A solar halo is a luminous ring that appears around the sun when sunlight passes through tiny ice crystals suspended in high-altitude cirrus clouds. The most familiar version sits at roughly 22 degrees from the sun, forming a pale circle with a faintly reddish inner edge. The phenomenon is entirely optical, produced by refraction through hexagonal ice prisms, and it is far more common than most people realize because it requires nothing more exotic than a thin veil of ice cloud between you and the sun.
How the 22-Degree Halo Forms
Cirrus clouds float at altitudes where temperatures drop well below freezing, and the water vapor there freezes into microscopic ice crystals rather than liquid droplets. Many of these crystals take a hexagonal prism shape, with six rectangular side faces and two flat hexagonal end faces. When sunlight enters one side face and exits through another that is angled 60 degrees away, the light bends. The geometry of a hexagonal prism means the minimum angle at which light can be deflected through this path is about 22 degrees. Because refraction cannot send light through an angle smaller than that minimum, a concentration of redirected light builds up right at the 22-degree mark. The result is a bright ring at that angular distance from the sun, with the sky inside the ring appearing slightly darker than the sky just outside it.
Early computational work confirmed this picture by tracing large numbers of parallel light rays through hexagonal prisms using geometric optics. The simulations reproduced both the expected forward-scattering peak and a prominent intensity maximum at the angle corresponding to the 22-degree halo.1Journal of Quantitative Spectroscopy and Radiative Transfer. A method for computing the transfer of solar radiation through clouds of hexagonal ice crystals The key requirement is that the ice crystals be much larger than the wavelength of visible light, so that ray-tracing geometry applies rather than wave-diffraction effects dominating.
Why the Ring Has Faint Color
A solar halo is not pure white. If you look carefully, the inner edge leans toward red and the outer edge fades through orange, yellow, and sometimes a hint of blue-green. This happens because ice refracts different wavelengths by slightly different amounts, just as a glass prism splits white light into a spectrum. Red light bends the least, so it appears closest to the sun. Violet and blue bend the most, landing at the outer edge.
The color spread in a 22-degree halo is modest. The angular difference between deep red and violet is less than a degree, which means the colors overlap heavily and wash each other out. Measurements of the halo angle at specific wavelengths show the total color spread across the visible spectrum is about 0.84 degrees for the 22-degree halo.2Optica Publishing Group. Polarization and intensity distributions of refraction halos That is narrow enough that most observers see the ring as mostly whitish with just a red fringe on the inner side. In contrast, the less common 46-degree halo has a color spread of about 2.4 degrees, making its colors noticeably more vivid when it does appear.
The 46-Degree Halo and Arcs That Accompany It
The 22-degree ring is not the only halo the sky can produce. A second, larger ring sometimes appears at 46 degrees from the sun. This one forms when light enters through a side face and exits through an end face of the hexagonal prism, passing through faces that meet at 90 degrees rather than 60. The minimum deflection through a 90-degree prism of ice is about 46 degrees, so the same concentration-of-light effect that creates the inner ring produces a fainter outer ring at roughly twice the angular distance. Computer ray-tracing through randomly oriented hexagonal prisms reproduces this 46-degree halo clearly.3PubMed. The 46 degrees Halo and Its Arcs
When ice crystals are not tumbling randomly but instead fall with a preferred orientation, portions of these halos brighten into distinct arcs. Hexagonal plate crystals that drift with their flat faces roughly horizontal produce two of the most striking effects: the circumzenithal arc, a brilliant splash of color high above the sun near the zenith, and the circumhorizontal arc, a broad band of spectral color that appears below the sun when it is high in the sky. These arcs show purer, more vivid color than a full halo ring because oriented crystals restrict the range of light paths, reducing the overlap that washes colors out.3PubMed. The 46 degrees Halo and Its Arcs Each arc’s appearance depends on how high the sun sits, so the same cloud layer can produce different visual effects at different times of day.
Crystal Orientation and Sun Pillars
Whether ice crystals tumble randomly or align as they fall makes a dramatic difference in what you see. Randomly oriented crystals produce full circular halos. When plate-shaped crystals fall with their flat faces nearly horizontal, they act like millions of tiny mirrors and reflectors, each tipped only slightly from level. This alignment gives rise not just to the arcs described above but also to sun pillars: vertical columns of light extending above or below the sun, created by reflections off the broad flat faces of those horizontal plates.
Simulations comparing sun pillars with light pillars from nearby artificial sources show that the visual appearance depends on how tightly the crystals cluster around horizontal. A very slight wobble produces a compact, intense pillar; a broader tilt distribution stretches and dims it.4Optica Publishing Group. Comparison of Sun pillars with light pillars from nearby light sources Column-shaped crystals falling with their long axes horizontal produce their own family of effects, and so-called Parry-oriented columns, which fall with a specific side face tilted upward, generate yet another set of rare arcs.5Journal of Quantitative Spectroscopy and Radiative Transfer. Statistical simulation of the angular distribution of reflected solar radiation in optically anisotropic ice-crystal clouds: Horizontally oriented particles The halo family is really a family of dozens of distinct optical effects, all tied to the same basic ingredients but differentiated by crystal shape and alignment.
What the Ice Crystals Actually Look Like
Halo theory traditionally assumes solid, smooth-faced hexagonal prisms, and the math works well under that assumption. Real cirrus clouds turn out to be messier. During a major field campaign in 1986, researchers flew an instrumented aircraft through four cirrus clouds that were actively producing 22-degree halos and collected ice crystals by impaction onto glass slides. Most of the identifiable crystals were columns and bullet rosettes with internal cavities and hollows, along with plate-column combinations. Solid, structurally pristine hexagonal crystals were actually rare.6PubMed. Effects of ice-crystal structure on halo formation: cirrus cloud experimental and ray-tracing modeling studies
This finding matters because internal structure and surface roughness scatter light differently than smooth faces do. Rough or hollow crystals blur the sharp intensity peaks that produce crisp halos, which is why many cirrus clouds pass overhead without generating any visible halo at all. The clouds that do produce halos tend to contain a sufficient fraction of crystals whose faces are smooth enough and whose geometry is close enough to an ideal hexagonal prism to concentrate light at the right angle. Later retrieval studies analyzing halo brightness patterns found that plate-like crystals often best matched the observed 22-degree halo, while columnar shapes with multiple elements fit the broader scattering pattern across both the 22-degree and 46-degree ranges.7Atmospheric Chemistry and Physics. Ice crystal characterization in cirrus clouds III: retrieval of ice crystal shape and roughness from observations of halo displays
Automated detection algorithms that scan all-sky camera images over long periods confirm the basic requirement: halos form when smooth-faced hexagonal ice crystals are present in the optical path between sun and observer.8Atmospheric Measurement Techniques. Analysis algorithm for sky type and ice halo recognition in all-sky images Cloud-top temperatures in halo-producing cirrus are typically colder than about minus 38 degrees Celsius.7Atmospheric Chemistry and Physics. Ice crystal characterization in cirrus clouds III: retrieval of ice crystal shape and roughness from observations of halo displays
Rare Halos from Pyramidal Crystals
Not all ice crystals are simple hexagonal prisms. Some develop pyramidal faces, tapering toward one or both ends like a sharpened pencil. These additional crystal faces introduce prism angles that differ from the 60-degree and 90-degree geometry of a standard hexagonal column, which means they can produce halos at unusual angular distances from the sun. Among the most discussed are halos at roughly 9, 18, 20, 24, and 35 degrees.
Modeling shows that the 9-degree halo can be as bright as the common 22-degree halo when pyramidal crystals with a specific apex angle are present. However, halos at 18, 20, 24, and 35 degrees are predicted to be difficult to see because the intense 22-degree halo dominates the scattering between about 18 and 35 degrees, drowning out nearby weaker features.9Annales Geophysicae. Pyramidal ice crystal scattering phase functions and concentric halos Pyramidal crystals falling with their long axes vertical generate an additional set of arcs, and a detailed treatment found twelve distinct halo phenomena from this orientation alone, six of which had never been formally described. The undiscussed halos were predicted to be of comparable intensity to the known ones, produced by reflections from the basal faces of pyramidal crystals.10Applied Optics. Halo arcs from airborne, pyramidal ice crystals falling with their c axes in vertical orientation
Sightings of these exotic halos are genuinely rare, partly because pyramidal crystals are uncommon in typical cirrus and partly because identifying a faint arc against a bright sky requires favorable conditions and a trained eye. Photographers and amateur atmospheric-optics enthusiasts have documented some of these features, but many remain more theoretical than observational.
How Researchers Simulate Halos
Because you cannot control the weather, much of what we know about halo formation comes from computer simulations and laboratory experiments. The standard computational approach is Monte Carlo ray tracing: virtual sunlight rays are fired at a model ice crystal, and the Fresnel equations (which describe how light splits into reflected and transmitted portions at each surface) are applied as probability rules at every face the ray encounters. Run enough rays through enough crystals in a given orientation distribution, and a simulated sky pattern emerges.11Journal of the Optical Society of America A. Monte Carlo simulation and analysis of halo phenomena These simulations can reproduce the relative intensities of halos, arcs, and bright spots, allowing researchers to match observed displays to specific crystal populations.
Laboratory work complements the simulations. In cold-chamber experiments, researchers grow ice crystals under controlled conditions and measure how they scatter laser light using specialized instruments. Clouds of plates and hollow columns generated in the lab have produced measurable 22-degree and 46-degree halo patterns consistent with the theoretical predictions.12Applied Optics. Light-scattering properties of plate and column ice crystals generated in a laboratory cold chamber At an even simpler level, a single commercially available hexagonal glass prism can be used to demonstrate halo optics in a classroom. By spinning the prism or building a mechanical device that rotates it through random orientations, you can recreate the angular distribution of light that produces a halo display.13American Journal of Physics. Artificial halos
Halos on Mars
Earth is not the only planet where halos have been spotted. In 2022, NASA’s Perseverance rover captured what appears to be the first observation of an optical halo on another planet. On sol 292 of its mission, during a cloudy and icy period, Perseverance recorded a 22-degree halo produced by water-ice clouds in the Martian atmosphere. The feature was detected across four cameras at six wavelengths and was observable over a roughly three-hour window of mid-to-late morning. Possible follow-up detections occurred on the next six sols during the same icy spell.14PubMed Central. Hexagonal Prisms Form in Water‐Ice Clouds on Mars, Producing Halo Displays Seen by Perseverance Rover
The Martian water-ice halo appeared at the same 22-degree angle seen on Earth, which makes sense because water ice forms the same hexagonal crystal structure regardless of which planet it is on. But Mars also has clouds made of carbon dioxide ice, and CO₂ crystals have a different molecular structure. Simulations of CO₂ ice scattering predict halos peaking at about 29 and 42 degrees rather than 22 and 46, reflecting the different refractive index and crystal geometry of dry ice. The CO₂ crystal shapes considered include cubes, octahedrons, and intermediate forms. A mixture of water-ice and CO₂ crystals with appropriate proportions could explain halo observations on Mars that do not neatly match the familiar 22-degree geometry.15Geophysical Research Letters. Simulations of Halos Produced by Carbon Dioxide Ice Crystals in the Martian Atmosphere
How to Spot One Yourself
Solar halos are more common than rainbows in many climates. Some surveys of all-sky camera images at atmospheric monitoring stations find 22-degree halos appearing on a substantial fraction of days, particularly in regions where high cirrus is frequent. You are most likely to see one when a thin, uniform veil of cirrus moves across the sky, the kind that makes the sun look slightly milky but does not block it. Thick overcast will not work because the ice crystals need to be in a relatively thin optical layer so that refracted light is not scattered too many times before reaching your eyes.
The easiest way to check is to block the sun with your thumb or a building edge and look for a ring at roughly the span of an outstretched hand held at arm’s length. The 22-degree radius corresponds to roughly the distance from the tip of your little finger to the tip of your thumb when your hand is spread wide. If the inner edge of the ring looks faintly reddish and the sky inside the ring seems darker than the sky just outside, you are looking at a genuine 22-degree halo. Polarized sunglasses can slightly enhance contrast, though they can also reduce the visibility of some halo arcs depending on their polarization properties, so it is worth checking both with and without them.
Halos carry no weather-prediction value in any scientific sense, despite the old saying that a ring around the sun means rain. The correlation exists loosely because cirrus clouds sometimes precede a warm front, but cirrus also appears on its own with no approaching storm. The optical display itself tells you only one thing with certainty: there are hexagonal ice crystals between you and the sun, and their faces are smooth enough to bend light into the pattern you see.