Circumhorizon Arc

The circumhorizontal arc is not seen often from this latitude (51 degrees north). The sun altitude needs to be at least 58 degrees and this only occurs around midday for a few weeks each year. The arc extends for many degrees parallel to the horizon and it can be as colourful as the circumzenithal arc.

Here it is only visible as a faint coloured band in the lowest pieces of cirrus.

This closeup clearly shows the colouration, although it is pale compared to what the arc can be like under good conditions.

In this image part of a bright 22 degree halo is seen curving up near the top. At lower centre a trace of colour in the cloud betrays the presence of the arc.

Although this is a very weak example it is still worth watching out for. Faint arcs are easily lost in the haze so clear air is essential.

This image shows the position of the circumhorizontal arc in relation to the Sun and the 22 degree halo.

About twice as far below the sun as the 22 degree halo is a slightly coloured wisp of cirrus.

A full arc would run parallel to the horizon for many tens of degrees either side of this.

Canon EOS 20D   19th June 2005
Wiltshire, England

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