A sundog — properly a parhelion — is a bright, often rainbow-tinted patch of light sitting 22° to the left or right of the Sun and level with it, and the 22° halo is a complete ring at that same radius. Both are produced by sunlight refracting through hexagonal ice crystals suspended in high cirrus cloud, typically 5–10 km up. The 22° figure is not a convention or an average; it is a direct consequence of the crystal's shape. Light entering one side face of a six-sided ice prism and leaving through an alternate face passes through an effective 60° prism, and the smallest angle by which such a prism can deflect light is close to 22°. Light piles up at that minimum, so that is where the sky brightens. Whether you see two isolated spots or a full ring depends on nothing more than whether the crystals are hanging in a preferred orientation or tumbling at random.
One Crystal, Sixty Degrees, Twenty-Two Degrees
Ice that freezes directly from vapour in the upper troposphere grows in the hexagonal system: flat six-sided plates and six-sided columns, depending on temperature and supersaturation. Both shapes present the same essential optical element. Any two alternate side faces of a hexagon meet at an effective angle of 60°, and a ray that enters one and exits the other has been refracted twice.
How far that ray is bent depends on the angle at which it struck the first face. Push the entry angle around through its whole range and the total deviation traces a curve with a minimum: for a 60° ice prism, about 21.8°. Crucially, the curve is flat near that minimum, so a large share of rays entering at a wide range of angles all emerge deflected by roughly the same 22°, while almost none are deflected by less. The result is a sharp inner edge at 22° with a diffuse outer fade — precisely the profile people notice as a ring.
The colour follows from the same mechanism. Ice refracts red light slightly less than blue, so the minimum deviation is marginally smaller for red. That puts red on the inside of the ring and blue on the outside, the reverse of a rainbow's ordering, and it explains why halo colours are usually pale: the blue end is smeared across a wider range of angles and overlaps everything else, leaving a distinct red inner edge and a washed-out remainder. The mechanism differs entirely from the water-droplet optics behind a rainbow, which is why the two never appear in the same part of the sky.
Why Sundogs Sit Beside the Sun and Halos Surround It
The single variable that separates a pair of sundogs from a complete ring is crystal orientation.
Large flat plate crystals falling through still air are aerodynamically stable lying flat, like a dinner plate settling through water, with their hexagonal faces close to horizontal. A population of crystals all oriented that way can only refract light toward the observer in the horizontal plane containing the Sun. The 22° deviation therefore shows up only at two places — one on each side of the Sun, at the same altitude — and those are the sundogs.
Small crystals, and columns tumbling in turbulent air, take no preferred orientation. Randomly oriented crystals deliver the 22° deviation in every direction around the Sun equally, filling in the complete circle. This is why the two forms often appear together but with different emphasis, and why sundogs drift outward as the Sun climbs: above about 30° solar altitude the geometry of a horizontally-oriented plate forces the parhelion further than 22° from the Sun, and by around 60° it has moved out and faded away. Sundogs are consequently a low-Sun phenomenon, best in the hours after sunrise and before sunset.
The Wider Family
Atmospheric-optics observers have catalogued dozens of distinct halo forms, nearly all built from the same hexagonal ice and distinguished by which faces the light uses and how the crystals hang.
- The 46° halo: a much larger, fainter ring, produced when light enters an end face and exits a side face — an effective 90° prism instead of a 60° one.
- The circumzenithal arc: a vivid, properly rainbow-coloured arc high overhead, curving away from the Sun, formed when light enters the top face of a plate crystal and exits a side. It only exists when the Sun is below about 32° altitude, and it is generally considered the most beautiful of the family. It is frequently mistaken for a rainbow, which it cannot be — rainbows never appear on the same side of the sky as the Sun.
- The parhelic circle: a white horizontal band running all the way around the sky at the Sun's altitude, produced by reflection off vertical crystal faces rather than refraction. It carries no colour because no dispersion is involved.
- Sun pillars: vertical shafts above or below a low Sun, made by reflection off the near-horizontal faces of plate crystals. The same physics operating on ground-level lights produces the light pillars seen above cold cities.
- The circumscribed halo and tangent arcs: curved caps sitting above and below the 22° ring, produced by horizontally-oriented column crystals; their shape changes systematically with the Sun's height.
Where the Sky Delivers Them Most Often
Halos require ice crystals along the line of sight, which means either high cirrus cloud or, in genuinely cold places, ice crystals suspended at ground level — the phenomenon known as diamond dust. The second case produces the richest and most complete displays, because the crystals are close, well-formed and abundant.
Antarctic stations report halo phenomena on a substantial majority of clear and partly cloudy days, and the most complete halo displays ever photographed, showing a dozen or more arcs simultaneously, have come from the South Pole and from high-altitude cold sites. Northern Canada, interior Alaska, Siberia, Mongolia, Finnish Lapland and the northern Great Plains all deliver frequent winter displays.
They are far more common in temperate latitudes than most people assume, and the reason they go unnoticed is worth naming: they appear close to the Sun, which everyone instinctively avoids looking at. The practical technique is to block the Sun itself behind a building edge, a signpost or an outstretched thumb, and then examine the sky roughly two outstretched-fist-widths to either side. Anyone who does this on days with thin, milky, high cloud will start finding sundogs within a week.
Three Suns Over a Battlefield
Because a good parhelion genuinely looks like two additional suns flanking the real one, the phenomenon has left a trail through the historical record wherever people wrote down what they saw in the sky.
The most consequential English instance came on 2 February 1461, before the Battle of Mortimer's Cross, when Edward, Earl of March, and his army reportedly watched three suns rise and merge into one. Edward interpreted it publicly as a favourable sign, won the battle, took the throne as Edward IV within weeks, and afterwards used the sun in splendour as a personal badge — a heraldic device that outlasted him by centuries and that Shakespeare later put to work in Henry VI, Part 3.
The scientific record starts in the same register. An elaborate display over Rome on 20 March 1629, with multiple rings and mock suns, was widely observed and described, and it was this event that prompted René Descartes to attempt a physical explanation of atmospheric halos in Les Météores, published in 1637. In 1661 the astronomer Johannes Hevelius recorded and engraved an extraordinarily complete display over Danzig, showing arcs that would not be satisfactorily explained for another two hundred years. The full theory, tracing every arc back to a specific crystal shape and orientation, only came together in the nineteenth and twentieth centuries — which means people had been drawing these things accurately for far longer than they had been able to say what they were.
The Warm Front Behind "Ring Around the Moon"
The proverb that a ring around the Moon means rain or snow within a day has a real mechanism behind it. Halos form in cirrus and cirrostratus, and in mid-latitudes that thin high cloud is characteristically the leading edge of an approaching warm front, running out ahead of the surface front by several hundred kilometres. The cloud deck then thickens and lowers over the following day as the front arrives.
The relationship is statistical rather than deterministic — plenty of cirrus passes without precipitation following, particularly in continental interiors and in summer. But the correlation was strong enough that it survived independently in European, Inuit and East Asian weather lore, and it remains a reasonable rule of thumb in the mid-latitude westerlies. The lunar version works because moonlight is ordinary sunlight and obeys the same refraction; a 22° halo around a bright Moon is exactly the same object as a 22° halo around the Sun, and unlike a sundog it is easy to look straight at.
Once the 22° geometry is familiar, the rest of the sky opens up: the same low, cold, clear conditions that produce a good halo display are also the ones that favour the green flash at sunset.
