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		Summertime Halo: A colourful circumhorizon arc spans the sky near Fredericton, New Brunswick, Canada. The halo lights brightest where the cirrus is thickest. Note the enormous size of the halo and its pure spectral colours. Imaged by Marc Sorensen in the summer of 2003. Image ©Marc Sorensen, shown with permission. Look for a circumhorizon arc near to noon near to the summer solstice when the sun is very high in the sky (higher than 58°). It lies well below the sun - twice as far from it (two hand spans) as the 22º halo. The arc is a very large halo and is close to, and parallel to the horizon. Usually only fragments are visible where there happen to be cirrus clouds.    Circumhorizon arc ray path   The arc (sometimes called a circumhorizontal arc) is usually produced by plate oriented crystals and is a close relation to the circumzenithal arc. Light rays enter the almost vertical crystal side faces and leave via the lower horizontal face (ray path 3-1). The refraction of the almost parallel sun's rays through faces inclined at 90° produces pure, bright and well separated prismatic colours ~ purer than those of the rainbow. The colours are at their best when the crystal tilts are smallest. Large crystal tilts produce more pastel hues. An infralateral arc can have a similar shape and appearance when the sun is high. The CHA is everywhere the same distance above the horizon whereas the infralateral arc is lowest beneath the sun then curves upwards.