Say you're in an ascending airplane as your 10 year-old son asks you: "Dad, why are these clouds wavy?" Now, say you know a little about gravity waves and the formation of wavy clouds(Maybe I should be linking to As instead? Are these wiki pages basically talking about the same thing?). How would you go about explaining it to your son?
PS. I'm looking for a simple and fluid explanation that would be able to give some intuition for the phenomena.
The wavy clouds and altostratus undulatus links describe specific types of cloud formation. I would guess that your son is asking why all clouds don't have smooth surfaces, so his question is more general.
Assuming this is the case, the clouds are wavy simply because the air flow in and around them is turbulent. If you pour Pernod into water you get a wavy white cloud for exactly the same reason.
If I'm wrong and your son is asking about specific cloud formations, then the waves are not that different in principle from waves on water. Waves on water are an oscillation at an interface, specifically the water/air interface, while the waves in clouds happen at the interface between layers of air at different temperatures and/or moving at different speeds.
