Four days ago, I discovered an amazing GIF featuring some weird clouds rolling like waves in the ocean, with the exception that there’s grass and cattles instead of sharks.

These weird clouds, also called gravity waves, were caught rolling over Tama, Iowa, on May 7, 2006. Pretty impressive aren’t they? Undulatus asperatus?
What are gravity waves?
According to NASA:
They are similar to waves on the surface of the ocean, but they roll through the air instead of the water. Gravity is what keeps them going. If you push water up and then it plops back down, it creates waves. It’s the same with air. […] Gravity waves get started when an impulse disturbs the atmosphere. An impulse could be, for instance, a wind shear, a thunderstorm updraft, or a sudden change in the jet stream. Gravity waves go billowing out from these disturbances like ripples around a rock thrown in a pond.
These gravity waves actually help meteorologists predict severe tornado storms, since when a gravity wave hits a rotating thunderstorm, it is sometimes transformed into a tornado.
Do not confuse gravity waves with gravitational waves (space).
Here some pictures of other strange gravitational wave clouds.