Few things stop people in their tracks quite like a vivid arc of color stretching across a stormy sky, or the ghostly twin suns flanking the horizon at dawn. These atmospheric light phenomena — rainbows, halos, and sundogs — are far more than beautiful accidents. They are the result of precise physics, playing out in real time across the atmosphere above us.
This article explores the science, structure, and significance of these three phenomena: what causes them, how they differ from one another, and why understanding them deepens appreciation for the natural world. Whether you spotted a double rainbow on your morning commute or noticed strange glowing patches beside the winter sun, this guide will give you the language and knowledge to make sense of what you saw.
The Physics of Light in the Atmosphere
Before diving into each phenomenon individually, it helps to understand the two fundamental optical processes at work: refraction and reflection.
Refraction occurs when light passes from one medium into another — say, from air into a water droplet — and bends as a result. Different wavelengths of light bend at slightly different angles, which is why white sunlight can be separated into its component colors. Reflection, on the other hand, occurs when light bounces off an internal surface without passing through it.
A third process, diffraction, also plays a role in some atmospheric optics, though it is less central to the three phenomena discussed here. What all three share is their dependence on tiny particles suspended in the atmosphere — water droplets, ice crystals, or both — acting as miniature optical instruments.
The specific shape, size, and orientation of these particles determines which phenomenon appears in the sky. This is why rainbows, halos, and sundogs look so different from one another despite sharing the same basic raw material: sunlight.
Rainbows: Refraction and Reflection in Raindrops
A rainbow forms when sunlight enters a spherical water droplet, reflects off the droplet’s inner surface, and exits at a specific angle. The refraction that occurs upon entry and exit separates white light into its spectral colors, producing the familiar arc of red, orange, yellow, green, blue, indigo, and violet.
The primary rainbow appears at an angle of approximately 42 degrees from the antisolar point — the point directly opposite the sun relative to the observer. Red light, which bends the least, appears on the outer edge of the arc; violet, which bends the most, appears on the inner edge.
The Double Rainbow
A secondary rainbow sometimes appears outside the primary arc, at roughly 51 degrees from the antisolar point. This second bow forms when light undergoes two internal reflections inside the droplet rather than one. Because of this additional reflection, the colors in the secondary rainbow are reversed — violet on the outside, red on the inside — and the bow appears noticeably dimmer than the primary.
The dark band between the two bows is known as Alexander’s dark band, named after Alexander of Aphrodisias, who described it in approximately 200 CE. Light that would otherwise illuminate this region is redirected into the two bows, leaving the band comparatively dark.
Why Rainbows Are Always Circular
Rainbows are, in fact, full circles — not arcs. The ground typically obscures the lower portion, which is why ground-level observers usually see only a semicircle. From an aircraft or elevated vantage point, however, a complete circular rainbow becomes visible. The center of the circle always aligns with the antisolar point, which means the sun must always be behind the observer for a rainbow to be seen.
Halos: Ice Crystal Optics in the Upper Atmosphere
Halos are optical phenomena produced not by liquid water droplets, but by hexagonal ice crystals suspended in cirrus clouds high in the upper troposphere, typically at altitudes between 5 and 10 kilometers. These crystals act as tiny prisms, refracting and sometimes internally reflecting sunlight to produce luminous rings, arcs, and spots around the sun or moon.
The most commonly observed halo is the 22-degree halo — a bright, slightly colored ring that appears to encircle the sun or moon at an angular radius of approximately 22 degrees. This halo forms when light enters one face of a hexagonal ice crystal and exits through another face inclined at 60 degrees. The minimum deviation angle for this geometry is close to 22 degrees, which is why the halo’s inner edge is sharp and often tinged with red, while the outer edge fades gradually into white.
The 46-Degree Halo and Other Halo Variants
A rarer and larger halo, the 46-degree halo, forms when light passes through the top face of an ice crystal and exits through a side face, producing a wider ring around the sun. Because it requires a specific crystal orientation and is considerably dimmer, the 46-degree halo is less frequently observed and often mistaken for atmospheric haze.
Beyond circular halos, the ice crystal optics system produces a remarkable variety of arcs and features, including:
- Circumzenithal arc: A colorful, upward-curving arc near the zenith, often described as an “upside-down rainbow.” It forms when sunlight passes through the horizontal top faces of plate-shaped ice crystals and exits through vertical side faces.
- Parhelic circle: A white horizontal ring at the same altitude as the sun, produced by internal reflection within ice crystals.
- Upper tangent arc: An arc that touches the top of the 22-degree halo, formed by pencil-shaped ice crystals with horizontal orientations.
The sheer variety of halo phenomena reflects the enormous complexity of ice crystal shapes and orientations found in cirrus clouds. Photographers and atmospheric optics enthusiasts frequently catalog these displays, as particularly rich halo events can produce dozens of distinct features simultaneously.
Sundogs: The Sun’s Luminous Companions
Sundogs — known formally as parhelia (singular: parhelion) — are bright, often colorful spots of light that appear to either side of the sun, at the same altitude on the horizon. They typically appear at an angular distance of approximately 22 degrees from the sun, placing them at the same position as the 22-degree halo’s left and right edges, and are indeed related to the same optical mechanism.
Sundogs form when plate-shaped hexagonal ice crystals drift through the atmosphere with their flat faces oriented horizontally. Sunlight enters through one vertical side face and exits through another, refracting at the 22-degree minimum deviation angle. Because the crystals are aligned — rather than randomly oriented as in the case of the circular halo — the refracted light concentrates at specific points to either side of the sun rather than forming a complete ring.
The Appearance and Colors of Sundogs
Sundogs are often vivid and striking. The inner edge, closest to the sun, typically displays red or orange coloration, while the outer edge grades through yellow, green, and sometimes blue — a result of the differential refraction of light wavelengths. In particularly well-developed displays, sundogs extend outward into bright white tails that merge into the parhelic circle.
Sundogs are most visible when the sun is low on the horizon — at sunrise or sunset — and are especially common in cold climates where ice crystal clouds frequently form at low altitudes. They have been observed and recorded throughout human history, appearing in ancient Chinese texts, Norse mythology, and Renaissance paintings. A famous historical sighting occurred in 1535 over Stockholm, Sweden, and was commemorated in a painting now considered one of the earliest known depictions of atmospheric optics.
Comparing Rainbows, Halos, and Sundogs
While all three phenomena involve sunlight interacting with particles in the atmosphere, several key differences distinguish them from one another.
|
Feature |
Rainbow |
Halo |
Sundog |
|---|---|---|---|
|
Particle type |
Liquid water droplets |
Hexagonal ice crystals |
Plate-shaped ice crystals |
|
Primary process |
Refraction + reflection |
Refraction (+ reflection) |
Refraction |
|
Angular size |
~42° from antisolar point |
~22° or ~46° from sun |
~22° to either side of sun |
|
Observer position |
Sun must be behind observer |
Sun in front of observer |
Sun in front of observer |
|
Color order |
Red outer, violet inner (primary) |
Red inner, fades outward |
Red inner, grades outward |
|
Typical conditions |
Rain or mist |
High cirrus clouds |
Cold, icy atmosphere |
Understanding these distinctions makes it considerably easier to identify a phenomenon when it appears. A glowing ring around the moon on a winter night is almost certainly a 22-degree halo. A vivid arc of color after a summer storm is a rainbow. Two bright, colorful patches flanking a low winter sun are sundogs.
The Cultural and Scientific Significance of Atmospheric Optics
These phenomena have captivated human observers for millennia, long before the physics underpinning them was understood. René Descartes published the first mathematical explanation of the rainbow in 1637, using the principles of geometric optics to calculate the 42-degree angle of the primary bow. Isaac Newton subsequently demonstrated that white light is composed of multiple colors, providing the framework for understanding spectral separation.
The systematic study of halos and sundogs advanced considerably through the work of 17th- and 18th-century natural philosophers, and continued into the 20th century with the development of computational modeling. Today, researchers use software to simulate the full range of halo displays produced by different crystal orientations, helping to predict and identify rare arcs.
Culturally, these phenomena have served as omens, portents, and symbols across diverse traditions. Viking navigators reportedly used sundogs and polarized light from the sky to navigate under overcast conditions. Halos around the sun or moon were frequently interpreted as harbingers of weather change — an intuition with some validity, since cirrus clouds often precede incoming weather systems.
Observing and Photographing These Phenomena
For those interested in observing atmospheric optics firsthand, a few practical considerations improve the experience considerably.
For rainbows, position yourself with the sun behind you and a rain shower or mist in front. Late afternoon, with a low sun angle and ongoing precipitation, provides ideal conditions. A wide-angle lens or camera with a broad field of view captures the full arc more effectively.
For halos and sundogs, scan the sky around the sun on days when thin, wispy cirrus clouds are visible. Never look directly at the sun without appropriate eye protection. Instead, block the solar disk with a hand or building edge and observe the surrounding region. Polarized sunglasses can sometimes reduce glare and make fainter halo features more visible.
Citizen science platforms such as the Atmospheric Optics website (atoptics.co.uk) and associated observer networks invite members of the public to submit photographs and sightings, contributing to a growing global dataset of these phenomena.
The Enduring Wonder of Light and Atmosphere
Rainbows, halos, and sundogs are reminders that the atmosphere is not merely a backdrop to daily life, but an active, dynamic optical environment. The same sunlight that illuminates a street or warms a windowsill is simultaneously being refracted, reflected, and redirected by billions of tiny water droplets and ice crystals overhead.
Learning to recognize these phenomena — and to understand why they appear where they do — transforms ordinary sky-watching into something closer to reading a language. A 22-degree halo suggests cirrus clouds at altitude. A sundog at dawn hints at cold, crystalline air near the surface. A bright primary rainbow means the sun is at your back and rain is ahead.
That knowledge does nothing to diminish the beauty of what you see. If anything, it deepens it.