Circle in the sky...

Back on Wednesday, Feb 9, 2011, around 1 pm, Dan Nathan looked up and saw what he described as a "fast-moving weird cloud" in the skies over Spring Hope. He took a picture (above), and asked us what could have caused such a cloud to form.

As some of you may surmise, the "cloud" in this case is most likely man-made, in the form of a contrail left in the wake of an aircraft. What makes it so visually interesting is the path the aircraft followed (a more or less horizontal loop), together with an atmosphere that happened to be favorable both for contrail formation and just as importantly persistence, the combination leaving a nice ring of ice crystals in the sky.

I don't have a good way to determine what specific aircraft may have been there at the time, but a number of contrails are evident in the photo, most of which appear to simply follow a typical more or less straight path, probably along one of the commercial flight corridors crisscrossing the region. One aircraft, though, be it commercial, private or military, banked its way through a circle, and perhaps a bit more than a circle, along the way. Its difficult to be certain of the path coming into and out of the loop, but I get the impression that the plane may have come in from the upper right hand portion of the picture (and possibly a higher altitude), made a circle to the left, and then continued on more or less parallel to the other contrails in the shot, thus making a turn of something like 380 degrees.

When conditions outside the aircraft are less favorable, we may see no contrail at all, or a short one that vanishes rapidly in the wake of the aircraft, or one that lasts a good while but is quickly distorted and/or spread out into a less recognizable form. In this case, though, there had to have been air that was cold enough for contrails (typically -40 degrees or lower, allowing them to form at all), had fairly high relative humidity at the altitude being flown by the circling plane (preventing them from evaporating quickly), was fairly stable at that altitude (to limit vertical mixing due to turbulence) and was lacking in vertical and horizontal wind shear (that would have distorted the shape of the circle and/or smeared the contrail out a good bit).

For an idea as to how things stacked up the day of the photo, I took a peek at archived upper air data available from the Storm Prediction Center web site. The 300 millibar map shown as the second image indicates temperatures near flight level 300 (30,000 feet) over Spring Hope would have been around -44 degrees Celsius, plenty cold, and while there was some horizontal wind shear indicated (west winds around 125 knits at Greensboro and 105 knots at Morehead City, over that distance this is fairly light shear. Likewise with vertical shear through that layer indicated on a radiosonde profile (3rd image), which also shows that relative humidity was reasonably high at that altitude, with an especially moist layer at about 350 mb. Finally, the air was rather stable through that layer, as is often the case.

While all of that addresses the most probable case that this was a fairly high-altitude contrail, Dan did note that his impression, based on referencing the cloud against foreground objects and using his past experience flying and as a parachutist, was that the cloud was quite low, perhaps around 5,000 feet above the ground or less. It seems possible this perception was fueled by the high speed of the winds aloft on that day, the fast progression of the cloud making it appear much lower than it actually was, but if his altitude estimate is on target then we have more of a mystery with this photo, since temperatures at that altitude would have been far too warm for contrail formation.

Thanks for the picture, Dan, and keep your eyes on the sky!