Recently the phenomenon know as 'Cloud Holes' have become more and more prevalent.  Many theories include weather manipulation, LIDAR, contrails, chemtrails and the like.  All of these may be plausible, however the recent UFO sighting at Chicago's O'Hare airport in late 2006 sheds some light on the anomaly of cloud holes.  Eye witness accounts describe the departure of the UFO's as straight up through the clouds, leaving a perfectly round hole or opening in the cloud cover, some with wispy vapor trails though the center.  These holes persisted for upwards of 30 minutes.  With worldwide sightings of UFO's on the rise it would be likely their departures would leave telltale signatures of these ever increasing cloud holes.