Circumzenithal and circumhorizontal arcs

The type of crystal responsible for parhelia can also form circumzenithal and circumhorizontal arcs. Circumzenithal arcs are much more frequent than circumhorizontal ones, though neither of them is as frequent as parhelia.

Figure 7.10 Circumzenithal arc. A circumzenithal arc is often mistaken for a rainbow, but, as you can see in this photograph, it differs from a bow in three important ways. In the first place a circum-zenithal arc is on the same side of the sky as the Sun. Secondly it curves away from the ground. Finally, the sequence of colours is opposite to that in a bow with the red edge being closer to the ground in the circumzenithal arc. (Photo Pekka Parviainen)

Arc Circumhorizontal
Figure 7.11 Circumhorizontal arc. This is only seen when the Sun is more than 58° above the horizon. (Photo John Naylor)

Circumzenithal arcs can form only when the Sun is less than 32° above the horizon. The cloud in which it is seen must be slightly more than 46° above the Sun. Thus they are seen only when the Sun is low in the sky, and there are suitable cirrus clouds near the zenith.

The relatively rapid movement of these clouds as they pass overhead means that these arcs are usually short-lived. Frequently the cloud in which a parhelion is seen will drift overhead to give a circumzenithal arc, thus giving you an opportunity to anticipate it, and prepare to photograph it.

Circumhorizontal arcs are due to light that has been refracted between a vertical face and the lower horizontal surface of an ice crystal. This is possible only for solar elevations of more than 58°. Therefore, at high latitudes circumhorizontal arcs are only seen around noon in midsummer.

Both circumhorizontal and circumzenithal arcs are the most highly coloured of all the phenomena due to refraction through ice crystals. Both rival the rainbow for colour and, indeed, are frequently mistaken for rainbows. The circumzenithal arc is slightly curved, with the edge closest to the Sun being red. The circumhorizontal arc is very slightly curved, though this may not be evident in a short segment, and forms a band of colour more or less parallel to the horizon, at least 32° above it.

Figure 7.12 Parhelic circle. Here we see a short segment of a parhelic circle and a strongly coloured parhelion. The Sun is out of the frame of the photograph on the left hand side. (Photo John Naylor)

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