A rare rainbow, called a circumhorizontal arc, presents itself to the skies of Idaho (Image: Brian Plonka/WPN)

Noon, Saturday 3 June. Over the fields of northern Idaho, a rare and beautiful sight could be seen. Known as a circumhorizontal arc, it forms as sunlight is refracted through hexagonal ice crystals in cirrus clouds several kilometres above the ground. Sunlight enters a near-vertical face of each crystal and leaves from a horizontal face at the bottom. “Effectively the crystals act as a 90-degree prism for the passing rays,” says Evelyn Hesse of the light-scattering group at the University of Hertfordshire in …