- Crystal orientation:
- rotate plates and oscillate columns
- Path of light:
- 3-7
- Occurrence:
- very rarely
EE 14c/15c
The upper lowitz arc touchs the konkave parry arc.
Simulation by Dr. Eberhard Tränkle (1937-1997) from MM 1/1996
Description and formation:
The Lowitz arcs in the sketch drawn by Tobias Lowitz of the St. Petersburg display are short arcs extending from the 22°-parhelia downward to the 22°-halo. The standard theory explains Lowitz arcs by rotating hexagonal plate-shaped ice crystals. (Tricker, Mueller, Greenler). For this case Greenler´s book shows three light paths generating the lower, the upper and the circle-shaped Lowitz arc. Greenler´s simulations show a sophisticated pattern varying considerably with changing sun elevations.
By the end of 1995 the observers of the "Sektion Halobeobachtung", several times observed an unusual arc, the upper part of which touches the Parry arc and the lower ends of which touch the 22°-halo. This arc was an upper circle-shaped Lowitz arc.
A survey made by the Finnish Halo Network shows 20 observations of this arc up to now. The first who observed it was J.M. Heighes, who registered this halo on May 11, 1965 at Earley/Great Britain. Even a photograph of this halo is said to exist, but nobody knows anything concerning the whereabouts of it. Only from August 11, 1985, we have another observation made by J. Fröhlich from Knau (Thuringia). It is conspicuous at all that about 30% of the reports concerning this kind of halo originate from Germany. So here it is worth while to continue observing the sky systematically.
Simulation by Riikonen and Ruoskanen, taken from MM3/96.
The simulation above which has been made by Riikonen and Ruoskanen (Finland) uses inclined column-shaped crystals, orientated plate-shaped and orientated column-shaped ice crystals. As a comparison, the sketches in the centre column also show the orientation necessary to generate a Parry arc. The programme used is HALOET by E. Tränkle.
At a sun elevation of 10° two Lowitz-sub-types appear above the 22°-halo. The upper one is the circle-shaped Lowitz arc already known theoretically. The other arc, also a kind of Lowitz arc, touches the V-shaped upper Parry arc and disappears together with this arc at a sun elevation of 15°. By following the series of simulations one can see how this arc flattens and finally merges with the 22°-halo.
The appearance of the upper circle-shaped Lowitz arc is also subject to the inclination of the crystals. The greater this inclination is, the earlier the halo disappears. At high sun elevations it may even become impossible to distinguish the halos caused by rotating plate-shaped crystals from those caused by inclined column-shaped crystals.
Picture: Oscillating column-shaped crystal (on the left) and plate-shaped crystal rotating completely around its axis (on the right).
Taken from MM3/96.
Oscillating column-shaped crystals seem to be the key to the second sub-type, provisionally called "unusual Parry arc". The main property of the family of Lowitz arcs is that they are generated by rotating or oscillating plate-shaped crystals. The exact appearance of the arcs is subject to the amplitude of the oscillations. If the column-shaped crystals oscillate less than 10°, the arc may more remind of a deformed Parry arc. Because the oscillation and the main direction of the curvature these halos are members of the Parry arc category.
So there are two different sub-types of Lowitz arcs above the 22°-halo. The upper one is a continuation of the same arc that extends from the parhelia (upper circle-shaped Lowitz arc). So this arc is not a new kind of halo, but the upper part of the Lowitz arc that had not been observed before. According to the theory the upper Lowitz arc should touch the upper concave Parry arc, a fact confirmed by the observations.
It is possible that there are such a few observations and photographs of the upper circle-shaped Lowitz arc just because it has been taken for a 22°-halo and/or 22°-parhelia. This might also fit for the photograph on page 51 in Tape‘s book. Even some of the photographs in our archives could later be identified as an upper circle-shaped (or even complete) Lowitz arc.
In the case of the two sub-types forming the upper V-shaped and the lower convex Parry arc, the situation is a bit different. These two Lowitz arcs got no name, neither by Mueller (1979) nor by Greenler (1980), because they did not know any appropriate observations of these arcs. In "Rainbows, Halos and Glories" Greenler just speaks about a "third component", but the light ray in question just shows the upper and lower nameless sub-types of Lowitz arcs.