A 9°-halo is a white ring of light with a radius of 9° around the sun. It often appears together with other halos caused by pyramid-shaped ice crystals. But occasions are few in which there are enough pyramid—shaped crystals to generate a halo. As the ring is situated rather near to the sun it is difficult to recognize it. Furthermore it can easily be taken for a corona.
Simulation of pyramidal halos.
Grafik: W. Tape; Atmospheric Halos; S. 89; Am. Geophysical Union; 1994
A 9°-halo is caused by randomly orientated pyramid-shaped ice crystals. It is generated by light entering a base face and leaving an adjacent pyramidal face.