Taking a punch
Posted December 16, 2014
Or at least, taking a photo or two of a cloud that's been punched, so to speak. One of our viewers named Jim Davidson sent in the featured photos, taken a couple of miles northwest of the Campbell University campus, wondering what could cause the odd appearance of the clouds there. As you can see, they nicely capture a neat phenomenon that is sometimes called a "hole punch" or "fall streak hole" cloud, in which an otherwise fairly uniform cloud sheet is interrupted by a circular or elongated clear area, sometimes with wispy cloud streaks in the middle.
This pattern was most likely initiated by passage of an ascending or descending aircraft through a shallow super-cooled layer of high altocumulus clouds. The disturbance caused by the aircraft wake sets off a "glaciation" process in which the supercooled cloud droplets evaporate in favor of rapidly growing ice crystals, which become heavy enough to fall (the beard-like streak in the middle of the hole - the presence of an elongated streak here may indicate that an aircraft descended at a slow rate through the cloud layer). Removal of moisture from the layer in the form of the falling crystals leaves the nice round or elongated "hole" in the cloud deck that previously existed. While this seems to occur most often due to aircraft "punching" through the clouds, it is also possible for it to occur naturally due to randomly concentrated areas of enhanced vertical motion, enhanced concentrations of ice nuclei, or some very small ice crystals from even higher clouds falling into the layer of supercooled water clouds and setting off the transfer of moisture from droplets to crystals. Sometimes, instead of punching through the cloud, a plane will pass through or above it at a constant altitude, so instead of a hole, we see a line of dissipated cloud, sometimes called a "distrail," in contrast to the condensation trails that are sometimes left behind high altitude planes.
In this particular photo, it's also interesting to note that the falling ice crystals give us a good visual indication that vertical wind shear is rather weak, as the falling streaks of ice crystals do not have a strong curvature that is often seen when the crystals fall from a level with stronger winds to a level with weaker ones, or into a layer with winds of a different direction. Also, in this case the clouds at the edge of the hole are not quite as sharp-edged as in some of the similar photos you can see around the internet by doing a search for "hole punch cloud" or "fall streak cloud."
We had a day a number of years back when several of these were observed around our area, and I posted a blog on the subject with some satellite images that showed what some of these holes and lines can look like from high above. The blog was titled "Holey Distrails!" and has a little more discussion regarding the processes involved with these clouds. If you're interested in reading it you can quickly find it by typing "holey" into the search box at the top of the page. Do note that there are a few references in that post to an animated satellite loop that is no longer available, but you can still see a series of images that were a part of the lapse, and for good measure I included a couple of those pictures here as well.
Our thanks to Jim for sending in his photos! Also note - to see all four images posted with this blog, just click on either of the ones you see above to open the gallery.