Infralateral arc

EE 22

Crystal orientation:
main axes horizontal
Path of light:
once in a year
Picture infralateral arc
The picture shows a infralateralarc and the parhelic circle.
Photo: Dipl.-Ing. Inge Fuhrberg, Saalbach/Hinterglemm, 01/31/1996


Infralateral arcs are also called lower lateral tangent arcs to the 46°-halo. Their appearance is subject to the sun elevation. With increasing sun elevation their contact points at the 46°-halo move downward along the 46°-halo. Later the infralateral arcs form a common arc. At a sun elevation of 68° this arc touches the 46°-halo vertically below the sun. When the sun gets still higher, the vertex of the arc lies some degrees below the 46°-halo. An infralateral arc can be as bright and colourful as a circumzenithal arc.

The diagram shows a computer simulation of the supralateral arc and the infralateral arc for different sun elevations. For a better orientation the 46°-halo, the infralateral arc and the parhelic circle are also drawn in. The infralateral arc is the arc which touches the 46°-halo at its lower part. It can form at all sun elevations. The supralateral arc is also shown. It touches the 46°-halo at its sides and at its upper part. At sun elevations of 32° or more the supralateral arc cannot form. Thus the diagram shows only the infralateral arc for sun elevations of 40° or more. Diagram taken from: R, Greenler; Rainbows, Halos and Glories; Cambridge University Press, 1980.


Infralateral arcs are caused by column-shaped ice crystals which are floating in the air. The light enters a vertical base face of the crystal and leaves a prism face.