Cloud-watching can be a supremely satisfying pastime -- that is, if climate change doesn't reduce them first. Sometimes, one can observe incredible, rare cloud phenomena like the undulatus asperatus, or this gorgeous "punch hole" cloud. In this photo taken by photographer David Barton of the skies over Victoria, Australia, we can see an opening in a cloudy sky, painted with a rainbow.
This rare cloud is also called a "fallstreak hole," says Wikipedia:
A fallstreak hole, also known as a hole punch cloud, punch hole cloud, skypunch, canal cloud or cloud hole, is a large circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water has not frozen yet due to the lack of ice nucleation (see supercooled water). When ice crystals do form it will set off a domino effect, due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud.
One can't help but marvel at the mind-boggling diversity of forms that clouds can take, with people forming "cloud appreciation" clubs to catalogue the different clouds they see. Strangely enough, the fallstreak hole cloud is also the type of cloud that many may mistake for an unidentified flying object (UFO).
[Via This Isn't Happiness]