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A rare rainbow of rare beauty

发布时间:2019-02-28 14:01:03来源:未知点击:

By Fred Pearce (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,