[cos2pi]

Gravity waves are waves in a fluid that obtain their restoring force from their buoyancy relative to the surrounding fluid. See [0] for the gory details.

Some examples of gravity waves in the atmosphere: [1] [2]

[0] http://glossary.ametsoc.org/wiki/Gravity_wave [1] http://cimss.ssec.wisc.edu/goes/blog/archives/2051 [2] https://www.youtube.com/watch?v=yXnkzeCU3bE