Ice halos are optical phenomena produced when sunlight (or moonlight) is refracted and reflected by millions of hexagonal ice crystals drifting in high-altitude cirrus clouds. The most common is the 22° halo — a bright ring around the sun at an angular radius of about 22°. Other phenomena include sun dogs (parhelia), the circumzenithal arc, and the rare 46° halo, each arising from a specific crystal orientation and ray path.
The 22° halo forms when light enters one prism face of a randomly oriented hexagonal ice column and exits through a non-adjacent face at minimum deviation. The 60° apex angle of the hexagonal prism produces a minimum deviation of 21.84° for water ice, creating the inner bright edge of the halo. Light rays that enter at larger angles undergo total internal reflection rather than refraction, explaining why there is no light inside the 22° halo.
The diversity of halos — over 100 documented types — depends on crystal shape (columns, plates, pyramidal crystals), orientation (random, horizontally floating, aligned by aerodynamics), and the specific ray paths through the crystal faces. Simulation programs like HaloSim and atmospheric optics software trace individual rays through millions of crystals to reproduce observed halo displays, enabling researchers to infer cloud microphysics from visible optical patterns.
The 22° radius is set by the minimum deviation angle for a ray passing through the 60° prism formed by two prism faces of a hexagonal ice crystal. For water ice with refractive index ~1.31 at visible wavelengths, minimum deviation occurs at ~21.84°, creating the sharp inner edge of the halo at that radius.
Rays entering the prism at angles that would produce deflection less than 22° undergo total internal reflection and do not exit — they cannot reach the observer from inside the minimum deviation angle. So no light reaches the observer from directions within 22° of the sun via this particular ray path.
Sun dogs are bright spots appearing 22° to the left and right of the sun, often with a reddish inner edge. They form when hexagonal plate crystals drift with their flat faces nearly horizontal, refracting light preferentially into a horizontal band at 22° from the sun.
Ice has a wavelength-dependent refractive index (dispersion), so red light is refracted less than blue. At the inner edge of the 22° halo, red appears first and blue appears just outside it, producing the characteristic reddish inner boundary. Stronger dispersion in pyramidal crystals creates more colourful halo arcs.
Yes. Lunar halos form by exactly the same mechanism as solar halos but appear around the moon. Because moonlight is much dimmer, lunar halos are less vivid and rarely show colour, but their geometry (22°, circumzenithal arc, etc.) is identical to solar halos.
This is a Monte-Carlo ray-tracer, not a canned image: thousands of light rays are launched at random incidence angles into a hexagonal ice-crystal prism every frame, refracted twice by Snell's law, and accumulated onto the sky wherever they land. The 22° halo, sun dogs and circumzenithal arc all emerge from that single physical model — nothing is drawn directly, only wherever enough refracted rays pile up.
Sunlight refracting through the 60° or 90° faces of a hexagonal ice-crystal prism, with red bending least and violet most (dispersion) to give the halo's coloured inner edge. Randomly tumbling column crystals spread rays into a full 22° ring, while horizontally floating plate crystals concentrate them into sun dogs either side of the sun.
Pick a Feature to isolate the 22° halo, sun dogs, the fainter 46° halo, or the circumzenithal arc (visible only below 32° sun altitude), or leave All features on. Drag Sun altitude (0-70°) to watch the CZA appear and sun dogs shift, raise Rays per frame for a cleaner image, and untick Dispersion to see the halo turn white.
The 22° radius is not arbitrary — it is the minimum deviation angle for a 60° ice prism at ice's refractive index of about 1.31. Because rays deviated by less than 22° are turned back by total internal reflection rather than exiting the crystal, the sky genuinely stays darker inside the halo than outside it.
Randomly oriented column crystals refract light at every azimuth around the sun, but the deviation angle for a given ray has a hard minimum near 21.8°. Rays cluster at that minimum-deviation edge, so brightness peaks in a ring at 22° and drops sharply just inside it, while fading gradually further out.
Sun dogs form from plate-shaped crystals that fall with their broad hexagonal face nearly horizontal, like leaves settling in still air. That preferred orientation restricts the refracted light to a narrow horizontal band at the sun's altitude, concentrating it into two bright patches rather than spreading it into a full ring.
The CZA forms when light enters a plate crystal's horizontal top face and exits a vertical side face at close to 90° geometry, and this ray path becomes geometrically impossible once the sun climbs above about 32°. The simulation enforces that same altitude cut-off, so raising Sun altitude past that point makes the arc vanish.
Ice's refractive index depends slightly on wavelength, so violet light bends more than red (dispersion). At the halo's sharp inner edge this produces a visible reddish fringe, while the outer edge fades so gradually that rays of every colour overlap and wash the colour out to white.
Each traced ray is a genuine Monte-Carlo sample of a random incidence angle, deposited onto the sky image and accumulated over time. More rays per frame converge on the true halo brightness pattern faster and with less speckle noise, at the cost of more computation each animation frame.