Halotra

Halotra, an ice spirit, wears a ring of light above her head, a playful nod to the real 22° halo that rings the Sun or Moon on a cold, hazy day.

The Physics

The real halo is made by millions of tiny hexagonal ice crystals in thin, high cirrus cloud, not by water droplets. Light entering one side face of a crystal and leaving by another passes through what is in effect a 60° prism. Every such prism has a minimum angle of deviation, and light piles up near it, so a bright ring appears at that angle from the Sun.

D_min = 2 arcsin(n sin 30°) − 60°

• D_min: minimum deviation, the halo radius (degrees)
• n: refractive index of ice, about 1.31 for visible light
• 30°: half the 60° prism angle between alternate faces

With n = 1.31, D_min = 2 arcsin(0.655) − 60° ≈ 21.8°, which is the 22° halo. No light is deviated by less than this, so the sky inside the ring is darker. Ice bends red light slightly less than blue, so the inner edge of a real halo is reddish.

Honest caveat: The halo is a ring around the Sun or Moon, about 22° in radius (roughly a hand-span at arm's length), not a ring above someone's head. It needs randomly oriented crystals; crystals that fall in a preferred orientation make sundogs and arcs instead.

On the sticker

Halotra, an icy-blue spirit, has a stylised halo ring floating above her head. Printed text: HALOTRA with the line 22°. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.