The Fire Rainbows (Circumhorizontal Arcs)
How These Rare Optical Phenomena Form in the Sky
A “fire rainbow,” or circumhorizontal arc, is a rare optical phenomenon where sunlight passes through ice crystals in high-altitude cirrus clouds, creating bands of vivid colors across the sky. Unlike typical rainbows that form after rain, circumhorizontal arcs occur when the sun is very high—higher than 58 degrees above the horizon—and its rays hit plate-shaped ice crystals at just the right angle.
These atmospheric displays can appear as shimmering, horizontal streaks of rainbow colors that sometimes stretch across large portions of the sky. While they are sometimes mistaken for ordinary rainbows, circumhorizontal arcs are entirely different in how they form and look, making their appearance both fascinating and unique.
What Are Fire Rainbows (Circumhorizontal Arcs)?
Fire rainbows, or circumhorizontal arcs, are vibrant optical phenomena that occur under specific atmospheric conditions. Understanding what sets them apart from traditional rainbows and similar ice halos helps clarify their unique appearance and causes.
Definition and Key Characteristics
A fire rainbow is formally known as a circumhorizontal arc. It is a type of ice halo, which forms when sunlight passes through hexagonal ice crystals in cirrus or cirrostratus clouds high in the atmosphere.
For a circumhorizontal arc to appear, the sun must be higher than 58° above the horizon. The ice crystals act like tiny prisms, refracting and dispersing sunlight to create a band of bright, rainbow-like colors that runs parallel to the horizon.
These arcs can span large portions of the sky and often show intense red, orange, yellow, green, blue, and violet hues. Unlike typical rainbows, their colors are strongly saturated, and the arc is often sharply defined.
How They Differ from Traditional Rainbows
Fire rainbows are distinct from traditional rainbows in both their formation and appearance. While typical rainbows result from refraction and reflection through raindrops, circumhorizontal arcs form via the refraction of light passing horizontally through flat, plate-shaped ice crystals.
Traditional rainbows appear as full or partial circles centered opposite the sun and occur regardless of the sun’s height. By contrast, circumhorizontal arcs only form when the sun is high in the sky, creating a horizontal band of color below the sun.
The color sequence in circumhorizontal arcs is similar to a rainbow, but the arc is much broader and often more striking in hue due to the uniform alignment of the ice crystals. This alignment gives the phenomenon its characteristic parallel-to-horizon shape.
Comparison With Other Atmospheric Phenomena
Circumhorizontal arcs are often confused with other atmospheric phenomena, especially other ice halos and cloud iridescence. The most common confusion occurs with 22° halos or sundogs (parhelia), which are also caused by ice crystals but have different shapes and positions relative to the sun.
Unlike circumhorizontal arcs, halos and sundogs generally appear as circles or bright spots around the sun. Additionally, cloud iridescence, which produces colorful patches in clouds, can occur in a variety of sky positions and usually has a softer, less structured appearance.
Table: Key Differences Among Optical Phenomena
Phenomenon Caused by Required Sun Height Appearance Circumhorizontal arc Ice crystals Sun > 58° above horizon Horizontal, rainbow-colored arc Traditional rainbow Raindrops Any Circular arc opposite sun Sundog (Parhelion) Ice crystals Any Bright spots beside sun Cloud iridescence Water droplets Any Irregular, colorful patches
Formation of Circumhorizontal Arcs
Circumhorizontal arcs, often called fire rainbows, appear under very specific atmospheric conditions. Their formation relies on the structure of ice crystals, the type of clouds present, the position of the Sun, and the effect of light refraction related to latitude.
Role of Plate-Shaped Ice Crystals
Plate-shaped ice crystals are the primary factor in the formation of circumhorizontal arcs. These hexagonal plates, typically found in the upper atmosphere, have flat surfaces that can precisely align horizontally as they float inside cirrus clouds.
The horizontal alignment of these crystals is crucial. When sunlight enters the vertical side face of these plate-shaped ice crystals and exits through the bottom, the light undergoes refraction at a set angle. This process, much like light passing through a prism, breaks sunlight into its various wavelengths, producing a spectrum of colors.
The more uniform the orientation of the crystal plates, the clearer and more intense the circumhorizontal arc appears across the sky.
Importance of Cirrus Clouds
Most circumhorizontal arcs are found in high-altitude cirrus or cirrostratus clouds. These clouds contain millions of small, plate-shaped ice crystals suspended in very cold air, usually above 20,000 feet (6,000 meters).
Cirrus clouds often form during fair summer weather, but can occur any time conditions at high altitudes allow. Their wispy, thin composition lets sunlight pass through with minimal scattering, maximizing the refraction process.
If the clouds are too thick or composed mainly of different crystal shapes, the arc will not form or will appear faint. Only cirrus clouds with a high concentration of horizontally-aligned plates provide the right setting for a visible and vivid circumhorizontal arc.
Influence of Sunlight and Refraction
Sunlight’s path through the plate-shaped ice crystals leads to the characteristic rainbow-like colors. The refraction occurs when sunlight enters one side of a crystal and exits another, bending the light similarly to a prism.
This bending separates sunlight into individual wavelengths—red, orange, yellow, green, blue, indigo, and violet—creating the distinct band of colors seen in the sky. The process is effective only when sunlight is strong and direct, so fire rainbows are more common on clear, sunny days.
A circumhorizontal arc typically runs parallel to the horizon, making it appear as a vivid, horizontal band of color across the sky beneath the Sun.
Required Angle and Latitude Conditions
A critical requirement for circumhorizontal arcs is the altitude of the Sun. The Sun must be higher than 58 degrees above the horizon for the correct refraction angle. This angle ensures that sunlight passes through plate-shaped crystals in the necessary orientation.
Because of this requirement, these arcs are only possible at certain times of year and at certain latitudes. They are most often seen in the summer when the Sun reaches its highest point in the sky, but only in regions between roughly 55° North and 55° South latitude.
Locations nearer to the poles rarely see them, since the Sun never rises high enough for the angle needed. The weather must also be cooperative, with high cirrus cloud coverage and minimal lower clouds to obscure the phenomenon.
Visual Appearance and Colors
Fire rainbows, more accurately known as circumhorizontal arcs, display some of the most distinct and vivid colors seen in atmospheric optics. Their appearance, shape, and sky position make them notably different from common halos and ordinary rainbow clouds.
Color Spectrum and Intensity
Circumhorizontal arcs show a brilliant spectrum, frequently described as more intense than many other optical sky phenomena. The colors range smoothly from red at the upper edge to violet at the lower edge, mimicking the order seen in a classic rainbow.
The intensity comes from the way sunlight refracts through flat, hexagonal ice crystals at high altitudes. These crystals act much like tiny prisms, dispersing sunlight very effectively.
In clear conditions, observers can often pick out separate bands of red, orange, yellow, green, blue, and violet. The sharpness and definition of these colors can be striking, especially when compared to the pastel tones sometimes seen in other cloud iridescence.
Shape and Position in the Sky
A circumhorizontal arc appears as a long, flat band or arc running parallel to the horizon. It is typically located well below the sun, which must be at least 58 degrees above the horizon for the phenomenon to be visible.
This arc can stretch across large portions of the sky, sometimes appearing beneath cirrus or cirrostratus clouds. The arc can be up to several hand-widths long when viewed from the ground.
Unlike a circular halo, which surrounds the sun, the circumhorizontal arc is a straight or gently curved horizontal band. Its position and shape depend on both the sun’s elevation angle and the orientation of ice crystals in wispy, high-altitude clouds.
Difference from Halo and Rainbow Clouds
Despite its common nickname, the fire rainbow is not a true rainbow. It is also different from halos that often encircle the sun or moon. Halos are caused by a broader range of ice crystal orientations and often display less saturated colors.
Rainbow clouds—also known as cloud iridescence—occur when sunlight diffracts around much smaller water droplets or ice crystals, producing softer and more patchy color patterns. These colors can be irregular and pastel, lacking the well-ordered spectrum of a circumhorizontal arc.
Circumhorizontal arcs are unique because they require precise alignment of sunlight and flat, plate-like ice crystals. This requirement gives them a distinct visual signature, setting them apart from ordinary halos and iridescent clouds in both color sharpness and spatial structure.
Geographical Distribution and Rarity
Fire rainbows, or circumhorizontal arcs, are a rare atmospheric phenomenon influenced mainly by latitude, season, and specific weather conditions. They are more likely to be seen in certain places and times where these factors align, making them uncommon in many regions.
Best Locations to Observe
Circumhorizontal arcs are most visible in locations between roughly 30° and 55° latitude, particularly in mid-latitude regions like the southern United States and southern Europe. For example, cities such as Los Angeles see the phenomenon during the right conditions far more often than places located at higher latitudes, such as London.
Regions closer to the equator can experience more frequent sightings because the sun reaches the required elevation above the horizon (at least 58°). High-latitude cities like London almost never see fire rainbows because the sun does not get high enough in the sky. In contrast, cities like Miami and Los Angeles have a favorable sun angle during late spring and summer.
Visibility is generally better in areas with wide, unobstructed skies. Urban areas with frequent cloud cover or tall buildings might reduce the chance to witness the arc.
Seasonal and Weather Requirements
The visibility of fire rainbows peaks during summer months, when the sun reaches its highest point in the sky. The phenomenon requires the sun (or sometimes the moon) to be at least 58° above the horizon, which happens most often from late May to early August in mid-latitude regions.
Clear weather is essential because circumhorizontal arcs only appear when sunlight passes through plate-shaped ice crystals in high-altitude cirrus clouds. Days with thin, wispy cirrus clouds and little lower cloud cover have the best prospects. Overcast or rainy weather blocks visibility entirely.
Places like Ohio may see these arcs, but only occasionally. Summer afternoons, when cirrus clouds are present and the sun is high, give the highest probability, but even then, favorable conditions are brief and hard to predict.
Notable Sightings Around the World
Notable sightings have been recorded in a range of locations, particularly in the United States. Cities like Los Angeles, Dallas, and Miami have reported multiple fire rainbows during particularly warm summers. The phenomenon has also been photographed in Ohio, though such events remain rare.
Sightings in London and other northern European cities are exceptionally scarce. The required solar elevation simply does not occur frequently at those latitudes. The rarity increases moving northward, making documented occurrences important for meteorology and astronomy enthusiasts.
Occasionally, fire rainbows have been reported from unexpected locations under exceptional weather patterns. However, the common thread remains a combination of high sun, cirrus clouds, and the right geographic setting.
Related Optical Phenomena
Various atmospheric optical phenomena are caused by the interaction of sunlight with water droplets or ice crystals. These effects can often be mistaken for each other, but their formation mechanisms and appearances differ in key ways.
Halos and Ice Halos
Halos, including ice halos, form when sunlight or moonlight interacts with ice crystals suspended in the upper atmosphere, typically within cirrus or cirrostratus clouds. The most common example is the 22-degree halo—a luminous ring encircling the sun or moon at a radius of about 22 degrees.
This effect occurs due to the refraction and reflection of light as it passes through the hexagonal ice crystals. The orientation and type of crystal determine the shape and intensity of the halo. Variations include parhelia (sun dogs) and circumzenithal arcs, both resulting from different angles of refraction.
Below is a simple comparison:
Phenomenon Cause Typical Appearance 22-degree halo Ice crystal refraction Circular ring Sun dogs Plate ice crystals Bright spots beside sun Circumzenithal arc Horizontally oriented ice crystals Upside-down rainbow
These halos differ from circumhorizontal arcs primarily by the required solar elevation and the direction of light passage through the crystals.
Double Rainbows and Their Causes
Double rainbows are a striking optical phenomenon resulting from sunlight interacting with water droplets in the atmosphere. They are produced when light is refracted, then internally reflected twice within a raindrop before exiting and reaching the observer's eyes.
The primary rainbow appears brightest and is located on the inner side, displaying the usual color order from red on the outside to violet on the inside. The secondary rainbow comes from the second internal reflection, appearing outside the primary arc with reversed color order—red on the inside, violet on the outside.
Double rainbows are less intense than single rainbows due to additional internal reflection leading to more light loss. The sky between the two bows, called Alexander’s band, often appears noticeably darker because of the way light is redirected away from this region. Unlike halos, rainbows and double rainbows depend on liquid water droplets rather than ice crystals.
Misconceptions and Common Errors
The term "fire rainbow" can be misleading and often causes confusion about what circumhorizontal arcs truly are. Many people mistake these phenomena for more common sights in the sky, such as airplane contrails or different types of clouds.
Fire Rainbows vs. Contrails
One frequent mistake is confusing circumhorizontal arcs with contrails left by airplanes. Contrails are formed when water vapor from aircraft engines condenses and freezes at high altitudes. These streaks are typically straight, thin, and white, lacking any spectral color separation or vibrant hues.
A circumhorizontal arc, or fire rainbow, forms in cirrus clouds when sunlight passes through ice crystals at just the right angle—usually when the sun is very high in the sky (58 degrees or more above the horizon). This angle is critical for the appearance of the vivid, rainbow-like colors.
In contrast, contrails appear regardless of sun position and never display the spectrum of colors seen in circumhorizontal arcs. The presence of a broad, multicolored band parallel to the horizon, rather than a line or streak, is a key distinguishing feature.
Feature Contrail Circumhorizontal Arc Shape Straight, thin line Broad, horizontal band Colors White or grey Vivid, rainbow-like colors Timing Any time, any sun angle Only with high sun, cirrus
Distinguishing from Altocumulus and Other Clouds
Some skywatchers mistakenly attribute the bright, iridescent colors of circumhorizontal arcs to altocumulus or other common cloud types. Altocumulus clouds are usually white or grey, and appear as puffy blobs or layers, often lacking the specific orientation and iridescence of circumhorizontal arcs.
Rainbows and “rainbow clouds” are also sometimes confused with circumhorizon arcs. Unlike typical rainbows, which arise from water droplets after rain, circumhorizontal arcs are linked to ice crystals in thin cirrus clouds at great altitudes.
Visual clues help differentiate these phenomena. Circumhorizontal arcs are always seen well below the sun, running parallel to the horizon. If colors are seen dispersed across puffy or patchy clouds, especially at lower altitudes, it is likely a different optical effect, such as cloud iridescence or a fragment of another arc, not a circumhorizontal arc.
The Science and Optics Behind Fire Rainbows
Fire rainbows, or circumhorizontal arcs, occur due to specific interactions between sunlight and ice crystals in the upper atmosphere. Their appearance is shaped by the way light is refracted and dispersed, which depends on both physical and atmospheric variables.
The Prism Effect and Light Dispersion
When sunlight encounters high-altitude cirrus clouds, it passes through plate-shaped ice crystals. These ice crystals act like tiny prisms.
Light entering the side of a crystal is refracted, or bent, and then dispersed into its individual spectral colors. This dispersion separates the colors much like visible light passing through a glass prism.
For a circumhorizontal arc to form, sunlight must hit the crystals at a particular angle—usually when the sun is above 58° in the sky. This precise angle allows for optimal refraction, so vivid bands of red, orange, yellow, green, blue, and violet become visible.
Each color seen in a fire rainbow emerges at a slightly different angle due to its wavelength. This combination of refraction and dispersion produces the bright, horizontally oriented arc typical of a circumhorizontal arc.
Role of Wavelengths and Atmospheric Conditions
The various colors in a fire rainbow arise because different wavelengths of sunlight bend at slightly different angles. Shorter wavelengths (like blue and violet) are bent more than longer ones (like red and orange). This separation by wavelength produces the distinct, rainbow-like spectrum.
Atmospheric conditions are crucial. Ice crystals must be thin, flat, and hexagonal to refract sunlight properly. If the crystals are irregular or conditions are not ideal, the arc may not form or can appear faint.
The phenomenon only occurs in the presence of cirrus or cirrostratus clouds at high altitudes—typically 20,000 feet or more. The clarity of the sky also matters, as excessive cloud cover can obscure the arc. This rarity of conditions explains why fire rainbows are unusual and often brief.
Appreciation and Photography Tips
Observing fire rainbows requires the right mix of weather, time, and sky conditions. Photographing them introduces unique challenges, but with the correct approach, anyone can document these rare displays of nature.
Best Practices for Viewing
Fire rainbows—circumhorizontal arcs—are most visible when the sun is high in the sky, typically above 58 degrees. They often appear in summer months and require the presence of cirrus clouds made of ice crystals.
Weather plays a crucial role: clear, sunny days boost the chances of spotting these arcs. Locations with unobstructed views of the sky, such as open fields or hilltops, provide the best vantage points.
For improved visibility, use polarized sunglasses to enhance the intensity of colors. It helps to check local weather forecasts for the presence of high-altitude ice crystal clouds. Be patient, as fire rainbows are rare and may not last long.
Capturing Fire Rainbows in Nature
Camera settings are vital for photographing these sky phenomena. A DSLR or mirrorless camera with a wide-angle lens captures more of the arc and surrounding clouds. Use a low ISO (100-200) to reduce image noise.
Adjust exposure settings carefully. Slightly underexpose to avoid washing out the colors. Using a tripod increases stability, especially in breezy weather.
A circular polarizing filter helps intensify the colors and reduce glare. Automatic white balance usually works, but manual adjustments can further emphasize the blues and reds.
Photographers should avoid shooting directly at the sun to protect both eyes and camera sensors. Compose shots with interesting landscape elements, such as trees or mountain ridges, to add context and scale to the phenomenon.
