I’ve talked about distrails and hole punch clouds before, but I’ve actually never seen one myself until this morning, when I saw two side by side. It’s quite rare to have this kind of weather condition in Los Angeles. I took a couple of photos at around 7:30AM today, Aug 19th, 2007. Santa Monica ATIS reports the sky is clear now, but Van Nuys reports broken cloud at 18,000 feet and 28,000,which is probably where these are, given they look like they are precipitating ice in artificial cirrus uncinus. The cloud layer that the holes formed in is cirrocumulus.
[UPDATED Dec 2012]
There’s an excellent article on them at the Cloud Appreciation Society:
We already knew that, for the hole to form, the droplets in the cloud layer need to be ‘supercooled’. This means that they are at a temperature below 0˚ Celsius, but have yet to frozen. In fact, cloud droplets can and do often remain liquid at temperatures well below the freezing temperature of a glass of water down here on the ground. When in the form of tiny droplets, water needs a nucleus of some sort to be encouraged to freeze – a tiny particle of some sort that it can get started on. Without one of these icing nuceli present, droplets will resolutely refuse to freeze until the air temperature drops to as low as –40˚ Celsius.
But what gets the freezing started in one region of the cloud? The research has confirmed the long-held suspicion that an aircraft flying through the cloud can be enough to set off the freezing process. The expansion of the air as it passes over the aircraft wings and (for certain types of plane) over the blades of its propellers, causes it to cool. This momentary cooling can be enough to take the temperature down low enough for the droplets to start to freeze spontaneously, even though there aren’t enough icing nuclei around for the droplets elsewhere to freeze.
The research also proposed a way by which the freezing water might cause a hole to expand outwards from the point where the plane flew through the cloud. When the droplets change into solid particles, they give off a tiny bit of heat – enough to cause the air to expand and rise a little. In reaction to this rising current, the surrounding air sinks slightly. As the air around the frozen fallstreak sinks and warms up, the droplets there evaporate. This seems to be the method by which the fallstreak of ice crystals produced by the plane climbing or descending through the cloud layer forms a circular hole that appears punched out of the cloud layer.
The larger ice crystals then drift downwards (as virga), since they are heavier, resulting in the characteristic “V” or “wing” cross-section of the central cirrus uncinus cloud, which looks a bit like a bird or an angel, if you are imaginative enough.
I like this second one, since it clearly shows the central contrail from the plane that made the hole (look by the palm tree). It’s not quite as nice a regular hole as some you see though. That’s probably to do with the location, being so close to the airport the climb/descent rate won’t be so steep at altitude.
(note, the above images have been contrast enhanced and sharpened, the original of the top one can be found here, for reference).
[UPDATE 2010/06] Here’s a paper that goes into great detail on the production of these holes:
[Update 2011/01] Here’s a video that actually show the clouds forming.
[Update 2011/06] New paper/article on hole punch formation, suggesting that it cna lead to increased precipitation near airports:
“It appears to be a rather widespread effect for aircraft to inadvertently cause some measureable amount of rain or snow as they fly through certain clouds,” Heymsfield says. “This is not necessarily enough precipitation to affect global climate, but it is noticeable around major airports in the midlatitudes.”
The researchers did not estimate the total amount of rain or snow that would result from such inadvertent cloud seeding. However, they analyzed radar readings that, in one case, indicated a snowfall rate of close to an inch an hour after several planes had passed through.