This is a scan of an interesting cloud type not often seen around Melbourne. The image is of a deck of altostratus / altocumulus moving across Melbourne in Jan 2002,a small patch had become glaciated a few minutes before this photo was taken, the result was a rapid infection of this thin layer of alto cloud by ice crystals the glaciation process then spread evenly outward to form a circular 'ice fallout hole', over the next few minutes, note the clear ring around the ice fallout area, over the next half hour all the supercooled droplet cloud surrounding the glaciated area evaporated leaving a progressively thinning patch of 'false cirrus' which was the remnants of the original ice hole,the alto stratus cloud deck was at about 17,000ft to 19,000ft.

Clyve Herbert 2002