Undular Bores

Posted by jns on 17 October 2007

Here is “Science @ NASA” again, sending me another interesting story with pretty pictures. This one — they say for shock value — is about “undular bores”. What they’re talking about is waves in the atmosphere that show up dramatically in cloud patterns.

I have a personal interest in all things waves because they were one of the things I studied in my former life as a physicist. They were waves in many forms that interested me, too, since I studied fluid motions, in which one can find physical waves, and field phenomenon, in which the waves are mathematically abstract but still represent real phenomenon.

In looking at these wave pictures coming up, it might be fun to know that in the ocean — or on any water surface — waves come in two types, call them “waves” (the big ones) and “ripples” (the little ones). Ripples are about finger sized, while waves are more body sized. Both are caused by disturbances, generally wind, but they undulate for different reasons. Ripples ripple because of the surface tension of the water; waves wave because of gravity, or buoyancy, in the water. Most water waves in the ocean, and those that break on shorelines, are generated by strong winds associated with storms that can be thousands of miles away.

Anyway, air also has buoyancy (remember: hot air rises, cool air sinks — the hot air is buoyant relative to the cool) and can sometimes show some very large scale waves, with wavelengths of about a mile. When there are clouds around they can make the waves dramatically visible. They can be caused by storms moving about as high-pressure centers collide with low-pressure centers and hot-air masses encounter cool-air masses.

Here’s Tim Coleman of the National Space Science and Technology Center (NSSTC) in Huntsville, Alabama:

“These waves were created by a cluster of thunderstorms approaching Des Moines from the west,” he explains. “At the time, a layer of cold, stable air was sitting on top of Des Moines. The approaching storms disturbed this air, creating a ripple akin to what we see when we toss a stone into a pond.”

Undular bores are a type of “gravity wave”—so called because gravity acts as the restoring force essential to wave motion. Analogy: “We’re all familiar with gravity waves caused by boats in water,” points out Coleman. “When a boat goes tearing across a lake, water in front of the boat is pushed upward. Gravity pulls the water back down again and this sets up a wave.”

[excerpt from "Giant Atmospheric Waves Over Iowa", Science @ NASA for 11 October 2007.]

There are two gorgeous visuals to see by clicking the link. The first is a video of the undular bores over Iowa. I’d suggest watching the animated gifs rather than the video because the gifs extract just the interesting parts between 9:25 and 9:45, and it keeps playing.

Then, just below that, don’t miss the radar photograph, which shows the train of waves with stunning clarity.

Now, as if that weren’t enough, I found this cool video on YouTube, intended to demonstrate “Gravity Waves” in the atmosphere, of which it is a beautiful example. But wait! Incredibly, this video also shows undular bores, over Iowa (but Tama, rather than Des Moines), as recorded by the same television station, KCCI!