The so-called “fire rainbow” creates one of the most stunning visual phenomena in the atmosphere. This rare event appears as a brilliant, pastel-colored band stretched low across the sky, often looking like a piece of a rainbow placed horizontally within a wispy cloud. Despite its common name, this display is purely an optical illusion that involves neither fire nor the liquid water droplets that form a true rainbow.
The Circumhorizontal Arc
The correct scientific term for this atmospheric event is the circumhorizontal arc (CHA). It is classified as a type of ice halo, not a rainbow. The phenomenon is not caused by rain or moisture low in the atmosphere; the colors result from light interacting with ice crystals at very high altitudes. When fully formed, the arc presents as a broad, bright band of color running parallel to the horizon, positioned far below the sun’s location in the sky.
The color spectrum is often purer and more saturated than in a typical rainbow. Red consistently appears on the uppermost edge and violet on the bottom edge closest to the horizon. This horizontal orientation and fixed color arrangement distinguish it from other optical phenomena like iridescence or sun dogs.
Physics of Formation
The formation of the circumhorizontal arc requires a precise combination of atmospheric conditions and solar geometry. The process begins with the presence of cirrus or cirrostratus clouds, which are high-altitude clouds composed entirely of ice crystals. The unique shape and orientation of these crystals are what separate the white sunlight into its constituent colors.
Specifically, the clouds must contain flat, hexagonal plate-shaped ice crystals. These crystals must be suspended in the air with their broad, flat faces oriented almost perfectly parallel to the ground. Sunlight is then required to enter one of the crystal’s vertical side faces and exit through the bottom horizontal face. This specific 90-degree pathway causes the light to refract, or bend, similar to how a glass prism works.
This refraction separates the different wavelengths of light, creating the vivid spectrum of colors visible to an observer below the cloud. The separation of colors is particularly distinct due to the large angle the light travels through the crystal. The final and most limiting physical requirement is the sun’s altitude. The sun must be positioned at least 58 degrees above the horizon for the light to enter the crystals at the necessary angle to produce the arc.
Geographic and Temporal Viewing Requirements
The stringent requirement for the sun to be at an elevation of 58 degrees or greater places significant geographical and temporal limitations on where and when the circumhorizontal arc can be seen. Due to the angle of the sun’s path, this phenomenon is impossible to observe at latitudes north of approximately 55 degrees North or south of 55 degrees South. In these polar regions, the sun never climbs high enough in the sky, even during the peak of summer.
For observers in the mid-latitudes, the arc is primarily a summertime event, as this is the only season when the sun reaches the required height around midday. For example, in locations near the 55-degree latitude line, the sun may only be high enough for a few hours around the summer solstice. The rarity of the CHA is tied directly to the need for high-altitude ice clouds to coexist with the specific solar angle.