Fire rainbows, technically called circumhorizontal arcs, aren’t actually rainbows or related to fire. They occur when sunlight interacts with high-altitude cirrus clouds containing hexagonal ice crystals.
These spectacular displays require specific conditions:
- The sun must be at least 58 degrees above the horizon
- Cirrus clouds containing plate-shaped ice crystals must be present
- The ice crystals must be oriented horizontally
When sunlight enters through a vertical side face of the crystal and exits through the bottom face, it separates into different colors, creating a brilliant horizontal spectrum that can stretch across the sky. They’re most commonly seen during summer months in mid-latitude regions.