A cloud over West Virginia looked like it had caught fire, but the effect came from ice.
NASA’s Astronomy Picture of the Day features “A Fire Rainbow over West Virginia,” a photograph taken near North Fork Mountain by Christa Harbig.
The APOD explanation says ice crystals in a distant cirrus cloud were “acting like little floating prisms.” The effect is known informally as a fire rainbow for its flame-like appearance, but it is a circumhorizon arc that appears parallel to the horizon.
According to the APOD text, a circumhorizontal arc can only be visible when “the Sun must be at least 58 degrees high” and cirrus clouds are present below, in this case “cirrus fibratus.”
It says the “numerous, flat, hexagonal ice-crystals” that make up the cloud must be aligned horizontally so they can refract sunlight in “a collectively similar manner.”
Because of those conditions, APOD says circumhorizontal arcs are “somewhat unusual to see.”
The image credit is listed to Christa Harbig. The APOD entry was written by Robert Nemiroff, Jerry Bonnell, Cecilia Chirenti and Keighley Rockcliffe.
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