Circumhorizon Arc Over Sampacho, Argentina

Photographer: Ariel A. Trepin


The photos above shows a remarkable circumhorizon arc as observed over Sampacho, Argentina, just before noon on November 19, 2016.

Photo Details: Top – Camera Model: Canon EOS REBEL T5i; Lens: 17-50mm; Focal Length: 17.0mm; Aperture: ƒ/8.0; Exposure Time: 0.0013 s (1/800); ISO equiv: 100;

 Focal Length: 47.0mm; Exposure Time: 0.0006 s (1/1600).

Look for the brightly coloured circumhorizon arc (also a circumhorizontal arc but never ‘fire rainbow’) when the sun is very high in the sky – higher than 58°. Near to noon in mid summer is a good time in middle latitudes. The halo is beneath the sun and twice as far from it (two hand spans) as the 22º halo.

It is a very large halo and always parallel to the horizon. Often only fragments are visible where there happen to be cirrus clouds – the individual patches of cirrus are then lit with colour that can be mistaken for iridescence.


The rarity, or otherwise, of the arc depends on where you are. At medium latitudes like much of the USA it is not rare – it can be seen several times each summer. In contrast, further north in much of Europe the circumhorizon arc is a rarity and impossible to see north of Copenhagen. See the charts in ‘How rare?’ for the visibility at your location.

So for errors associated with the phenomenon of UFOs. Confused with the anomaly, because that looks very unusual.

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