This is a time-lapse of a gravity wave taken by the Tama, Iowa KCCI-TV webcam on 6 May 2007. Gravity waves form when buoyancy pushes air up, and gravity pulls it back down. Very cool stuff!

Gravity Wave – Videosift

In physics, a gravitational wave is a fluctuation in the curvature of spacetime which propagates as a wave, traveling outward from a moving object or system of objects.

Gravity Wave – Wikipedia