Or terrestrial transmitters. It's an interference pattern, a moire pattern. Slight variations in electric charge cause nucleation and condensation as anon's picture clearly shows. Haven't we all seen these non-natural cloud banks form, the grid pattern change as we watch, and then rapidly dissipate? I used to phograph the grids and have tons of photos. I don't bother photographing them anymore, but still point them out to others in the house when they are observed.