Feature Sound: My Life as a Raindrop

Image by Robert Simmon, NASA GSFC

My life begins in a cloud. A cloud contains not only water, but also tiny particles such as dust, smoke and salt. These particles are called condensation nuclei. I wrap myself around one of these tiny particles and depending on its size, I become a droplet ranging anywhere from 0.0001 to 0.005 centimeters in diameter. At this point I’m too light to fall out of the cloud. However, every time I bump into another, smaller droplet, I get larger. This is called coalescence. Soon I am so heavy that the cloud can no longer hold me and gravity pulls me toward the Earth. I start to fall, bumping into smaller droplets and becoming even larger. When my size becomes 0.5 millimeters or larger in diameter I am called a raindrop. My final size will depend on the size of my condensation nuclei and my rate of coalescence. Would you believe that I am now about one million times larger than I was when I was a cloud droplet! If I get any larger than 4 millimeters, chances are I’ll split into two separate drops. Most people think that I am tear-shaped. Actually, I only look like this after I have splattered on a window. If my radius is less than 1 millimeter, I am spherical, when I am larger my shape looks more like a hamburger bun!

Did you know that rain falling onto water causes one of the loudest sources of underwater sound? Depending upon my size, I make different sounds as I hit the ocean. This is because some sizes of raindrops produce bubbles and others do not. * There are two parts to the sound of my splash, the splat (or impact) of my drop and the bubble formed underwater during my splash. The bubbles I create have 2 stages, the infant or “screaming” stage and the quiet adult stage. When my bubble is formed, the pressure inside it is not equal to the pressure of the surrounding water. As the water pushes against it, the bubble is compressed and the air trapped inside increases in pressure. The pressure inside the bubble is now higher than the pressure of the water. The bubble expands to equalize the pressure, but goes too far. As it goes back and forth between high and low pressure, at a high speed or frequency, the bubble creates a very distinctive sound. Quickly, just after tens of milliseconds, my bubble becomes a quiet adult and now, instead of producing a sound, absorbs sound. Bubbles are one of the most important components of underwater sound!

-Janet Alden

Large raindrops in wind offshore of Westport, WA. Sound courtesy of Jeffrey A. Nystuen, Applied Physics Laboratory, University of Washington.