A hole in the cloud, but not in the the­ory….

Cape Breton Post - - WEATHER - CINDY DAY Cindy Day is SaltWire Net­work’s chief me­te­o­rol­o­gist

I don’t spend a lot of time on Face­book dur­ing the week­end, but I like to check it a few times. When I did yes­ter­day, I couldn’t be­lieve my eyes. Not one, but sev­eral pho­tos of a fairly un­usual cloud for­ma­tion. It ap­pears that many peo­ple in the Syd­ney area were look­ing up Satur­day af­ter­noon and no­ticed a huge hole in the clouds! More than one per­son de­scribed it by say­ing “it looks like some­one punched a hole in the cloud”. Not only is that a great de­scrip­tion, but it also names the cloud. The cloud is of­ten re­ferred to as a punch cloud, a hole-punch cloud or fall­streak hole.

A fall­streak or punch hole is a large cir­cu­lar gap that can ap­pear in the clouds; most of­ten in al­tocu­mu­lus clouds. That cloud layer can be com­posed of small wa­ter droplets that are be­low freez­ing (su­per-cooled wa­ter droplets). If ice crys­tals can form in the layer of su­per-cooled droplets, they will grow rapidly and shrink or pos­si­bly evap­o­rate the wa­ter droplets com­pletely.

This is known as the Berg­eron process. Air­craft can trig­ger this process too. Stud­ies have shown that air­craft pass­ing through these cloud lay­ers can trig­ger the for­ma­tion of the heav­ier ice crys­tals, which fall to earth and then leave the cir­cu­lar void in the blan­ket of clouds. De­pend­ing on the amount of mois­ture in the air, one of two things can hap­pen. grow • It at can the ex­pense snow: Ice of the par­ti­cles su­per­cooled wa­ter droplets and fall out as snow. The snow changes to rain on its way down to earth. • Or a hole re­mains in the sky: if the cloud layer is thin or if the wa­ter is not re­plen­ished, the snow leaves a hole in the cloud. Sci­en­tists first re­ported ob­serv­ing hole-punch clouds in the 1940’s. They of­ten lead to false re­ports of UFOs or rocket launches.

So glad that Heather Lewis was look­ing up on Satur­day! This hole in the cloud ap­peared over Whit­ney Pier, NS...

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