18° lateral arcs (EE67)

When rings with unusual radii, such as the 9°-, 18°- or the 24° ring are visible, attention should be paid to the lateral arcs of these rings. The 18° lateral arcs are at the level of the sun on the 18° ring. They are, so to speak, the sun dogs of the 18° ring. The lateral arcs are white and curve slightly away from the sun with ideally vertically oriented crystals. The lateral arcs have been observed very rarely so far. If the pyramidal crystals fall without a perfectly vertical main axis, the lateral arcs blur, become longer, and transition smoothly into the 18° ring with even poorer vertical orientation. At a sun height of more than 60°, the 18° lateral arcs become very faint and cannot form at a very high solar altitude.

The 18° lateral arcs are formed at pyramidal crystals with a vertical main axis. The main axis always passes through the two base surfaces. This orientation occurs mainly when the lower pyramidal section is quite long, whereas the upper one is only short.