Circumzenithal Arc Over Erlensee, Germany

July 20, 2005


Provided by: Thomas Herrmann
Summary authors & editors: Thomas Herrmann

Sitting in a garden in Erlensee, Germany on May 27, I realized that cirrus clouds were visible all across the sky. Knowing that these clouds can provide an opportunity to observe atmospheric phenomena, I kept an eye on the sky and was rewarded by seeing the amazing circumzenithal arc, shown above. With just a little imagination, you can see seahorses floating inside the arc. Circumzenithal arcs are perhaps the most colourful of all halo-related phenomena. In fact, many people who witness them believe they're rainbows because of their beauty and arc-like shape. However, contrary to a rainbow, which forms a circle around the antisolar point, a circumzenithal arc appears, as its name implies, at the zenith. Only the portion pointing towards the Sun is visible. Normally a quarter of a circle appears, but in special cases you may be able to discern half of it. Its vertex is at about 48º above the Sun. At a solar elevation of 32 º, the circumzenithal arc disappears in the zenith -- when the Sun is higher than 32º, it cannot arise. However, at a solar elevation of 22.1º it attains its greatest brightness. Thus, it's observed in most cases at solar elevations, usually between about 15º and 25º.

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