Kelvin-Helmholtz wave clouds are manifest from two different velocities... an unstable cloud experiences one velocity, the air rushing over the surface is moving at another velocity, and this instability creates waves in the 'interface'.
The same thing occurs to create waves on water; the water moves with one velocity, the air with another, and waves are created in that water/air interface.
Kelvin-Helmholtz instability waves in clouds however, are fairly uncommon, and typically a very short life span, mere seconds. This is only my third sighting in my life, and not really a very pronounced one.
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