Have you ever seen this happen in the sky? It looks like a certain type of cloud, but with a huge hole in the middle of it?

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Image credit: Leslie Becker, WDRB viewer, Dec 21st, '21, 

For many years, we have seen photos like this that show an odd hole in the clouds. The name of this phenomena is a hole punch cloud. This phenomena can only happen in a very specific set of circumstances. The cloud deck must be a mid or high level cloud known as a  "alto-cumulus" clouds or "cirro-cumulus" clouds. These clouds must contain water that is below freezing, but has not quite frozen yet because the particles necessary (condensation nuclei) for the water to freeze on are not present. To produce one of these hole punch clouds, something has to introduce the necessary cloud condensation nuclei for the supercooled water to form into ice crystals. The second these particles are present, a domino effect known as the Bergeron process occurs allowing tons of ice crystals to form within this cloud layer. As the ice crystals form, they immediately fall out as snow from the clouds.

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A hole punch cloud observed on August 17, 2008 near Linz, Austria. Credit: H. Raab, NASA

The process seems easy enough, but what could introduce these necessary particles (cloud condensation nuclei) to create this process and why does it look like a hole? The answer is airplanes! When planes fly through these mid/high cloud layers where the ideal conditions are present (supercooled water that has not condensed into ice crystals yet), the plane exhaust introduces the necessary particles for the supercooled water to condense into the ice crystals. These ice crystals immediately fall away from the cloud deck and you get the fall streaks you see in the following picture...