What’s that falling out of the sky? That’s rain, right?

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Rain has fallen so infrequently in the Tennessee Valley this fall, it almost feels like we’ve forgotten what rain is, right?

Well, here’s a reminder (many thanks to Michelle Owens for sharing this photo):

Michelle jokes that she took a picture of the rain because she had forgotten what it looked like, since we’ve been in a dry pattern for so long.

But let’s take a closer look at what rain looks like as its falling from the sky. Many of us think it looks like this:


But in reality, it looks more like a hamburger bun:

Schematic of a falling raindrop. Source: NASA Precipitation Measurements Missions.

Schematic of a falling raindrop. Source: NASA Precipitation Measurements Missions.

Why the funky shape? It all has to do with physics.

Raindrops form when water vapor condenses into liquid water around particles of dust or salt, known as cloud condensation nuclei. The tiny liquid droplets (about 1 millimeters in diameter, or about 0.04 inches) eventually coalesce together and grow bigger in size.

(Source: USGS)

(Source: USGS)

The force of gravity pulls them towards the ground, resulting in air rushing against the bottom of the raindrop. Surface tension — a force that occurs due to the hydrogen bonds within water — hold the drop together and maintain the round shape.

As the raindrop falls, higher air pressure pushes against the bottom of the drop compared to the top. The surface tension keeps the drop intact, but the pressure from the rushing air causes the the bottom to flatten out, making it resemble a hamburger bun.

The drop continues to grow in size, and soon the flat bottom gets punched inwards, making the drop resemble a kidney bean. Eventually, the raindrop grows to a size that is large enough such that the rushing air pressure overcomes the surface tension within the raindrop (greater than 4.5 millimeters, or about 0.18 inches). The result is that the raindrop splits into smaller drops, and the process begins all over again.

So there you go! Whether you forgot what rain looks like because it’s been so dry around here — you didn’t know that rain actually looks more like a hamburger — I hope this helps reacquaint you with the liquid form of precipitation. Now if only it would return to the Tennessee Valley…