Photo: Colorado Uerling
Dark Roasted Blend has a neat post about all sorts of weird cloud formations. This one above is a Punch Hole Cloud:
Punch Hole Clouds may appear as a circular or oval holes in a layer of supercooled clouds; sometimes they assume a form of a perfect circle and persist for quite a long time, drifting together with the cloud layer. One explanation seems to blame the air traffic (the jet contrail intersections) combined with a thermal inversion (a circular motion of a rising warm air).