Eric Zrubek and Michael Carmody

How did you first observe the hole-punch-cloud phenomenon?

Three years ago during a field project in Colorado, we saw this very narrow band of precipitation that we had never seen before. It was very unusual for winter. It looked like it could be lined up with the flight path of two turboprop planes that had just flown by above us, and, sure enough, it matched.

And you happened to be in a plane with instruments that could study the precipitation?

This was the first time a fully instrumented aircraft had been able to fly through snow falling from other aircraft, and this allowed us to characterize hole-punch clouds in a study we published a year ago.

The thing that intrigued me was that these holes seemed to be long-lived. It's amazing that something that starts out as small as a pencil can spread throughout a whole cloud layer.

Basically, you say, aircraft create hole-punch clouds when they fly through a supercooled cloud layerwhere water is below its normal freezing temperature but remains a liquidand induce the formation of ice. How do they do that?

Propeller planes create them at the propeller tips, and jets [create them] over their wings. Propellers move the plane forward by pushing air behind. As the propeller pushes the air outward, the air expands, which causes a lower pressure and cooling. With jets, there is a lower pressure above the wing than below. In both cases, air is cooled, which creates condensation, and you get a stream of ice crystals behind the plane. Under the right conditions, the expansion process kicks in to create hole-punch clouds.

How do these holes expand through the cloud layer?

This phase change (from supercooled water to ice) produces some latent heat that causes some buoyancy. That bubble of air rises like a hot-air balloon. As the air rises, it cools and causes more condensation, more ice, and so on. This pattern of circulation spreads with time as the ice particles move farther and farther out. [The fact] that the holes are so symmetrical shows this pattern of circulation occurs.

What conditions are necessary for hole-punch clouds to form?

We think for propeller aircraft, surrounding temperatures need to be between -10 degrees Celsius and -40. For jet aircraft, -15 C to -40 C. Hole-punch clouds can be differentiated from jet contrails, which basically occur only below -40 C. Drops of liquid water spontaneously freeze at this temp. Commercial jet aircraft typically fly at altitudes where temperatures are below -40 Ctoo cold for hole-punch clouds.

What's new in this study?

This study shows that many types of planespropellers, jets, commercial and military aircraftcan all produce hole-punch clouds. We also further characterized how these things spread and use data from satellites to map out how often low-level clouds conducive to this effect occur around major airports.

Tell us more about the airport factor. How many airplane takeoffs produce hole-punch clouds?

Around major airports, this phenomenon happens 3 to 5 percent of time on average per year, and about 15 percent of the time in the winter. This generates a little bit of additional snow that could affect operations like de-icing. In our paper we showed that planes departing Denver International Airport created bands of snow whose rate was 3 inches more per hour than its surroundings. It might be enough to cause a need for more de-icing than might be expected due to added snowfall at the airport.

How long do hole-punch clouds persist?

They can last for 4 hours or more. They spread for about the first hour and then slowly dissipate.

Why hasn't this phenomenon been well-understood before now?

I'm not exactly sure, but certainly the recent advances in satellite imagery have helped make it possible. We were also lucky to fly through precipitation caused by the hole-punch phenomenon.

How is this different from cloud seeding?

It is exactly [a type of] cloud seeding. With that, you try to deliver ice nuclei, using a chemical like silver iodide, for vapor to condense onto and create snow that falls out or melts into rain. We found that planes inadvertently do this without releasing special chemicals.

What percentage of hole-punch clouds are visible?

I don't know, but I hope to come up with a good estimation in the future.

Why should people care about hole-punch clouds?

You see a phenomenon that absolutely captivates people's attention. There could be misinterpretations.

Have you been confronted by people who think the clouds are actually UFOs?

I have. I generally explain the science. Now that this study is coming out, I'm probably going to get a flood. But I'm not worried, because the science is solid.

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