I can't guarantee too many things, but I can pretty much guarantee this...Unless you are a studied meteorologist, you have probably never heard of Kelvin-Helmholtz instability and the clouds that result from that instability. One of my favorite weather phenomena (up there with ball lightning and St. Elmo's Fire), it is an extremely rare find, and you are very lucky if you ever see them. Often found near mountain ranges, they are formed by vertical wind shear, meaning that two layers of air are moving in opposite directions at different speeds. Just like what happens with waves at the beach, the lower layer of air slows due to friction with the earth, but the upper layer of air still moves at the same speed. This causes the upper layer of air to move out ahead of the lower layer and fall in front of it, creating a wave-like appearance.
Note: Blogger is being stubborn again and it won't let me upload a photo of these awesome cloud features...in the mean time, you can always do a google images search and see for youself :-D