FIRE RAINBOWS

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 as it hung over northern Idaho near the Washington State border (map of Idaho).

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° 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, according to the London Daily Mail.

A circumhorizontal arc or circumhorizon arc (CHA), also known as a fire rainbow, is a halo or an optical phenomenon similar in appearance to a horizontal rainbow, but in contrast caused by the refraction of light through the ice crystals in cirrus clouds.

It occurs only when the sun is high in the sky, at least 58° above the horizon, and can only occur in the presences of cirrus clouds. It can thus not be observed at locations north of 55°N, except occasionally from mountains.^[1]

The phenomenon is quite rare because the ice crystals must be aligned horizontally to reflect the high sun. The arc is formed as light rays enter the horizontally-oriented flat hexagonal crystals through a vertical side face and exit through the horizontal bottom face. It is the 90° inclination that produces the well-separated rainbow-like colours and, if the crystal alignment is just right, make the entire cirrus cloud shine like a flaming rainbow.^[2][1]

A circumhorizontal arc can be confused with an infralateral arc when the sun is high in the sky; the former is however always oriented horizontally where the latter is oriented as a section of a rainbow, e.g. as an arc stretching upwards from the horizon.^[2]

One particularly fine example was photographed over northwestern Idaho on June 3, 2006, and was reported in both the New Scientist^[3] and the Daily Mail (the latter under the caption "flaming rainbow"). As the event was eventually featured on National Geographic News^[4], the news quickly spread over the internet.