Sun Dogs: Parhelia and the Hexagonal Plate Crystals That Make Them

What a Sun Dog Is and Why It Matters

A sun dog is a concentrated bright spot — often a small, colored miniature of the Sun — that appears at the same elevation as the Sun and about 22° to one or both sides of it. When both flank the Sun, they can look like a pair of luminous companions escorting it across the sky, which is the likely origin of the folk name "sun dogs" (the formal term parhelion is Greek for "beside the Sun"). They frequently coincide with a faint 22° halo, sitting where that ring crosses the horizontal line through the Sun.

Beyond their beauty, sun dogs are a direct diagnostic of what is happening inside a cloud. Their very existence tells you the cloud contains ice, not water droplets; their strong color and position tell you the crystals are plate-shaped and horizontally oriented; and their sharpness reports on how well-formed and steadily falling those crystals are. They are, in effect, a free remote-sensing signal of high-altitude cloud microphysics visible to the naked eye — one reason atmospheric-optics researchers catalog them so carefully.

The Mechanism, Step by Step

Start with the crystal. Cirrus ice grows as hexagonal prisms; when the crystal is much wider than it is thick — a flat hexagonal plate — aerodynamic drag makes it fall like a leaf, broad face down, with its six side faces vertical. Now follow a ray. Sunlight enters one vertical side face and exits not the adjacent face but the one beyond it — the two refracting faces meet at an internal wedge angle of 60°. The plate therefore behaves exactly like a 60° glass prism laid on its edge.

A prism bends light, and the bending is least at the angle of minimum deviation. For a 60° prism of ice (refractive index n ≈ 1.31), that minimum deviation is ≈ 21.8°. Rays entering at other angles deviate more than 22°, so light piles up at the 22° minimum and thins out beyond it — producing a bright inner edge and a diffuse outer "tail." Because a floating plate spins freely about its vertical axis, the whole population sends this 22°-bent light to fixed points at the Sun's own elevation, on the left and right: the two sun dogs.

Characteristic Numbers, Scales, and the Key Relation

The governing relation is the prism minimum-deviation formula. For apex angle A = 60° and index n, the minimum deviation is δ_min = 2·arcsin(n·sin(A/2)) − A. With n = 1.31, δ_min ≈ 21.8°. Because ice is dispersive — n is smaller for red light (~1.307) and larger for blue (~1.317) — red is bent least (~21.8°) and violet most (~22.4°). Red therefore lands closest to the Sun and blue farthest out, exactly the reverse of a primary rainbow's ordering, and the colors overlap into a whitish outer tail.

Scales: the crystals are tens to a few hundred micrometers across; the host cirrus sits at ~5–12 km altitude where temperatures run −20 °C to −60 °C. Crucially, the 22° figure holds only when the Sun is near the horizon. As solar elevation rises, the ray must slice through the plate at a slant, the effective prism angle grows, deviation increases, and each sun dog migrates outward beyond the 22° halo — reaching ~30° or more, and fading, once the Sun climbs above ~50–60°.

How Sun Dogs Are Observed and Identified

Sun dogs are a naked-eye phenomenon — no instrument required — but observing them safely means blocking the Sun itself behind a building, tree, or thumb. The identifying signature is a bright patch at the Sun's elevation, offset horizontally, with an ordered color sequence (red toward the Sun, blue-white away) and often a horizontal white streak (the sun-dog "tail") pointing away from the Sun along the parhelic circle. That color ordering and the fixed ~22° starting distance distinguish a genuine parhelion from a random cloud brightening or a lens flare.

They appear whenever plate crystals are present: in thin cirrus and cirrostratus, in aircraft contrails, and — most vividly — in diamond dust, ground-level ice fog that lets observers stand inside the halo-forming crystal cloud in polar and high-altitude cold snaps. Photographers document the full display (halo, parhelic circle, tangent arcs, circumzenithal arc) to reconstruct the crystal population, and researchers use ray-tracing simulations (e.g., HaloSim, HaloPoint) to match a photographed display to specific crystal shapes and orientation distributions.

Where They Operate and How They Differ From Related Effects

Every ice halo is made by hexagonal crystals, but the display you get depends on crystal habit and orientation. The plain 22° halo comes from randomly tumbling crystals (columns and plates), which smear the 22° minimum deviation into a full ring. Sun dogs need the extra ingredient of oriented plates falling face-down, concentrating that same 22° light into two spots. Swap plates for horizontally floating columns and you get tangent arcs above and below the halo; the rare Parry arc needs columns doubly oriented (main axis and side faces both horizontal).

Reflection, not refraction, produces the colorless parhelic circle (a white band around the whole sky at the Sun's altitude) and sun pillars (vertical shafts) — which is why those are white while sun dogs are colored. The vivid, upside-down-rainbow circumzenithal arc at 46° above the Sun shares the plate-crystal population with sun dogs and often appears alongside them, making sun dogs a reliable herald of a richer halo display.

Significance and Open Questions

Sun dogs sit at the intersection of classical optics and modern cloud science. Their occurrence, brightness, and color purity encode the degree of crystal orientation — how flat, how well-aligned, and how steadily the plates fall — which in turn depends on crystal size, turbulence, and fall speed. Because horizontally oriented plates are also what make cirrus clouds specularly reflective to lidar and to satellite radiometers, understanding the same crystals matters for how ice clouds are represented in climate radiative-transfer models, where cirrus remains a stubborn source of uncertainty.

Open questions are largely microphysical: what governs the fraction of perfectly oriented plates versus wobbling or tumbling ones; how surface roughness, hollowing, and complex polycrystals blur real halos relative to idealized ray-tracing; and how displays differ in exotic settings. Water-ice clouds on Mars can form hexagonal prisms too — the Perseverance rover imaged halo-producing crystals — extending the same 60°-prism physics to other worlds. Historically, unusual halo complexes recorded in medieval and early-modern chronicles are now being reinterpreted as parhelic displays rather than portents, turning sun dogs into data points in the history of the sky.

Sun dogs versus related ice-halo phenomena (all from hexagonal ice crystals but distinguished by crystal shape, orientation, and ray path)

Phenomenon	Crystal type & orientation	Angular position	Color / appearance
Sun dog (parhelion)	Plate crystals, flat faces horizontal	≈22° to sides of Sun (moves outward as Sun rises)	Strong color; red inner, blue-white outer + white tail
22° halo	Randomly oriented columns & plates	Full 22° ring around Sun	Faint color; red inner edge, sky darker inside ring
Parhelic circle	Vertical side faces (reflection)	White band through Sun, parallel to horizon	White, no color (external reflection)
Upper tangent arc	Column crystals, long axis horizontal	Tangent to top of 22° halo	Colored; shape changes with solar altitude
Circumzenithal arc	Plate crystals (top/side refraction)	46° above Sun, near zenith	Vivid, rainbow-like, purest colors of any halo
Sun pillar	Plate/column crystals (reflection)	Vertical shaft above/below Sun	White or Sun-colored, no refraction color