OPOD - 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!