain falling onto water is one of the loudest sources of underwater sound, therefore we can measure oceanic rain by listening to it from below the ocean surface.

next How do raindrops make sound underwater?

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

  Scientists use the distinct sounds made underwater by different size raindrops to measure rainfall. Hydrophones in remote stretches of ocean record the sound of rain falling on water and transmit it to laboratories where the recordings are analyzed to augment our undersdtanding of global rainfall patterns. Click on each image to hear the underwater sound of rain (˜500kB MP3). (Images by Robert Simmon, NASA GSFC)

  How do Raindrops Make Sound Underwater?   Page 1Page 3
 

There are two components to the sound generated by a raindrop splash. These are the splat (impact) of the drop onto the water surface and then the subsequent formation of a bubble under water during the splash. The relative importance of these two components of sound depends on the raindrop size.
 

   
 

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  Different sizes of raindrops produce different sounds underwater. The most distinct of these sounds are generated by the bubbles formed by small and large-sized drops. (Animations by Susan Gonnelli, NASA Television)
 

Surprisingly, for most raindrops, the bubble is by far the loudest source of sound. Bubbles are one of the most important components of underwater sound (Clay and Medwin 1977). They have two stages during their lifetimes: “screaming” infant bubbles and quiet adult bubbles. When a bubble is created, the pressure inside it is not at equilibrium with the pressure of the surrounding water. The water pushes against the bubble, compressing it. As the bubble shrinks, the air trapped inside increases in pressure. This occurs so rapidly that the pressure inside the bubble becomes higher than that of the water, so it expands to equalize, again overshooting. The bubble oscillates between high and low pressure at a high frequency, creating a distinctive and well-quantified sound. The sound radiates energy, so the bubble eventually reaches equilibrium with its surroundings.
 

   
 

The frequency of the sound is well defined (Minnaert 1993) and depends on bubble radius, local pressure, local water density, and a geophysical constant. The important observation is that the size of the bubble is inversely proportional to its resonance (ringing) frequency. Larger bubbles ring at lower frequencies and smaller bubbles ring at higher frequencies. The sound radiated is often loud and narrowly tuned in frequency (a pure tone). But quickly, after just tens of milliseconds, a bubble in water becomes a quiet adult bubble and changes its role—it absorbs sound and is especially efficient at absorbing sound at its resonance frequency.

Naturally occurring raindrops range in size from about 300 microns in diameter (a drizzle droplet) to more than 5 millimeters in diameter (often at the beginning of a heavy downpour). As the drop size changes, the shape of the splash changes and so does the subsequent sound production. In laboratory and field studies (Medwin et al. 1992; Nystuen 1996), scientists identified five acoustic raindrop sizes (see Table 1). For tiny drops (diameter less than 0.8 mm), the splash is gentle and no sound is detected. On the other hand, small raindrops (0.8—1.2 mm diameter) are remarkably loud. The impact component of their splash is still very quiet, but the geometry of the splash is such that a bubble is generated by every splash in a very predictable manner (Pumphrey et al. 1989). These bubbles are relatively uniform in size, and therefore frequency, and are very loud underwater. Small raindrops are present in almost all types of rainfall, including light drizzle, and are therefore responsible for the remarkably loud and unique underwater “sound of drizzle” heard between 13—25 kHz, the resonance frequency for these bubbles.
 

  The relationship between the size of a bubble and the frequency of sound it emits is well known, and is calculated from the following formula:

Bubble Equation

 
drop
size
diameter (mm) sound source frequency
range
(kHz)
splash
character
tiny < 0.8 silent  gentle
small 0.8–1.2 loud bubble 13–25 gentle, with bubble every splash
medium 1.2–2.0 weak impact 1–30 gentle, no bubbles
large 2.0–3.5 impact, loud bubbles 1–35
2–35
turbulent, irregular bubble entrainment
very large > 3.5 loud impact, loud bubbles 1–50
1–50
turbulent, irregular bubble entrainment, penetrating jet

 
  Acoustic raindrop sizes. The raindrop sizes are identified by different physical mechanisms associated with the drop splashes.
Graph Scale

Graph of the Underwater Sound of a Thunderstorm

Interestingly, the splash of the next larger raindrop size, medium (1.2-2.0 mm diameter), does not trap bubbles underwater and, consequently, medium raindrops are relatively quiet—much quieter than the small raindrops. The only acoustic signal from these drops is a weak impact sound spread over a wide frequency band. For large (2.0-3.5 mm diameter) and very large (greater than 3.5 mm) raindrops, the splash becomes energetic enough that a wide range of bubble sizes are trapped underwater during the splash, producing a loud sound that includes relatively low frequencies (1 - 10 kHz) from the larger bubbles. For very large raindrops, the splat of the impact is also very loud with the sound spread over a wide frequency range (1-50 kHz). Thus, each drop size produces sound underwater with unique spectral features that can be used to acoustically identify the presence of drops of a given size within the rain.

next Detection and Measurement of Rain at Sea
back Listening to Rain

 

 
An example of the underwater sound field generated by a heavy thunderstorm recorded in Miami, FL, is shown at left.

sound level palette

The variations in the sound field are associated with changes in the drop size distribution. During the heavy convective downpour, with rainfall rates reaching 150 mm/hr, very large raindrops are present and the sound field is loud across the entire spectrum (1–50 kHz). At the end of the convective downpour, a long drizzle begins. This phase of the storm has few large drops. The sound generated by small drops dominates the sound field producing the distinctive 13–25 kHz peak in the sound field associated with drizzle. At the end of the event, a few large drops are again present and once again the sound field becomes elevated below 10 kHz.

rain rate palette

Because the sound signatures for each drop size are unique, it is possible to invert the underwater sound field to acoustically estimate the drop size distribution within the rain. Once an acoustic drop size distribution is obtained, a variety of interesting features associated with the rain can be calculated, for example, rainfall rate or median drop size.

The observed drop size distribution in the thunderstorm and the acoustical inversion based on the unique sound signatures for each drop size. Very large raindrops are present during the heavy downpour. During the following drizzle, only small and medium raindrops are present and the sound of drizzle is heard between 13–25 kHz. Still later, a few large raindrops are present and the sound levels below 10 kHz become higher once again.

(Figure by Jeffrey A. Nystuen, University of Washington Applied Physics Lab)

  Detection and Measurement of Rain at Sea   Page 2Page 4
 

In order to measure rain at sea, scientists at the University of Washington’s Applied Physics Laboratory designed and built Acoustic Rain Gauges (ARGs). The ARG consists of a hydrophone (underwater microphone), some electronic circuitry, a low-power sampling computer, and a battery package designed to operate the ARG without servicing for up to a year. The ARG is attached to a mooring line, and can be placed at any depth in the ocean, although practically the depth is limited by the crushing strength of the instrument case. Every few minutes the ARG "wakes up" and evaluates and records the underwater sound field. Currently, the ARG design is autonomous from the surface float, and the recovery of data awaits recovery of the mooring. In the future, real-time transmission of the data will be needed to provide useful data for weather forecasting.

When listening for rain in the ocean, the first step is to identify the sound as rain. There are lots of other sounds underwater, including the sounds of waves breaking, man-made sounds and biological sounds. Biological and man-made sounds are sometimes very loud and, if they contain frequency components that overlap the rain-generated sound, then they can prevent acoustical measurement of rain. These noises are usually intermittent or geographically localized. Some locations where persistent "noise" is present includes harbors (shipping and industrial activity) and snapping shrimp colonies. Snapping shrimp are from a family of shrimp species that make very loud "snaps" and that inhabit shallow tropical waters. Fortunately, the frequency content of most sounds is unique to their sources, and can be used to identify the sources, including rain, drizzle, and whitecaps. Some examples of oceanic sound spectra are shown in the graph below.
 

  Acoustic Rain Gauge
Dr. Jeffrey A. Nystuen holding an Acoustic Rain Gauge (ARG). This instrument is designed to be clamped onto an oceanic mooring and will record the underwater sound for one year. (Photograph courtesy Jeffrey A. Nystuen)

graph label

sound level vs. frequency

Most of the time it is not raining and no man-made or biological noises are present. When this is true, the sound is from the whitecaps generated by wind and can be used to quantitatively measure wind speed (Vagle et al. 1990) as the number of whitecaps is proportional to wind speed. The shape of the sound spectrum generated by breaking waves is controlled by the distribution of bubble sizes generated by the breaking wave (Medwin and Beaky 1989). An interesting feature of the wind-generated signal is an apparent limit to the loudness of the sound at higher frequencies. This is due to quiet adult bubbles absorbing the higher frequency sound levels (Farmer and Lemon 1984). Because of their smaller size, bubbles that absorb high-frequency sound stay in the water longer and can form effective layers of sound-absorbing bubbles.
 

 

The graph at left shows examples of underwater sound spectra recorded from an oceanic mooring in the South China Sea. The sound spectra from wind-only conditions (green) show a uniform shape and a sound level which is proportional to wind speed. The sound of drizzle (light blue) shows the characteristic peak associated with the sound generation mechanism of the small raindrops. The sound of heavy rain (dark blue) is louder and includes lower frequencies. The sound of extreme rain includes sound generated by very large raindrops and is very loud. It also shows the effect of "quiet adult bubbles." Two spectra from extreme rain (200 mm/hr) are shown. The first (dark blue) shows extremely high sound levels at all frequencies. The second (burgundy) shows relatively lower sound levels above 10 kHz. This spectrum was recorded five minutes after the first, and yet the rainfall rate was still the same. A layer of bubbles had been injected into the sea surface. New "rain sound" has to pass through the bubble layer to reach the ARG sensor, and is partially absorbed by the bubbles. Since smaller bubbles (higher resonance frequency) are less buoyant than larger bubbles, they stay in the water longer and thus this bubble effect is most noticeable at higher frequencies. (Graph by Jeffrey A. Nystuen)

  Using the graph above, the differences between wind-only and rain-generated spectra often appear to be subtle. However, by presenting the data in a different manner (below), acoustic identification of different weather conditions becomes apparent. The sound of rain and drizzle contains relatively more high frequency sound than the sound from wind-only conditions. Furthermore, rain is much louder. Even drizzle, under low wind speed conditions, has sound levels which can be orders of magnitude louder than wind-only conditions. The characteristic sound of drizzle, the 13-25 kHz peak, is sensitive to wind and has not been detected when the local wind speed is more than 8-10 m/s. On the other hand, the sound from heavy rain is very robust and can be detected even in very high wind speed conditions (over 20 m/s) (Nystuen and Farmer 1989). Extreme rain (over 100 mm/hr) is even louder, and can generate an ambient bubble layer that will distort the recorded sound spectrum.
 
   
Graph Label

Graph comparing sound level at two different frequencies

An example of the acoustic interpretation of the underwater sound field is shown below. During this three-day period a strong atmospheric front passed over the location of an ARG. When it was not raining, the acoustic estimate of wind speed matched a nearby mechanical anemometer to within +/- 1 m/s (very good agreement). Because rain is so loud underwater, acoustical wind speed measurements are only possible when is not raining. During the peak of the storm, heavy rain was detected. This acoustic observation was ’confirmed” by near-surface (1 meter depth) salinity measurements. Similar records of acoustic measurement of rainfall have been obtained from ARGs on drifting buoys (Nystuen and Selsor) and from an oceanic mooring in the South China Sea.
 

  Acoustic weather classification uses features of the underwater sound spectrum to identify the sound source: wind (green), drizzle (light blue), rain (medium blue), extreme rain (dark blue) and to detect ambient bubbles. (Graph by Jeffrey A. Nystuen)

Graph Label

Ambioent Sound Record Graph

next Conclusions and References
back How do raindrops make sound underwater?

 

 
A history of the sound field over three days at four different frequencies from the North Atlantic Ocean (ASREX Experiment, sponsored by Office of Naval Research). The lowest frequency (500 Hz) is not affected by precipitation, while the highest frequency (20 kHz) is affected by rain, drizzle and ambient bubbles. The acoustic interpretation of the sound record measures wind speed (light green), rainfall rate (dark blue) and detects drizzle (light blue). Comparison data for wind speed (dark green) and near-surface salinity (green, bottom graph) from a nearby surface mooring are also shown. Note that the surface salinity dropped during the rainstorm. (Graph by Jeffrey A. Nystuen)

  Conclusion   Page 2
 

The sound of rain underwater is a loud and distinctive signal that can be used to detect and measure rain at sea. Individual raindrops make sound underwater by two distinct mechanisms: the impact of the raindrop onto the ocean surface and sound radiating from any bubbles trapped underwater during the splash. For most raindrops, the sound radiation by bubbles is, by far, the louder sound source. Because the geometry of their splashes regularly traps a bubble of uniform size, small raindrops (0.8-1.2 mm diameter) are unexpectedly loud underwater. These drops are responsible for the remarkably loud "sound of drizzle" heard between 13-25 kHz. Medium raindrops (1.2-2.0 mm diameter) are relatively quiet, while large (2.0-3.5 mm diameter) and very large (> 3.5 mm) raindrops have energetic splashes which can trap larger bubbles. These bubbles radiate sound at frequencies as low as 1 kHz. Because the different raindrop sizes produce sound with distinctive features, the sound field can be "inverted" to measure the raindrop size distribution within the rain. This is a good measure of rainfall rate, or other interesting features of rainfall.

Although there are sometimes man-made or biological noises that are loud and could potentially interfere with the acoustical measurement of rain, these noises are generally intermittent or geographically localized. When rain is present, the sound from rain dominates the underwater sound field. There are two features of rain- and drizzle-generated sound that allow detection of rain at sea. These are the relative level (very loud) and the relatively higher sound levels at higher frequency (over 10 kHz) when compared to wind. By monitoring for these distinctive spectral features, it is possible to detect and then quantify rainfall at sea.

New ARGs are currently being deployed on several of the moorings that form the Tropical Atmosphere Ocean (TAO) deep-ocean mooring array deployed by NOAA in the tropical Pacific Ocean (McPhaden et al. 1998). Data from these ARGs should become available for scientists beginning in the year 2000. By learning to listen to the ocean, we can make important rainfall observations that will help meteorologists, oceanographers and climatologists to better understand the distribution and intensity of this important component of climate.
 

   
 

References
S.P. Anderson, S.P., R.A.Weller and R. Lukas, 1996: Surface buoyancy forcing and the mixed layer of the western Pacific warm pool: Observations and 1–D model results. J Climate 9, 3056–3085.

C.S. Clay and H. Medwin, 1977, Acoustical Oceanography, Chapter 6, Wiley, New York, 544p.

D.M. Farmer and D.D. Lemon, 1984: The influence of bubbles on the ambient noise in the ocean at high wind speeds. J. Phys. Oceanogr. 14, 1762–1778.

M.J. McPhaden, A.J. Busalacchi, R. Cheney, J.R. Donguy, K.S. Gage, D. Halpern, M. Ji, P. Julian, G. Meyers, G.T. Mitchum, P.P. Niiler, J. Picaut, R.W. Reynolds, N. Smith and K. Takeuchi, 1998: The Tropical Ocean–Global Atmosphere (TOGA) observing system: A decade of progress. J. Geophys. Res, 103, 14,169–14,240.

H. Medwin and M.M. Beaky, 1989: Bubble sources of the Knudsen sea noise spectrum. J. Acoust. Soc. Am. 83, 1124–1130.

H. Medwin, J.A. Nystuen, P.W. Jacobus, L.H. Ostwald and D.E. Synder, 1992: The anatomy of underwater rain noise. J. Acoust. Soc. Am. 92, 1613–1623.

M. Minnaert, 1933: On musical air bubbles and the sounds of running water. Philos. Mag. 16, 235–248.

J.A. Nystuen, 1996: Acoustical rainfall analysis: Rainfall drop size distribution using the underwater sound field. J. Acoust. Soc. Am. 13, 74–84.

J.A. Nystuen and D.M. Farmer, 1989: Precipitation in the Canadian Atlantic Storms Program: Measurements of the Acoustic Signature. Atmosphere–Ocean 27, 237–257.

J.A. Nystuen, M.J. McPhaden and H.P. Freitag 1999: Surface Measurements of Precipitation from an Ocean Mooring: The Underwater Acoustic Log from the South China Sea", submitted to J. Applied Meteor.

J.A. Nystuen and H.D. Selsor, 1997: Weather classification using passive acoustic drifters. J. Atmos. and Oceanic Tech., 14, 656–666.

H.C. Pumphrey, L.A. Crum and L. Bjorno, 1989: Underwater sound produced by individual drop impacts and rainfall. J. Acoust. Soc. Am. 85, 1518–1526.

S. Vagle, W.G. Large and D.M. Farmer, 1990: An evaluation of the WOTAN technique for inferring oceanic wind from underwater sound. J. Atmos. and Ocean. Tech. 7, 576–595.

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  Small Drop Animation
 

next How do Raindrops Make Sound Underwater?

  Small raindrops (0.8—1.2 mm diameter) are remarkably loud. The impact component of their splash is very quiet, but the geometry of the splash is such that a bubble is generated by every splash in a very predictable manner. These bubbles are relatively uniform in size, and therefore frequency, and are very loud underwater. Small raindrops are present in almost all types of rainfall, including light drizzle, and are therefore responsible for the remarkably loud and unique underwater “sound of drizzle” heard between 13—25 kHz, the resonance frequency for these bubbles. (Animation by Susan Gonnelli, NASA Television)

drizzle recording

small drop animation
medium drop animation
large drop animation