The sky occasionally reveals a rare and captivating atmospheric optical display known informally as a “fire rainbow.” This phenomenon often draws immediate attention due to its striking visual appeal, presenting a vibrant, colorful band stretched across the horizon. Despite its common name, this celestial spectacle is neither fire nor a true meteorological rainbow, sparking curiosity about its actual origins.
Unveiling the “Fire Rainbow”
Scientifically termed a circumhorizontal arc, this optical phenomenon appears as a bright, spectrum-colored band running parallel to the horizon. Its full form displays distinct colors, with red at the top and violet at the bottom, creating an appearance reminiscent of a horizontal, fiery rainbow. The term “fire rainbow” is a misnomer; it is not caused by combustion or related to fire, nor is it a true rainbow, which forms when sunlight refracts through water droplets and always appears opposite the sun.
A circumhorizontal arc is categorized as an ice halo, distinguishing it from other atmospheric optical events. Unlike common 22-degree halos, which form a ring around the sun, a circumhorizontal arc is located significantly farther below the sun. It also differs from cloud iridescence, which results from light diffraction and shows randomly distributed colors, whereas a circumhorizontal arc displays pure, ordered spectral colors in a fixed position relative to the sun. Its unique characteristic is its distinct horizontal orientation, unlike the curved shape of a traditional rainbow or the inverted arc of a circumzenithal arc.
The Atmospheric Recipe for Formation
The formation of a circumhorizontal arc relies on a precise combination of atmospheric conditions. The sun must be very high in the sky, at an elevation of at least 58 degrees above the horizon. This high sun angle is essential for sunlight to enter the ice crystals at the necessary path. The arc often reaches its maximum intensity when the sun is around 68 degrees high.
The presence of high-altitude cirrus or cirrostratus clouds is another prerequisite. These clouds, which form at extremely low temperatures, are composed of tiny ice crystals rather than water droplets. These ice crystals must possess a specific hexagonal, plate-like shape.
These hexagonal plate-shaped ice crystals must be oriented flat and parallel to the ground as they drift through the atmosphere. Sunlight then enters one of the vertical side faces of these aligned crystals and exits through the near-horizontal bottom face. This 90-degree inclination causes the sunlight to refract and disperse into its constituent colors, much like a prism, creating the spectral display.
Where and When to Witness This Spectacle
Witnessing a circumhorizontal arc is a relatively rare event due to the precise atmospheric conditions required. The strict sun angle requirement means they are primarily observed in mid-latitudes, where the sun can reach the necessary elevation. For instance, these arcs are impossible to see in regions north of 55 degrees North or south of 55 degrees South latitude, as the sun never climbs high enough.
The phenomenon is more commonly reported in areas like much of the United States, where it can be seen several times during the summer months. In contrast, it is a much rarer sight in northern Europe, and cannot be observed north of locations like Copenhagen.
The optimal time for sightings is during the warmer months, particularly summer, when the sun’s path allows it to reach its highest point. Observing a circumhorizontal arc is best around midday, when the sun is at its peak elevation, aligning with the 58-degree minimum sun angle. The presence of wispy cirrus clouds at the right altitude is also necessary, as these clouds provide the ice crystals for light refraction.