The circumhorizontal arc, also known as the fire rainbow, is a rare optical phenomenon that occurs in the Earth’s atmosphere. It is caused by the refraction, dispersion, and reflection of sunlight passing through ice crystals in high-altitude cirrus clouds.
Unlike a traditional rainbow, which forms when sunlight is refracted and reflected by water droplets in the atmosphere, the circumhorizontal arc occurs when the sun is high in the sky (at an elevation of 58 degrees or more) and the ice crystals in the cirrus clouds are shaped like flat hexagonal plates.
When sunlight enters these ice crystals, it undergoes a complex process of refraction and dispersion. The light is bent, or refracted, as it enters the crystal, then dispersed into its component colors due to the different wavelengths of light. This dispersion is similar to what happens in a prism, creating a spectrum of colors.
The dispersed light is then reflected internally within the ice crystal, bouncing off the inner surfaces of the crystal. Finally, the light exits the crystal and is refracted again, resulting in a concentrated beam of colored light that forms the circumhorizontal arc.
The arc appears as a long, colorful band that is parallel to the horizon. It is typically characterized by vibrant shades of red, orange, yellow, and green, with blue on the bottom. The colors are more intense and vivid than those of a traditional rainbow because of the more efficient dispersion of light in ice crystals compared to water droplets.
Circumhorizontal arcs are relatively rare because they require specific conditions to form: the sun must be high in the sky, typically at midday, and the cirrus clouds must contain horizontally oriented ice crystals. The arc can only be observed in locations between 55 degrees north and 55 degrees south latitude during specific times of the year.
Due to its striking appearance, the circumhorizontal arc is often mistaken for a cloud iridescence or a rainbow. However, it is important to note that the circumhorizontal arc is distinct in its formation and position in the sky. It is a fascinating natural phenomenon that captivates observers lucky enough to witness it.
