Posted November 3, 2008
Updated November 4, 2008
No, we're not talking pugilism here, but a description of an interesting cloud formation that occurs under just the right conditions. One of our viewers named Travis Buckingham sent in the featured photo recently taken by his uncle Gary Ayotte in Harbor Beach, Michigan, wondering what could cause the odd appearance of the clouds in the picture. It's a very nice shot, as you can see, and nicely captures a neat phenomenon that is sometimes called a "hole punch" 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 droplets evaporate in favor of rapidly growing ice crystals which become heavy enough to fall (the beard-like streaks in the middle of the hole - the presence of two distinct streaks here may indicate that more than one aircraft passed through the original cloud). 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.
In this particular photo, it's also interesting to note that the falling ice crystals give us a good visual indication of some moderate vertical wind shear aloft, in which winds are most likely blowing faster toward the right hand side of the image at the top of the streaks than at the bottom. If winds were calm throughout the layer, or the same speed and direction from the top of the streaks to the bottom, they would extend straight down rather than curving off to the left toward the bottom.
We had a day some time 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.
Our thanks to Travis and his uncle for sending in the picture!