Supralateral Arc (EE21)

The supralateral arc changes its shape with the solar altitude. It always lies above the horizontal circle. If the sun is lower than 15°, then the supralateral arc touches the 46° ring at the sides. When the sun is on the horizon, the contact points with the 46° ring are at solar height. At a solar altitude of 15°-27°, the supralateral arc almost coincides with the 46° ring, making it very difficult to distinguish. In fact, many observations of the 46° ring seem to actually be sightings of the supralateral arc. It is only from a solar altitude between 27° and 32° that the supralateral arc is significantly higher than the 46° ring. Moreover, the circumzenithal arc also provides a clue for differentiation. It only occurs at solar altitudes below 32° and always touches the supralateral arc. It touches the 46° ring only at a solar altitude of about 22°. Between 15° and 27° it lies very close to the ring. Another distinguishing feature can be the stronger coloration of the supralateral arc. The 46° ring is usually only white or has a weak reddish inner edge. The supralateral arc, on the other hand, is one of the most colorful halos, like the circumzenithal arc. Red lies on the side facing the sun. Blue and green are also clearly visible. Its coloration and brightness can match that of the rainbow. In the halo key, EE21A denotes the left and EE21B the right supralateral arc.

The supralateral arc forms in floating column crystals. Light enters through a prism face and exits through a vertical base face. Since the tangent arcs of the 22° halo form in the same floating column crystals, the presence of a strongly pronounced upper tangent arc can indicate that the arc above is the supralateral arc and not the 46° ring.