Thanks to Jim Ward who sent us this photo that he took Thursday afternoon. He said he took the picture about three hours after the storms ended, and that there were three rows of these clouds. He wanted to know what they were.They're what's called an undular bore... or, more simply, a type of gravity wave. To visualize how they form, think of a pebble that's dropped into a pond. Once the pebble makes impact on the water, waves form and move across the water's surface. Atmospheric gravity waves work in essentially the same manner. A "piece" of air is somehow displaced (usually due to a thunderstorm) out of its normal, stable condition. It bobs up, then bobs back down, then back up, then down, and so on, until it runs out of the energy it takes to move up and down. We can sometimes see the effects of the air moving up and down - such as in the picture that Jim sent us. Where the cloud formed, the air was moving upward, cooled, and condensed. Where there's no cloud, the air was coming back down.
Note that these can only form when the atmosphere is stable - such as in the wake of a thunderstorm.
You can read a lot more about undular bores, including how they can impact tornado development, by clicking here.
Here are some timelapse videos of gravity waves and undular bores that have been captured by KCCI's central Iowa webcam network.
- Saylorville Lake undular bore (same as the one in the article above)
- Tama undular bore
- Chariton undular bore
- Indianola undular bore
- Tama gravity wave (circulated around the Internet en masse a while back)
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