In the last 2 weeks, you may have heard about an important discovery involving “Gravitational waves”; ripples in space-time that are potential remnants of the immediate aftermath of the big bang (see our sister page TU for more: https://www.facebook.com/AstrophysicsAndAstronomy/posts/659723177426191).
Fluid dynamics uses a disturbingly similar, easily confused term; gravity waves. Those type of waves are featured here in Earth’s atmosphere, represented by the undulating clouds.
These types of waves are formed at boundaries where there is a difference in density, such as layers in the atmosphere or between the air and the ocean. In this case, air in the atmosphere is being pushed up; either because of heating from below or because of nearby topography.
As the air rises, it eventually hits a layer that it cannot penetrate fully. The rising air runs into this overriding layer, which pushes back against the air as it rises, creating a restoring force that sets off ripples in the boundary layer. The air above these ripples will move up and down, causing this visible pattern of cloud waves to form if there is enough moisture in the air.
Any system where gravity serves as a restoring force can be called a gravity wave. When waves crash down on the ocean, they are gravity waves as well. A pyroclastic flow, which occurs when the density of a volcanic column becomes greater than that of the air below it, is another example of a wave caused by gravity, trying to create a stable density structure.
Image credit: Anders Sandberg