A "hole punch" cloud!

August 27, 5:18 PMSF Weather ExaminerJan Null

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Looking west from the Souh Bay this afternoon it looked like something had punched a gigantic circular hole through the layer of clouds above the Coast Range.  What was being seen was what has indeed been labeled as a "hole punch cloud".

This relatively rare occurenvce is the result of an aircraft flying through a  layer of high clouds that have precisely the right temperatre and moisture.  As the jet aircraft flies through the layer it contributes just enough additonal moisture and exhaust particles for the ice crystals in the cloud to grow large enough to fall out as "fall streaks".  This happens in a circular pattern around the path of the jet with a hole in the cloud layer being the result. 

Jan Null
SF Weather Examiner

For additional information see:
http://www.srh.weather.gov/mob/121103hole_punch/holepunch-main.html
http://apod.nasa.gov/apod/ap040112.html