A Kelvin-Helmholtz wave has a similar appearance to ocean waves but they are in the sky. These cloud patterns form at the interface between layers of air that contrast in density and wind speed over a short distance in the vertical. The more dense air is below the less dense air just like as an analogy to the ocean where dense water is under less dense air. The stronger wind in the less dense air creates waves on top of the more dense air in a similar way to the wind causing ocean waves to be generated on the denser ocean water. Clouds need to be present in order to visually see the waves in the sky. Thus, saturated air near the top of the denser cold air is needed. This is most likely to occur in the morning when the lower atmosphere is not heated by the sun and the boundary layer air is at its coldest. It is possible to see this cloud formation at any time of the day though, thus have your camera ready! A Google search of Kelvin-Helmholtz waves will yield beautiful photos of this phenomenon. Below is a sketch of generally what this cloud formation looks like.