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Punch a hole in the sky

Aircraft can punch cloud holes that are much larger than the plane itself.

Back in the distant past, when air planes still graced the skies, you might sometimes see a huge hole in the middle of high cloud. These are hole punch clouds, also known as fallstreak holes and cloud holes. To understand how they form, we need to take a quick tour through supercooling and atmospheric ice crystal formation.

We all know that water freezes at 0°C. Well, that’s not precisely accurate. Water can be cooled well below that temperature without any ice forming, even at normal atmospheric pressure. For ice to appear, there need to be not one but two conditions: sub-zero temperature, and the right “seed” to start the ice crystallisation process. 

Think of ice as a rigid crystalline structure – lots of water molecules forming stable clusters. That structure has to begin somewhere: in technical terms, it must have a nucleus from which to spread out. The chemical structures of ice (and indeed all crystals) are contagious, in that as they spread the ice conveys the same crystalline form to the surrounding ice. 

(Side note: you may remember from an earlier post that there are at least eighteen different phases of ice, including electric viral space ice.)

If no nucleus is present, water below 0°C won’t freeze, it’ll just become supercooled. At about -35°C an ice nucleus will spontaneously appear. Much more commonly a particle of dust or ash or sand will trigger the ice nucleation. Once ice begins to form in one place, the surrounding water will very quickly freeze. There’s a nice demonstration here:

Okay, back to cloud holes. High-altitude clouds are effectively huge masses of supercooled water that lack a nucleus. When an aircraft passes through such clouds carrying ice on its wings, that supercooled water suddenly gets a nucleus. The result: rapid ice formation in a circle around the original ice. And that’s the cloud hole.

[Thanks to Gareth E. for suggesting this topic.]

Categories: Earth & sky Physics & chemistry Sciences

The Generalist

I live in Auckland, New Zealand, and am curious about most things.

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