R. Sussmann is with the Fraunhofer-Institut für Atmosphärische Umweltforschung (IFU), Kreuzeckbahnstrasse 19, D-82467 Garmische-Partenkirchen, Germany.
Photographs of a 120° parhelion and a 22° parhelion within
persistent contrails are presented. These phenomena result from hexagonal
plate-shaped ice crystals oriented horizontally with diameters between 300 µm and 2 mm. From our observations and reinvestigation of previous
reports, we conclude that a subset of the population in persistent contrails
can consist of highly regular, oriented, hexagonal plates or columns
comparable to the most regular crystals in natural cirrus clouds. This is
explained by measured ambient humidities below the formation conditions of
natural cirrus. The resulting strong azimuthal variability of the scattering
phase function impacts the radiative transfer through persistent
contrails.
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Singly oriented columns and oriented plates and further crystals with no distinct typeb
This paper
Jan. 1996
Photograph with angle measurement
120° parhelion
Oriented plates
This paper
June 1996
Photograph
22° parhelion
Oriented plates
The second statement could be taken as an indication of the
occurrence of the 22° parhelia rather than parts of the 22°
halo.
Nearly all randomly oriented crystals show the 22° halo;
however, circular halos do not necessarily require random
orientation.16
The manual drawing points to a parhelic circle, which forms in
the north opposite the Sun. Contrary to the verbal note, we exclude
circumzenithal arc and circumhorizontal arc, which would be centered around
the Sun-azimuth angle (we calculate aziSun = 215° from the observation time of
13:00 UTC on 3 April 1942, Augsburg, Germany) and extend at most a third
around the horizon; furthermore, the latter are restricted to elevation angles
above 58° and below 32°, respectively. This does not fit the Sun
elevation of 47.5° that we calculate from the observation
time.
The 22° parhelia (mock Suns) usually contain all colors,
whereas the 22° halo is just red inside the
ring.
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