A circumhorizonal 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 degrees above the
horizon, and can only occur in the presences of cirrus clouds. It can thus not
be observed at locations north of 55 degrees N or south of 55 degrees S, except
occasionally at higher latitudes from mountains. To be visible the sun must be
at an elevation of 57.8 degrees (90 -32.2 degrees) or more and if cloud
conditions are right it is seen along the horizon on the same side of the sky as
the sun. It reaches its maximum intensity at a sun elevation of 67.9 degrees.
The phenomenon is quite rare because the ice crystals must be aligned
horizontally to refract 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 degrees inclination that
produces the well-separated rainbow-like colours and, if the crystal alignment
is just right, makes the entire cirrus cloud shine like a flaming rainbow.
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.
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 degrees above the
horizon, and can only occur in the presences of cirrus clouds. It can thus not
be observed at locations north of 55 degrees N or south of 55 degrees S, except
occasionally at higher latitudes from mountains. To be visible the sun must be
at an elevation of 57.8 degrees (90 -32.2 degrees) or more and if cloud
conditions are right it is seen along the horizon on the same side of the sky as
the sun. It reaches its maximum intensity at a sun elevation of 67.9 degrees.
The phenomenon is quite rare because the ice crystals must be aligned
horizontally to refract 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 degrees inclination that
produces the well-separated rainbow-like colours and, if the crystal alignment
is just right, makes the entire cirrus cloud shine like a flaming rainbow.
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.
