During a total solar eclipse, gradually changing snake-like "shadow bands" may be seen on the ground just before & after totality:pic.twitter.com/o34H8FAMbE
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The comment about refraction can be put better: you can think of regions of turbulence as weird, tiny little lenses, bending light,
focusing it in some areas, and causing it to be more diffuse in others.
But as to why it's in bands, I don't know. That's a surprise to me.
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