Atmospheric
Optics
A rarely seen Parry arc
Heli Kesti saw this ice halo display between Oulu and Helsinki, Finland.
"Fortunately, by chance we got a place just in front of the wing and a man gave me a place by the window."
Fortunate indeed. The ~30° high sun shone through cirrus crystals to give a bright lower tangent arc (or maybe a circumscribed halo for the latter forms at 29°).
But the gem is the fainter arc inside the lower tangent - A rarely visible lower sunvex Parry arc. Parry would have liked the chance to see this!
All images ©Heli Kesti
A HaloSim ray tracing for a 30° high sun. The approximate sun elevation is obtained from the shape of the lower tangent arc, the separation of the Parry arc and the distance of the sundogs from the 22° halo.
Parry columns have their long axes horizontal AND a prism face horizontal. A rare orientation offset by the high efficiency with which they generate halos.
Four ray paths give arcs near the 22° halo. Whether they are possible or not depends on the sun elevation. They are shown versus sun elevation here and here.
Run HaloSim with its ray filters active to identify the ray that generated Heli's arc.
The Parry arc came and went. At right a colour subtraction enhancement of the centre image.
