High Sun Lower Suncave Parry Arc
A very rare sighting by Nick Beck at Jacksonville, Florida May 25, '11.    ©Nick Beck, shown with permission.

There were cirrus clouds thickening around noon, and a 22 degree halo (or circumscribed halo) and weak circumhorizon arc showed up. At 1:45, the sun was 79° high and I noticed not only the 22° halo (or a circumscribed halo, to be more accurate because when it became more complete, it was sharply defined and slightly elliptical), but another arc outside of it!   When the display became more complete, I noticed the arc also extending from the bottom also. Immediately I knew it was a lower suncave arc, which I knew were very rare. Later, the Parry arcs went away, but an infralateral arc and parhelic circle showed up. The parhelic circle was very small too.

The Parry arc is arrowed in the two images. The inner coloured arc is a circumscribed halo from singly oriented column crystals. The display is simulated in the HaloSim ray tracing at right. Rather long column crystals c/a=10 were used to weaken the parhelic circle which cannot be discerned in the images at that time.

The lower suncave Parry arc is 'lower' because its ray path is [4,6]- see crystal face numbers. The upper Parry ray enters the top face of the crystal [3,5]. The weak arc in the simulation between the circumscribed and infralateral arc is another Parry arc, this time with an internal reflection within the crystal [4,5,6]. Well oriented crystals would be required to see it in nature.


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Change in appearance of Parry (and other)
arcs with solar altitude.

LP - Lower Parry [4,6]
UP - Upper Parry [3,5]

CA - Circumscribed arc
TA - Tangent arc

PC - Parhelic circle

That the lower Parry has a greater radius than the circumscribed halo at 90° sun arises because the Parry rays cannot enter the crystal at the minimum deviation angle.