The circumscribed halo
is an ice-based atmospheric effect
. It appears in the sky
a white line that surrounds the sun
Its exact shape changes as the sun rises between 29° and 90° degrees in the sky. At 29° it looks like a sagging oval—a giant ski mask, lima bean, or Tragos mouth. At the point where the circumscribed halo touches the top and bottom and the 22° halo, it thickens and displays a stronger chromatic aberration, with its sharper red band towards the sun and its more diffuse blues away from the sun. As the sun rises to 90°, the lobes of the shape slowly constrict to form a circle. At zenith it thickens the 22° halo, adding its stronger colors and brightness.
As the sun leaves zenith and returns towards the horizon, the halo expands and sags again, until it splits to become the softly writhing upper and lower tangent arcs. Atmospheric optics make a strong distinction between arcs and halos, and so these phenomena, while related, have separate names.
When the circumscribed halo is less strong, it may appear as bright bulges in the 22° halo.
Circumscribed halos occur when hexagonal needle-shaped ice crystals in cirrus clouds are distributed before the sun, with their long axes horizontal. Sunlight passes into one edge face of the crystal and diffracts out the next adjacent face plus one, making a 60° bend from sun to eye.
Similar conditions can produce parhelia and the 22° halo, depending on the orientation of the crystals.
- An interactive animation is available at the time of noding at