What is a circumhorizontal arc?
The circumhorizontal arc, often referred to as a “fire rainbow,” is a rare and breathtaking optical phenomenon that occurs in Earth’s atmosphere under very specific conditions.
Unlike a traditional rainbow, which forms when sunlight interacts with water droplets, this phenomenon is created by sunlight passing through ice crystals found in high-altitude cirrus clouds.
How the “fire rainbow” forms
The formation of a circumhorizontal arc involves a complex interaction of refraction, dispersion, and reflection of sunlight inside hexagonal ice crystals.
These crystals act like tiny prisms. As sunlight enters them, it bends and splits into its component colors, similar to how light behaves when passing through glass.
The light is then internally reflected within the crystal before exiting and bending once again, producing a vivid spectrum of colors that appears as a horizontal arc in the sky.
Why it looks different from a normal rainbow
Unlike standard rainbows, which are typically curved and formed by water droplets, circumhorizontal arcs appear as long, colorful bands parallel to the horizon.
Their colors are often more intense and vivid, displaying bright reds, oranges, yellows, and greens, with blue hues along the lower edge.
This increased intensity is due to the more efficient way ice crystals disperse light compared to water droplets.
Why it is so rare
Circumhorizontal arcs are relatively uncommon because they require very precise atmospheric conditions.
The sun must be positioned high in the sky, at an elevation of at least 58 degrees, and the cirrus clouds must contain flat, horizontally aligned ice crystals.
They can only be observed in regions between approximately 55° north and 55° south latitude and typically appear around midday during certain times of the year.
A spectacular but often misunderstood phenomenon
Due to its striking appearance, the circumhorizontal arc is often mistaken for a rainbow or cloud iridescence.
However, it is a distinct optical phenomenon with unique formation conditions and positioning in the sky.
For those lucky enough to witness it, the “fire rainbow” offers a stunning display of nature’s beauty and atmospheric physics.
