Red Arrows & Halos
The RAF Aerobatic Team echo the curve of a circumscribed halo.

The right hand sundog interestingly coincides with the circumscribed halo disproving that sundogs are always 22° from the sun.

Imaged at Duxford, Cambridgeshire, England by Mark McCaughrean. ©Mark McCaughrean, shown with permission


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The sun was ~42° high and at that altitude the "22° parhelia" sit concidentally at where the circumscribed halo crosses the parhelic circle. The inner halo is the 22° radius halo. Sundogs move further and further from the sun as it increases in altitude. Sundogs are only 22° distant when the sun is on the horizon.

Sundogs are a minimum deviation effect. Rays enter the side face of a horizontal plate crystal and leave through another side face inclined 60° to the first. Rays can be deflected through a variety of angles but not less than the minimum one that occurs when the ray crossing the crystal is parallel to the intermediate side face. Many rays bunch at the minimum deviation condition and these produce the brightest part of the sundog.

The minimum deviation angle is 22° when the sun is at the horizon and rays cross the crystal in a horizontal plane.

Rays from a high sun pass through plate aligned ice crystals in a plane skewed from horizontal. The angles of incidence and exit are larger and the minimum deviation angle is correspondingly greater than 22°. the sundog is farther from the sun.

Rays from a moderately high sun passing through the crystal in this skew fashion are several times reflected from the upper and lower horizontal faces before emerging to form the 'dog'.