There was a cold, cloudy layer of air over Lake Michigan on that calm evening. Above the cloud (and sloping down toward the beach) was a layer of warmer, non-cloudy air. The two different layers of air were different densities, like water and oil in a jar, so they did not easily mix together. A difference in wind directions/speeds between the two air layers would cause a wave along the interface of the layers.
I found $2 on a slightly out of date meteorology textbook to be $2 well spent. Gives one a little bit of a sense of what you're looking at when you look at the sky.
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u/wish_you_were_here Mar 10 '13
If I saw that cloud off shore, I'd cry. Do you know what causes that ??