WRAL WeatherCenter Blog

Circle in the sky...

Posted February 21, 2011

Circular contrail photographed from Spring Hope around 1 pm on Feb 8, 2011, courtesy of Dan Nathan.

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!


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  • Mike Moss Feb 23, 2011

    Irishiii, Thanks for the link to your "punch" post. Nice! Not sure if you've seen them before, but I've also done a couple of blogs in the past on the same phenomenon with a little additional info in case you or anyone is interested. They are located at





  • Mike Moss Feb 23, 2011

    Goobnav, Not sure what reference you're using regarding contrails being "designed out in the 60s," but that's certainly not the case. However, part of most military missions is a weather briefing to assess the likelihood of contrails in the operating area and what altitudes they can likely be avoided. Also, since weather forecasts aren't perfect, there are procedures pilots can use to verify what altitudes they will or will not "mark" by way of the trails. Thunderbirds fly most of their shows in the warmer months and the bulk of the flying is done at altitudes below those that would support contrails, not to mention that the atmosphere doesn't support contrails every day anyway, thus the need to use smoke generators. Regarding the SR-71, you might like to scan the last paragraph on page 85 of the book "SR-71 Revealed" by a former Blackbird pilot. Googling "SR-71 Revealed Graham contrails" should take you right to an online excerpt on the first link.

  • goobnav Feb 22, 2011

    Sorry, Mike,

    I definitely have to disagree with you. Knowing that such maneuvers are against FAA regulations and are only allowed by military aircraft in VFR conditions. Aircraft from the modern equipment of USAF don't produce such trails because the design was figured out in the 60's. Otherwise, a plane known as the SR-71 would have been shotdown during the flights over the USSR, Vietnam, China, Laos, Cambodia and Iraq and Afghanistan even at night. Other aircraft to reference, B-2, F117A, F/A 22, F-35 Lighting II and Global Hawk. The last is a UAV that has the same signature of a commerical aircraft and none have been recorded of being spotted due to a so called con-trail. The Thunderbirds have to use smoke dispensers to show their flight paths at air shows otherwise you wouldn't see them unless they go supersonic and even those shockwaves evaporate after 100 feet max.

  • NCSUPackfan Feb 22, 2011

    How cool. I blogged about a "hole punch" in the clouds on the GOLO side of WRAL:


  • treki70 Feb 22, 2011

    Ever heard of Cemtrails?-just check it out ok?

  • elon1206 Feb 22, 2011

    I saw this above the Crossroads area also. I did see the plane and it was commercial, looked like a larger commuter jet. It was pretty cool to see.

  • Mike Moss Feb 22, 2011

    Elon and Mulecity, Thanks for chiming in and glad you caught a glimpse!

    Jimm, Did you happen to send us pics back then? I remember a case where someone sent us some photos of several curving contrails taken from somewhere around Cary Crossroads. I think I used it in a blog post but can't find copies of the images now.

    Goobnav, I suspect this kind of circle is well within the capabilities of most commercial aircraft, although we can't say from the image whether it was one of those, or a private plane or a military aircraft. The conditions under which contrails are most likely to form and persist have been studied ever since they became a problem for military aircraft (making them highly visible to AA gunners on the ground) and although there are even now difficulties in precisely forecasting their presence or lack thereof, it's well established that regular engine exhaust products are sufficient for their formation. Nice FAQ at http://asd-www.larc.nasa.gov/GLOBE/faq.html.

  • Jimm57 Feb 22, 2011

    About 8 yrs ago I saw three rings one afternoon. I have pictures someplace. I just thought it was 3 pilots from a nearby air station flying a maneuver.

  • goobnav Feb 22, 2011

    This is not commercial jet. They cannot maneuver that well, Mike you know that being a former Air Force officer. Also, since when do clouds form in less dense air with little to no humidity.

    Why don't we see trails on the ground or at lower altitudes where the air is denser and has more humidity? This was chemically created any self respecting physicist would back this up.

  • mulecity007 Feb 21, 2011

    I actually saw this while it was happening! Pretty neat to watch! Kind of figured the pilot was lost or most likely trying to kill some time before their next stop.......