Middle Lowitz Arc
Image ©Gary Saunders, reproduced with permission.   Middle Lowitz arc over the Atlantic Ocean. Imaged by Gary Saunders 12th September 2005 from the flight deck of an aircraft at 33,000ft. A rare combination of Lowitz, sunvex Parry, upper tangent arc and 22° halo.    Mouse over the image for an enhanced view and halo key.     The relative positions and shapes of the Parry and upper tangent arcs fix the solar elevation at ~4°. At this sun height the middle Lowitz arc curves upwards and over the sun but it remains tangential to a sunvex Parry arc.    In the HaloSim4 simulation, the rectangle shows the area covered by the image. Arcs formed by classically oriented Lowitz plates appear in red.    The simulation used 38% Lowitz oriented plates, 1% ordinarily oriented plates to produce the parhelia, 5% Parry oriented columns, 15% singly oriented columns and 41% randomly oriented columns. The figures illustrate the relatively large concentrations of Lowitz oriented plates that are necessary to produce a display and perhaps partly accounts for the paucity of sightings. Upper and lower Lowitz components are clustered around the sundogs but they do not appear in several of Gary Saunder's images of the left hand parhelion. This could be taken as evidence for semi-regular hexagonal Lowitz crystals that enhance the intensity of the middle arc as suggested by Riikonen et.al.  However, other factors such as the known transience of the Lowitz appearance in this display and variations in crystal concentrations across the sky could be responsible.