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 Hatfield, UK.
Circumhorizontal arcs occur when the sun is at more than 58 degrees above the horizon. "They are most unusual, and this is an impressive example," Hesse says.
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