Sunday, March 13, 2011
A Fire Rainbow Over Whiting, New Jersey, USA in late May 2008.
Credit & Copyright: Paul Gitto (Arcturus Observatory)
Explanation: What is that inverted rainbow in the sky? Sometimes known as a fire rainbow for its flame-like appearance, a circumhorizon arc is created by ice, not fire. For a circumhorizon arc to be visible, the Sun must be at least 58 degrees high in a sky where cirrus clouds are present. Furthermore, the numerous, flat, hexagonal ice-crystals that compose the cirrus cloud must be aligned horizontally to properly refract sunlight like a single gigantic prism. Therefore, circumhorizon arcs are quite unusual to see.
Summertime Halo: A colourful circumhorizon arc spans the sky near Fredericton, New Brunswick, Canada. The halo lights brightest where the cirrus is thickest. Note the enormous size of the halo and its pure spectral colours.
Imaged by Marc Sorensen in the summer of 2003. Image ©Marc Sorensen.
Rare "Rainbow" Spotted Over Idaho - It looks like a rainbow that's been set on fire, but this phenomenon is as cold as ice.
Known in the weather world as a circumhorizontal arc, this rare sight was caught on film on June 3, 2006, as it hung over northern Idaho near the Washington State border. The arc isn't a rainbow in the traditional sense - it is caused by light passing through wispy, high-altitude cirrus clouds. The sight occurs only when the sun is very high in the sky (more than 58-degree above the horizon). What's more, the hexagonal ice crystals that make up cirrus clouds must be shaped like thick plates with their faces parallel to the ground.
When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus's crystals are aligned just right, the whole cloud lights up in a spectrum of colors.
This particular arc spanned several hundred square miles of sky and lasted for about an hour.
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