parhelier

Parhelion, plural parhelia, is a halo phenomenon commonly known as a sun dog. In English, parhelion (singular) and parhelia (plural) are the standard terms, while parhelier is used in some languages to refer to the same effect.

Formation and causes: Parhelia form when sunlight enters hexagonal ice crystals suspended in high-altitude clouds such as cirrus or cirrostratus. The crystals act as prisms and mirrors, bending light by about 22 degrees from the Sun and projecting bright spots to the left and right of the Sun. The effect is often seen in conjunction with the 22-degree halo surrounding the Sun. The light can be dispersed by wavelength, giving a reddish inner edge near the Sun and a cooler blue or purplish tint toward the outer edge, though color may be faint.

Appearance: The most recognizable feature of parhelia is a pair of bright spots at roughly the same altitude as the Sun, symmetrically placed to its left and right. They may occur alone or accompany other halo features, and can sometimes be accompanied by fainter peripheral patches within the halo. The phenomenon is typically more vivid when the Sun is low on the horizon and when colder air supports well-aligned plate-like ice crystals.

Context: Parhelia are part of the halo family and are most commonly observed in mid-latitudes during cold weather. They are a seasonal and weather-dependent optical display produced by atmospheric ice-crystal halos.