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Graphic: Symbol MemoPad.


Circumhorizontal Arcs

The circumhorizontal arc is the counterpart to the colourful circumzenithal arc. Both halos are formed by plate crystals. Circumhorizontal arcs are very rare in Middle Europe because the Sun must be higher than 58°. This leaves only a few weeks a year to observe it around noon.

Like circumzenithal arcs, circumhorizontal arcs change their appearance with solar elevation. As the sun rises, the arc gets thinner and thinner. Near the equator, when the sun is almost overhead, the arc vanishes.

Right hand side: circumhorizontal arc on June 23rd, 2001.

Below: circumhorizontal arc at a solar elevation of 62°. Simulation with HaloSim 3.6 by © Les Cowley and Mike Schroeder, shown with permission.

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