December 15, 2017

Strange Holes in Clouds Explained

On Wednesday, I received nearly a dozen emails with pictures of strange circular holes in the cloud deck around Seattle.  Here are a few samples:

 Courtesy of Lawrence Wallman

  Courtesy of Lawrence Wallman

Courtesy of Justin Green

These strange features, known as hole-punch clouds...among other names, are caused by the penetration of aircraft through a cloud deck made of supercooled liquid water.  

Believe it or not, many middle-level clouds are below freezing but are still made  of liquid water.  Turns out that clean water can take its time to freeze when temperatures are below 0C.   

We had a cloud deck over Seattle around 3 PM Wednesday that was made up of such supercooled water.  A vertical sounding (vertical ascent from a weather balloon) at Quillayute on the WA coast at 4 PM suggested that the cloud was at roughly 450 hPa pressure or around 6600 m (22,000 ft) and a temperature around -22C (-8F).

We were dealing with an altocumulus or cirrocumulus clouds of liquid water.  A past colleague of mine, Peter Hobbs, and one of his past associates (Art Rangno) studied this situation and called the holes APIPS (Aircraft Produced Ice Particles).   Let me tell you why.

Look closely at the picture shown above (repeated below).  Do you see the fibrous looking material in the middle of the hole. Those are ice crystals! You can tell because they have a wispy, less distinct look to them.  

But what caused the transition between liquid water clouds (with much sharper edge  to ice clouds?   The passage of an aircraft!

When air goes across the wings of an aircraft the air is speed up and that causes pressure to fall on the upper side of the wing (that is what keeps a plane in the air!).  When air parcels experience lower pressure they expand and cool (this is called adiabatic cooling).  

Now clouds can be comprised of liquid water at -10C to around -22C, but if temperatures drop further, it is harder to remain liquid.  Also air can become saturated when it cools on the air frame and go directly to ice if the air is cold enough (-30C to -40C).

It turns out that ice and liquid water find it difficult to be in the same environment, water vapor leaves the liquid water and heads over to the ice crystals (this is often called the Bergeron process).  The ice crystals grow rapidly, become heavy, and fall out of the cloud, producing the hole.  You can see that happening in the cloud picture above.

Mystery solved....


  1. Hi Cliff,
    Here is something I have always wondered about. When they send up these weather balloons for soundings, what sort of instrumentation or package goes along and do they recover it, and if so, how?


  2. There's a recent episode of Radiolab that delves in to super-cooled water. Not sure if this is exactly what is going on here, but it sounds like it could be... water will remain liquid until it has a particle of dust, bacteria, etc. to begin the process of forming crystals. It could be that the jets are trailing a cloud of particles that allow the water for crystallize. In the podcast they explore the possibility that some bacteria evolved the ability to produce proteins large enough to form ice crystals as a way of getting themselves back down to Earth.

  3. Cool. I'm going to apply this phenomenon to something I observed a few days ago.

    We have had an inversion in E. Washington for a week now right. Well it's been a little unusual because in some sense it has been "dry." So we didn't see fog, freezing fog, drizzle, flurries etc. A shallow, but relatively high stratus base layer. Also made for a "brighter" dark cloudy inversion. Roads have been very dry and good to drive on which isn't always the case in this scenario.

    One day driving thru town I came across a wall of reduced visibility. I was like omg snow. It was legit snowfall. Coming down. Everything was white, cars slowing down. Yes it was just a dusting. Then in a few more feet, bam clear again. It was a real head scratcher. No rhyme or reason but something caused it and I thought, an airplane. Sure enough around that time an airplane left Yakima to Seattle and I was NW of the airport, the direction of the snowband. Very cool example how an airplane may have inadvertently ice seeded a cloud to make it precipitate.

  4. I was going to ask the same question as Eric regarding the affect of exhaust particles from the jet in affect seeding the super-cooled clouds.

  5. Layer up...install more insulation...stock up the wood pile...get snow tires and chains...while they are cheap. Consider moving to a lower latitude. Here we go into a long, long cold spell.

  6. I wasn't going to suggest that it's aliens...but it's aliens.

  7. You leave out the explanation of why the hole in the cloud should be a large circle. Surely an aircraft would give a long slot?


  9. Could there be an additional explanation for this phenomenon?

    Liquid water can be super cooled at room temperature and held there as a liquid as long as it is held in a vibration free environment. When energy is applied, this energy is sufficient to induce crystallization. Anyone else in high school science cool a beaker of water below 0 deg C. in a vibration free environment, only to cause it to freeze up by scratching the inside of the beaker with a glass stirring rod?

    Apparently, sound waves can also cause this to happen - check out some patent filings for this approach.

    So, could the sound waves ahead of the forward edges of the wings or just plain friction be providing sufficient energy to trigger the start of crystallization?

  10. Here's an interesting video on aircraft icing:

  11. JimGower: the aircraft are usually only passing through the cloud layer as they climb to (or drop from) cruising altitude...
    The usual climb/descent rate is 1000 feet per minute, so they're only in the cloud layer for a short time. If you look at the photos, many of the holes are oval-shaped instead of circular, reflecting the airplane's time in the particular moisture environment. So there is a "slot" effect, but it's only a passing fancy.

  12. Please say "Hi" to Art for me.


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