When there are halos with unusual radii as the 9°-, the 18°- or the 24°-halo visible, you should also watch out for the lateral arcs to these rings. The 18°-lateral arcs are at the same elevation like the sun on the 18°-halo. One can say they are something like the 18°-sundogs or parhelia. Lateral arcs are white and when the crystals are orientated vertically in an ideal way, they slightly curve away from the sun. Up to now, lateral arcs have been observed at very few occasions. When the pyramid-shaped crystals do not fall with their main axes orientated - exactly vertical, the lateral arcs become diffuse, longer,and when their vertical orientation becomes even worse, they fluidlv fade into the 18°-halo. (Diagram valid for a sun elevation of 30°.)
Simulation of halos which are caused by pyramidal ice crystals. The 18°-lateral arcs are showed violet. (Sunelevation 30°)
18°-lateral arcs are caused by pyramid-shaped crystals with their axes orientated vertically. The main axis always passes the two base faces (Numbers 1 and 2 in the sketch above) This orientation mainly occurs when the lower pyramid—shaped extension is rather long and the upper one rather short.