Parhelion

A parhelion is a localized bright patch that may appear to the left or right of the Sun, often with a reddish inner edge and a paler or bluish outer side. The plural is parhelia. Sun dogs are commonly seen as parts of a halo display, but they are distinct bright concentrations rather than simply a complete circular ring. The World Meteorological Organization classifies optical phenomena produced by reflection, refraction, diffraction or interference of sunlight or moonlight as photometeors.

The usual explanation involves sunlight passing through hexagonal ice crystals suspended in the atmosphere, often in thin cirrus clouds. When the crystals are oriented with their broad faces approximately horizontal, light entering and leaving through vertical side faces is refracted. Different wavelengths bend by slightly different amounts, producing colour separation; rays near the minimum deviation angle concentrate into a bright area roughly 22 degrees from the Sun. The red is typically on the side nearer the Sun.

The crystals need not form a solid cloud visible to an observer below. Conditions such as crystal orientation and abundance, cloud thickness and the observer’s position all affect how distinct the phenomenon appears. Sun dogs are often prominent when the Sun is low, especially near sunrise or sunset, but their visibility depends on atmospheric conditions rather than a fixed schedule.

The familiar display is a pair of spots at approximately equal angular distances on either side of the Sun. Yet the two need not be equally bright: the ice crystals and thin clouds may be distributed unevenly, one side may be obscured by other clouds or objects, or the viewing geometry may make a patch difficult to detect. A single visible sun dog therefore does not necessarily indicate a different optical process.

A sun dog is not the same as a 22-degree halo, although both are associated with ice-crystal optics and may occur together. A halo forms a circular ring around the Sun, while parhelia are bright side regions. Looking directly at the Sun can injure the eyes; observations should be made without staring at it.

Parhelia are examples of atmospheric optics: they show how the shape, orientation and optical properties of ice crystals can organize and separate sunlight. They are also useful visual clues that ice-bearing high clouds are present, although a sun dog alone does not provide a complete forecast of weather. Their form varies with the distribution of crystals and cloud layers, so photographs and observations may capture partial, faint or asymmetrical displays.