A circumhorizontal arc, sometimes called a ‘fire rainbow,’ is a stunning optical phenomenon that appears as a bright, multicolored band parallel to the horizon. Unlike true rainbows, these arcs form when sunlight refracts through high-altitude ice crystals in cirrus clouds.
Key characteristics include:
- Only occurs when the sun is very high in the sky (above 58 degrees)
- Requires specific hexagonal ice crystal orientation
- Colors appear in the same order as a rainbow but are typically more vibrant
- Most commonly seen during summer months
While often confused with rainbows, circumhorizontal arcs differ because they’re caused by ice crystal refraction rather than water droplet reflection and refraction.