Sometimes clouds form in parallel lines; long, straight lines of clouds with clear sky in between. These clouds are often called billow clouds or buoyancy clouds, but their technical name is altocumulus undulatus. They look like waves on a lake because that's almost exactly what they are. When air moves, it behaves like water with a fluid motion. Waves happen on the surface of a lake when wind blows across the water faster than the water can move. The resulting drag force sets up waves.

Waves in the air happen at the boundary between two layers of air composed of differing temperature and density. The colder, denser air acts as a drag on the warmer, lighter air and waves are formed just like on a lake. When the wave crests are cold enough to condense moisture, the tops of the waves become visible clouds.

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