High Sun Halo
|A ‘circular’ halo around a high sun is not necessarily the common 22° halo. It could be a circumscribed halo.
Randomly oriented hexagonal prisms, probably in clusters, make the 22° halo. It is circular at all sun altitudes. Hexagonal prisms aerodynamically aligned with their long axes horizontal – ‘singly oriented columns, - generate the circumscribed halo. When the sun is 40° high or so the halo is a saggy oval. As the sun climbs it approaches a circular shape.
Both halos are side by side in the HaloSim ray tracings at right. The sun altitude was set to 77°.
The 22° halo has diffuse colours with extensive brightness outside the main ring. The circumscribed halo has more saturated colours and its ‘white’ intensity beyond the colours falls off more quickly. The 22° halo is 'less efficient', three times as many crystals were needed to generate the simulation.
The differences are obvious when compared directly in the ideality of a simulation but they can be much less so in the sky. Search for other clues. Was there a bright upper tangent arc at lower sun? That would indicate horizontal column crystals and a circumscribed halo at high sun. Horizontal columns also produce a parhelic circle by glinting light from their near vertical hexagonal end faces. The right-hand ray tracing shows the diminutive PC when the sun is high. Its brightness depends on the crystals’ aspect ratio, long crystals have a relatively smaller area of end faces and give a dimmer PC compared to the circumscribed halo. The end faces might be imperfect, they frequently are, and the PC is then weak or absent. And a PC is not definite proof of horizontal column crystals, plate crystals also make it.
What was Alfredo’s halo? You decide!