Doughnuts On A String -- Sorry, But Plane's Mysterious Rings Are Natural Phenomenon

In response to the articles on the "mystery airplane" which appeared in The Times Sept. 22 and 24, I have some information on contrails and wake vortices to share. I dislike spoiling a good mystery, but the "doughnuts on a string" contrails that have been observed in the Southwest are a perfectly natural phenomenon. As an airplane flies, it leaves behind two counter-rotating vortices, one from each wingtip. These vortices are invisible, just as the wind is invisible. However, if the airplane is also making a contrail, which consists of water vapor from its engine exhaust that has frozen into ice crystals, the contrails are drawn into the tip vortices, in effect making the vortices visible.

The two parallel vortices are unstable, and over the course of several minutes, they break down into a series of ring vortices, which form a stable configuration. This set of ring vortices gradually decays over time, due to friction. If the vortices have entrained contrails, they will form a series of visible ring vortices, or doughnuts on a string.

About 20 years ago, NASA did some research on wingtip vortices. They used smoke instead of ice crystals to make the vortices visible. NASA was able to photograph the breakdown of wingtip vortices into the series of ring vortices, mainly because the smoke was much more persistent than the contrails are. The contrails tend to evaporate and disappear.

At the time, NASA also did some mathematical calculations to simulate the decay of two parallel vortices. The mathematics give exactly the same result. We seldom see the phenomenon of "doughnuts on a string" for two key reasons. One, the contrails usually evaporate before the rings have had enough time to develop. Two, if the winds aloft are strong, the vortices are literally blown apart before the process can be completed.

I have no such hard information on the pulsing engines and the strange lights that were also mentioned in The Times articles. However, I would not be surprised to learn that P.T. Barnum, wherever he is, has a little smile on his face. - Don West, Mercer Island