A Fire Rainbow over West Virginia

APOD: 2021 August 30 - A Fire Rainbow over West Virginia

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2021 August 30
The picture shows a solar circumhorizontal arc -- nicknamed a fire rainbow
-- over West Virgina, USA. 
Please see the explanation for more detailed information.

A Fire Rainbow over West Virginia
Image Credit: Christa Harbig

Explanation: What's happening to this cloud? Ice crystals in a distant cirrus cloud are acting like little floating prisms. Known informally as a fire rainbow for its flame-like appearance, a circumhorizon arc appears parallel to the horizon. For a circumhorizontal arc to be visible, the Sun must be at least 58 degrees high in a sky where cirrus clouds present below -- in this case cirrus fibrates. The numerous, flat, hexagonal ice-crystals that compose the cirrus cloud must be aligned horizontally to properly refract sunlight in a collectively similar manner. Therefore, circumhorizontal arcs are somewhat unusual to see. The featured fire rainbow was photographed earlier this month near North Fork Mountain in West Virginia, USA.

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