Australian Weather Calendar: July 2016

July photograph by David Barton

David Barton and his wife spotted a rare phenomenon over eastern Victoria on 3 November 2014. Fallstreak holes, or hole punch clouds, form in clouds that contain supercooled water droplets—at a temperature below freezing point, but not frozen. A trigger such as a passing aircraft causes ice crystals to form, and a chain reaction makes water droplets around those crystals evaporate, leaving a ‘hole’ around the wispy patch of ice crystals.

Sunlight passing through this thin part of the cloud is refracted, or bent, with different wavelengths (i.e. colours) spreading out in a similar way to how rainbows form. This is called cloud iridescence or irisation.

David took this shot from his front porch, with a Nikon D7100 and an 18–140 mm lens.