|Linear 'Hole Punch'
Imaged by Wolfgang Hinz (Atmospheric Phenomena) at Chemnitz, Saxony, Germany 22nd October '97. ©Wolfgang Hinz, shown with permission
|This magnificent 'hole punch' spanned the sky.
Large ice crystals falling from it formed a circumzenithal arc and (lowest image) a sundog.
Hole punches are thought to result from an instability causing supercooled droplets already in the cloud layer to suddenly convert to large ice crystals. The ice crystals fall out of the cloud and are in any case less opaque – we therefore see a hole.
A trigger for the instability might be disturbance by an aircraft. Another trigger could be ice crystals that act as nuclei, seed crystals, tumbling downwards from higher cloud or a contrail.
Clouds with supercooled droplets arise because water does not always freeze at zero degrees Celsius. If no nuclei are present on which ice crystals can grow, the cloud water droplets remain and cool to sub-zero temperatures. The condition is metastable in that ice is the more stable state. Introduce enough nuclei or otherwise disturb the cloud and ice crystals grow very rapidly at the expense of the droplets.
This is a normal. Rain can form that way from an initial formation of ice crystals coagulating into larger snowflakes which subsequently melt as they descend. What is strange is that in hole punches and their like, the supercooled droplet to ice transformation takes place only in small patches.
This linear version could be induced by passage a higher aircraft or there could be another interpretation to the whole phenomenon.