During the summer, if you're very lucky, you may notice a cloud in the sky that looks just like a rainbow. These fire rainbows can occur when the sun has risen higher than 58° in the sky. There must be high altitude cirrus clouds with plate-shaped ice crystals, and sunlight has to enter the ice crystals at a specific angle. This is why fire rainbows is such a rare phenomenon. For those living at mid-latitudes, the best chance to see this phenomenon is in the middle of summer. The rarity of the event is highly dependent upon latitude and weather conditions. The potential for these arcs to form in Los Angeles is 5-10 times higher than in London.
“Fire Rainbows” are neither fire, nor rainbows,
but are so called because of their brilliant pastel colors and flame like
appearance. Technically they are known as circumhorizontal arc - an ice halo
formed by hexagonal, plate-shaped ice crystals in high level cirrus clouds. The
halo is so large that the arc appears parallel to the horizon, hence the name.
The position of the observer is also
important. Fire rainbows cannot be seen in locations north of 55°N or south of
55°S. Likewise there are certain times of the year when they are visible. For
example, in London, England the sun is only high enough for 140 hours between
mid-May and late July. While in Los Angeles, the sun is higher than 58 degrees
for 670 hours between late March and late September.
Fire rainbows should
not be confused with Iridescent clouds, which can produce a similar effect.
While fire rainbows occur only in cirrus clouds, iridescence often occur in
altocumulus, cirrocumulus clouds and lenticular clouds but very rarely in
cirrus clouds. Fire rainbows are so large that
sometimes we see only parts of them where they happen to 'light' fragments of
cirrus cloud. Circumhorizon arcs are also huge
that their colours sometimes appear to be those of the sky itself rather than
an ice crystal.
I think it’s so amazing and I wish I could see fire rainvows,
but it’s very rare soooooo… I think it’s hopeless. But these a few picture of
circumhorizon arc/fire rainbows, it’s cool, right?
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| Aletenjo, Portugal, 2006 |
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| Dublin, Ohio, May 2009 |
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| Switzerland, June 2007 |
