Horizontal hexagonal ice crystal

The refractive index of ice is 1.32 ... a little less than the refractive index of water which is 1.33. The exceptionally bright 22° halo formed by refraction through horizontal hexagonal crystals (seen when the Sun is overhead) is called a circumscribed halo. This halo becomes distorted as the elevation of the Sun is reduced. Below 20° elevation the circumscribed halo decays to upper and lower tangent arcs.

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