The 22-Degree Halo: Ice-Crystal Refraction Rings Around the Sun and Moon

What it is and why it matters

The 22° halo is a ring of light centred on the Sun or Moon with an angular radius of about 22°. It is the flagship member of the ice-halo family — a class of optical phenomena caused not by liquid raindrops (which make rainbows) but by hexagonal ice crystals in high, cold cloud. Whenever you see a diffuse ring at roughly two hand-spans from the light source, with a sharp reddish inner edge and a noticeably darker sky inside, you are seeing sunlight sorted by ice geometry.

It matters on several levels. Practically, the halo is a weather signpost: it means a broad deck of cirrostratus has moved overhead, often the leading edge of an approaching warm front, so folklore that 'a ring around the Moon means rain' has real physical backing. Scientifically, halos are a natural probe of cloud microphysics — the shape, size, and orientation of ice crystals aloft — and the same refraction physics is used to interpret halos reported in the atmospheres of other worlds. It is also simply the most accessible piece of quantitative optics in the everyday sky.

The mechanism, step by step

A hexagonal ice crystal has six long side faces. Pick any face light enters through and skip one to the face two positions over: the two faces meet at a 60° apex angle, so the crystal behaves as a 60° glass prism. Sunlight entering such a prism is refracted at the first surface, crosses the crystal, and refracts again on exit, emerging deviated from its original path.

Because the crystals tumble in essentially random orientations, light exits at a whole range of deviation angles — but not evenly. As you sweep the entry angle, the total deviation reaches a minimum and then increases again. Near that minimum the deviation changes very slowly with input angle, so rays from a huge range of crystal orientations all emerge within a degree or two of the same angle. Light therefore piles up at the angle of minimum deviation, ≈22°, and thins out beyond it. No light is refracted to smaller angles at all, which is why the sky inside the ring looks conspicuously dark. Since ice bends short wavelengths more, violet deviates a bit more than red, painting the ring red-inside, blue-outside.

The characteristic numbers and the key relation

The governing quantity is the angle of minimum deviation D_min of a prism, set by its apex angle A and the refractive index n via D_min = 2·arcsin(n·sin(A/2)) − A. For ice, the nominal n ≈ 1.31 and A = 60° give D_min = 21.84°; using the true wavelength-dependent index, D_min ≈ 21.5° for red light (n ≈ 1.306, ~700 nm) and rises to about 22.37° for violet (n ≈ 1.317, ~400 nm). The mean ~22° fixes the ring radius; the ~0.8° spread across the visible spectrum sets its colour width and reddish inner edge.

The dark inner void follows directly: with A = 60° and n = 1.31, geometry forbids any refracted ray closer than ~22° to the source, so the interior receives no refracted light. The full 44° diameter, the ~2° radial thickness, and the modest colour saturation all fall out of the same prism relation. Contrast the 46° halo, which uses a 90° prism (a side face to an end/base face): the larger apex angle yields a bigger minimum deviation (~46°) and much wider colour dispersion, making it fainter and rarer.

How it is observed and measured

The 22° halo is a naked-eye phenomenon, best seen through a thin, uniform veil of cirrostratus — ice cloud at roughly 5–10 km altitude and temperatures near −20 to −40 °C, cold enough that the crystals are solid ice rather than water. To view it safely you block the Sun itself behind a building, tree, or outstretched thumb; the ring then reveals its colours and the dark interior. Around the Moon the same physics operates on faint moonlight, appearing whitish because the eye's colour sensitivity is poor at low light.

Quantitatively, observers measure the angular radius with a simple fist-at-arm's-length rule (a fist ≈10°, a spread hand ≈20°) or with cameras and wide-angle lenses whose geometry is calibrated. Dedicated halo enthusiasts and networks (such as the community around Les Cowley's Atmospheric Optics resource) photograph, stack, and model events, and simulation codes like HaloSim ray-trace millions of crystals to reproduce the exact brightness profile, confirming crystal shapes and orientations from the observed pattern.

Where it operates and how to tell it apart

The 22° halo requires randomly oriented hexagonal ice crystals, so it forms wherever high ice cloud exists — over most of the planet, in every season, whenever cirrostratus passes. This is why it is far commoner than a rainbow, which needs a rain shower opposite the Sun. When crystals instead settle into preferred orientations, the halo family diversifies: horizontal plate crystals concentrate light into sun dogs (parhelia) — bright, often coloured spots at the same 22° distance but only to the left and right of the Sun — and into the circumzenithal arc high overhead; vertically drifting columns give tangent arcs and, at solar elevations above ~29°, a circumscribed halo hugging the ring.

Distinguish the 22° halo from a corona (a much smaller, diffraction-made ring right at the Sun/Moon from water droplets), from a 42° rainbow (liquid water, opposite the Sun), and from the rarer 46° halo further out. The ring's fixed 22° radius, red-inside ordering, and dark interior are its signatures.

Open questions and broader significance

The core optics of the 22° halo were settled centuries ago — Descartes puzzled over halos, Huygens worked on the refraction geometry, and by the 19th century the minimum-deviation explanation (with contributions from workers such as Venturi and later Bravais for tilted crystals) was secure. Yet live questions remain in the microphysics: exactly which crystal habits (columns, plates, bullet rosettes, pyramidal forms) dominate a given cloud, how they orient in real turbulence, and why some rare halos (like the elusive odd-radius pyramidal halos at 9°, 18°, 20°, 23°, 24°, and 35°) appear only under special conditions.

The significance reaches beyond Earth. The same ice-prism physics predicts halos in the water-ice and CO₂-ice clouds of Mars, and exotic species in the ammonia, methane, or diamond-dust atmospheres of the giant planets and Titan — an optical diagnostic for crystal composition on other worlds. Closer to home, halo displays help validate cloud models and remote-sensing retrievals of cirrus, which are important for Earth's radiation budget and climate.

The 22° halo compared with related ice-crystal optical phenomena

Phenomenon	Angular position	Crystal / geometry	Cause
22° halo	22° radius ring around Sun/Moon	Randomly oriented hexagonal prisms; 60° prism faces	Refraction, minimum deviation
46° halo	~46° radius, faint	Same crystals; 90° face-to-base prism	Refraction, larger deviation
Sun dogs (parhelia)	~22° left/right, at Sun's height	Horizontal plate crystals	Refraction through 60° faces
Circumzenithal arc	Near zenith, above Sun	Horizontal plates, top-entry ray	Refraction, upside-down 'rainbow'
Sun pillar	Vertical shaft through Sun	Plates/columns tilting slightly	Reflection off crystal faces
Rainbow (contrast)	42° radius, opposite Sun	Liquid water droplets	Refraction + internal reflection