Perfect Circumscribed Halo

Perfect Circumscribed Halo - Imaged at Shore Acres, Oregon, USA by Phillip Bonn. This outstanding display had other ice halos - some rare. More about those in the next OPOD. ©Phillip Bonn, shown with permission.

Two halos surround the cirrus hazed sun, the inner one truly circular and the outer an oval that has slightly sagged. Through the sun passes the parhelic circle.

The outer halo is a 'circumscribed halo'. It always touches the inner one - a 22 degree halo - directly above and below the sun.

Its shape changes with sun height. When the sun is below 29� it splits into two parts, the upper and lower tangent arcs.

When the sun is high it draws inwards to the 22 degree halo and becomes increasingly circular. The circumscribed halo is often mistaken for the 22 degree under these conditions and especially when produced by a tropical sun. They can be hard to distinguish but a guide is that the circumscribed halo is sharper and has more saturated colours.

The circumscribed halo and 22 degree halos are examples where the ray path through the ice crystals is the same and the different halo shapes arise purely from differences in crystal alignment.

More of this display's halos in the next OPOD.