A circumscribed halo is a type of halo

Halo (optical phenomenon)

A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...

, an optical phenomenon

Optical phenomenon

An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....

 that circumscribes a related phenomenon, the 22° halo

22° halo

A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...

 centred on the sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

.

When observed, a circumscribed halo is normally oval in shape and, at the points directly below and above the sun, lies tangent

Tangent

In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...

ial to the 22° halo. It form when, as the sun rises, the upper tangent

Upper tangent arc

An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low it appears as an arc over the sun forming a sharp angle...

 and lower tangent arc

Lower tangent arc

A lower tangent arc is a rarely observable halo, an optical phenomenon appearing under and tangent to a 22° halo centred around the sun.Just like upper tangent arcs, the shape of a lower tangent arc is dependent on the altitude of the sun. As the sun slips over the horizon the lower tangent arc...

s extend into each other and close in on the 22° halo.
As the sun rises above 70° it essentially covers the 22° halo.

A circumscribed halo is more intense in colour than the 22° halo. Like many other halos, it is reddish on the inner edge, facing the sun, and bluish on the outer edge.

External links

• www.paraselene.de - HaloSim Computer simulations of a circumscribed halo.
• Atmospheric Optics - Circumscribed Halo - solar altitude - an animation showing how the shape of the phenomenon changes as the sun rises.