OCCURRENCE OF TWISTERS ON MOUNTAIN RIDGES

1. Observations

Twisters of great intensity were observed during an ascent of the Watzmann, a mountain in the northern limestone belt of the Alps, south of Berchtesgaden, Germany, on August 1st of this past summer.

The ascent of the Watzmann was made from the North by way of the Watzmann House (5,500 ft. altitude), from which point, already above the tree line, a sharp ridge rises to the Hocheck (8,000 ft.). Beyond this peak the ridge remains essentially horizontal for some 1,000 yards, then descends again to 6,000 ft. Consequently, this ridge, essentially straight, extending in a north-south direction, flanked toward the west by a great and rather smooth-sloping shelf of bare rock and toward the east by a precipice of many hundred feet, resembles a shed roof of rather symmetrical lateral profile.

On the day of the ascent, a fairly steady wind was blowing from the west, i.e., normal to the extent of the ridge and up the western slope. The wind velocity at the ridge was perhaps 20 to 30 miles, varying but slightly with the onsurge and departure of clouds.

As the upper part of the ridge was approached, occasionally, at intervals of about 10 minutes, a great noise was heard, as from a falling avalanche. Soon it was found that this noise arose from local twisters which occurred right on the ridge and toward the lee of it. The intensity was so great that rocks as large as chicken eggs were lifted up and strewn about. Local wind velocities quite in contrast to the steady wind were so high that it was not considered safe to proceed along the crest of the ridge close to the precipitous eastern slope. Equally impressive as the intensity and the turmoil of this phenomenon was the suddenness of its occurrence and disappearance.

Interpretation of the Phenomenon

It appears quite plausible that the sudden occurrence of twisters, as observed along the Watzmann ridge just north of the Hocheck, is related to the formation of vortices arising at the point of separation of the air stream approaching the mountain ridge essentially at right angles. If these vortices are of the nature of a Theodorsen horseshoe vortex, a fact which eluded observation but which, by the shape of the mountain ridge, is well within possibility, then the great increase of local wind intensities within the twister may well be explained by the stretching of the horseshoe vortex and the consequent increase of vortex velocities.

The occasional occurrence may very well be related to a veering of the direction of the wind.

John R. Weske

Editor’s Note: Dr. John Weske is at present holding the position of Visiting Research Professor at the Institute for Fluid Dynamics and Applied Mathematics, University of Maryland. He has carried on research in experimental turbulence for the National Advisory Committee for Aeronautics. His discussion of twisters on mountain ridges appears to agree with and constitute an integral part of Dr. Theodorsen’s theory, explained in the latter’s Mechanism of Turbulence, Proceedings of the Midwestern Conference on Fluid Mechanics, Ohio State University, 1952.

Dr. Phrixos J. Theodorides, Research Professor at the Institute for Fluid Dynamics and Applied Mathematics, University of

Maryland, and A. A. C. member since 1951, in transmitting the above note writes:

Dr. Weske is a friend and colleague of ours at the Institute. His remarkable observations are corroborated by a similar pattern of wind flow past mountain ridges which I had noticed myself in Greece under extraordinary conditions.

It was in the 1920’s. We were aloft in a direction SE. to E. of the Mount Olympus Range—highest elevation 2918 meters (Pantheon-Mytika), flying in a heavy windstorm that was blowing mainly from N. or NW. Our airplane, a Handley Page airliner of Imperial Airways (predecessor of B.O.A.C.), was struggling to keep on schedule for the first regular airmail from India to London, though adverse weather that day had actually downed even the military planes. Aboard the plane were the pilot, an experienced flyer who had previously flown the first airliner from London to Paris, the engineer, and three invited official guests, Sir Oliver Harvey, Counselor of the British Legation in Greece, with Lady Harvey, and myself, at the time Consultant of Aeronautics to the Government of Greece.

Unfortunately, my notes and sketches on that remarkable flight have joined a host of other losses during the ordeals of World War II. Yet, as far as my memory goes, I can state that the extraordinarily strong down currents which we met in the wake of Mount Olympus reflected clearly a pattern of flow similar to the outer region of a big horseshoe vortex, as set down in Dr. Th. Theodorsen’s new theory of turbulence.