The non-coastal city of Birmingham, Ala., caught some massive waves last week, but they weren't from a body of water. All residents had to do to glimpse the waves was look at the sky to see very distinctive wave-shaped cloud formations.
Check out this video footage of the clouds:
The cloud phenomenon caused by the Kelvin-Helmholtz instability. A recent Reddit discussion started by alison_bee (via Gizmodo) on the topic prompted this response from a meteorologist as to how they are formed:
These are indeed Kelvin-Helmholtz waves. What is happening is that the nocturnal near-surface layers (lowest 50-100m) of the atmosphere are much more stable than the layers above it in the mornings. Until the ground heats up due to daytime heating, the surface layers stay more stable than the air over it. Kelvin-Helmholtz waves occur when the wind shear between the layers destabilizes the topmost portion of that stable layer, and entrains the air into the unstable layer. What you see is stable air being lifted, cooled, and condensed so that this process becomes visible, though this commonly happens many places without being visible.
I'd also like to note that this is different from gravity waves as stated elsewhere as these are completely shear induced while gravity waves are usually from lifting buoyant air into a stable region and gravity pulling that air back down. When that air is pulled back down, it can overshoot it's location of being stable, and a wave pattern forms.
The Kelvin-Helmholtz instability is also what causes waves on water, as well as on the sun. Learn more about the waves on the sun: