There are all sorts of weird-looking clouds. Mammatus is my particular favourite, but you can also get Morning Glory clouds, clouds shaped like waves formed by Kelvin-Helmholz instability and noctilucent clouds, just to name a few. They’re all pretty rare, but they do happen from time to time, and, of course, they’re all quite natural and can be explained using sound scientific principles.
Naturally though, like our friend Liara in Hawaii who craps her pants at the sight of (relatively common, in NZ) lenticular clouds, Clare over at Northland NZ Chemtrails seems to see the presence of a hole-punch cloud as evidence of ‘full-time weather modification’. You’d think if ‘full-time weather modification’ was actually happening and resulted in hole-punch clouds, then we’d see this type of cloud all the time. I await further pictorial evidence of the increased commonality of hole-punch clouds with bated breath. (It goes without saying that attempts to point out to Clare just what type of cloud this was were deleted in short order).
Of course, hole-punch (or fall-streak) clouds are perfectly normal. The wikipedia entry has a nice succinct explanation on their formation…
A fallstreak hole (also: hole punch cloud, punch hole cloud, canal cloud) is a large circular 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 particles. When a portion of the water does start to freeze it will set off a domino effect, due to the Bergeron process, causing the water vapor around it to freeze and fall to the earth as well. This leaves a large, often circular, hole in the cloud.
It is believed that a disruption in the stability of the cloud layer, such as that caused by a passing jet, may induce the domino process of evaporation which creates the hole. Such clouds are not unique to one geographic area and have been photographed from the United States to Russia.
Because of their rarity and unusual appearance, fallstreak holes are often mistaken for or attributed to unidentified flying objects.
It’s interesting in this instance that, to an extent, hole-punch clouds can sometimes be triggered by human interfence — passing planes, as mentioned in the wikipedia entry, can set off the process whereby a hole-punch cloud is formed. You can see some good examples of this in the gallery of hole-punch clouds over at the Cloud Appreciation Society — they really are quite spectacular.