An 18°-halo is a white ring of light with a radius of 18° around the sun. In most cases it appears together with other halos caused by pyramid-shaped ice crystals.
Simulation of pyramidal halos.
Grafik: W. Tape; Atmospheric Halos; S. 89; Am. Geophysical Union; 1994
18°-halos are caused by randomly orientated pyramid-shaped ice crystals. The path of light can be seen in the diagram above.