Sun dog in the context of Horizontal coordinate system

Atmospheric optical phenomena include:

• Afterglow
• Airglow
• Alexander's band, the dark region between the two bows of a double rainbow.
• Alpenglow
• Anthelion
• Anticrepuscular rays
• Aurora (northern and southern lights, aurora borealis and aurora australis)
• Belt of Venus
• Brocken Spectre
• Circumhorizontal arc
• Circumzenithal arc
• Cloud iridescence
• Crepuscular rays
• Earth's shadow
• Earthquake lights
• Glories
• Green flash
• Halos, of Sun or Moon, including sun dogs
• Haze
• Heiligenschein or halo effect, partly caused by the opposition effect
• Ice blink
• Light pillar
• Lightning
• Mirages (including Fata Morgana)
• Monochrome Rainbow
• Moon dog
• Moonbow
• Nacreous cloud/Polar stratospheric cloud
• Rainbow
• Sprite (lightning)
• Subsun
• Sun dog
• Tangent arc
• Tyndall effect
• Upper-atmospheric lightning, including red sprites, Blue jets, and ELVES
• Water sky

A halo (from Ancient Greek ἅλως (hálōs) 'threshing floor, disk') is an optical phenomenon produced by light (typically from the Sun or Moon) interacting with ice crystals suspended in the atmosphere. Halos can have many forms, ranging from colored or white rings to arcs and spots in the sky. Many of these appear near the Sun or Moon, but others occur elsewhere or even in the opposite part of the sky. Among the best known halo types are the circular halo (properly called the 22° halo), light pillars, and sun dogs, but many others occur; some are fairly common while others are extremely rare.

The ice crystals responsible for halos are typically suspended in cirrus or cirrostratus clouds in the upper troposphere (5–10 km (3.1–6.2 mi)), but in cold weather they can also float near the ground, in which case they are referred to as diamond dust. The particular shape and orientation of the crystals are responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split into colors because of dispersion. The crystals behave like prisms and mirrors, refracting and reflecting light between their faces, sending shafts of light in particular directions.

An anthelion (plural anthelia, from late Greek ανθηλιος, "opposite the sun") is a rare optical phenomenon of the halo family. It appears on the parhelic circle opposite to the Sun as a faint white spot, not unlike a sundog, and may be crossed by an X-shaped pair of diffuse arcs.

How anthelia are formed is disputed. Walter Tape, among others, has argued they are not separate haloes, but simply where various haloes caused by horizontally oriented column-shaped ice crystals coincide on the parhelic circle to create a bright spot. If this theory is correct, anthelia should only appear together with these other haloes.