Hole In Cloud Deck

July 23, 2003


Provided by: Jeff Higgins
Summary authors & editors: Jim Foster

The photo above shows what appears to be a large opening in an altocumulus cloud deck above Toronto, Canada. This gap or "hole punch" is likely caused by localized sublimation of ice crystals. If ice crystals are introduced into a cloud layer containing minute liquid droplets that exist well below freezing (supercooled), they'll grow at the expense of the droplets. When these crystals become sufficiently large to fall, they often fall into drier air and evaporate. Thus, a gap may appear in the cloud deck. This is what is occurring above -- ice crystals have descended into drier air and are sublimating. Virga (precipitation that doesn't reach the ground) can be seen falling from the opening.

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