Las Vegas is underneath some major aircraft routes. During the non summer months here (the few that we have) the temperature way up high is cool with very low humidity, and the jets overhead leave some major stripes behind them way up there. Some of the first photos I took after moving here were of the interesting patterns in the sky.
Driving to downtown early last week I looked up and saw an interesting checkerboard pattern overhead. The camera in my phone doesn’t capture all of the strips overhead, but even so it still is pretty.