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No. 123: May-Jun 1999

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Puzzling Shadows

August 3, 1997. Dooey, West Donegal, Ireland. The observer, A. Evans, was on a north-south beach.

"It was a glorious day with some broken cloud over the land but none out to sea. Over the period between 3.00 and 6.00pm, a total of five jets passed overhead, all leaving vapour trails. Two showed a very interesting phenomenon, because while the vapour trails extended eastwards, the trails were continued westwards in front of the planes as dark lines which stretched all the way to the horizon. It was as if the plane was running on an aerial railway. I think it significant that only two of the planes showed this phenomenon as I suspect it was heightdependent. At this time the sun was high and shining from the south-southwest."

J.O. Mattsson, Lund University, surmised that the black streaks in front of aircraft were shadows of the condensation trails behind the two planes. The shadows were cast forward ahead of the planes upon the hazy, though cloudless, atmosphere above the ocean.

(Evans, Alun; "Condensation Trail Shadows," Weather, 53:371, 1998.)

Comment. Since the sun was high in the sky, it is difficult to visualize how a the vapor-trail shadow could be cast directly ahead of the aircraft. Hummm! We suppose that the vapor trails acted like those sunset clouds that produce crepuscular rays. The geometry, though, is a bit different.

Contrail phenomenon Digitized sketch of contrail phenomenon. Apparently, the contrail's shadow was projected on the misty atmosphere ahead of the aircraft.

From Science Frontiers #123, MAY-JUN 1999. � 1999-2000 William R. Corliss