This is well known from water waves at the interface between sea water and the atmosphere, but can also take place at the interface of waters with different density (e.g. cold water and warm water (stratified) layers in the ocean), or in the atmosphere between different layers of air.
Here is an example of waves at the interface between thick clouds and fast-moving air higher in the sky.
The clouds are examples of "Kelvin-Helmholtz waves". This type of turbulence forms when a fast-moving layer of fluid slides on top of a slower, thicker layer, dragging its surface. When the difference between the air and cloud speed increases to a certain point, the waves "break" — their crests lurch forward — and they take on the telltale Kelvin-Helmholtz shape. The clouds were moving from left to right (i.e. from west to east). I saw these clouds above the Eyjafjallajökul on 11 January 2012.
Fascinating isn’t it? — my best guess is that you have never seen such rare cloud waves, for most of us they are only for “once in a lifetime”.
And here is a nice collection of photos better than mine (but I suppose I'll never get a second chance):