The ‘summertime halo’, the ‘fire rainbow’, the ‘circumhorizon’ or the ‘circumhorizontal arc’… Whichever of its names you decide call it, this enormous, rare, pastel-coloured optical effect will only appear up in the sky when several different factors happen to occur at the same moment.

Firstly, there needs to be the right sort of cloud around, for this is one of the many ‘halo phenomena’ that can only appear as the light shines through ice-crystal clouds, such as this fine Cirrus. The sun also has to be high in the sky (hence the ’summertime’ name); in fact, it needs to be at least 58° above the horizon (hence the ‘horizon’ name).

In order to refract the high sunlight in the right way, the ice crystals that make up the Cirrus cloud must be shaped like microscopic hexagonal plates. They must also be aligned more or less horizontally (hence the ‘horizontal’ name), for the colours are formed as light rays enter the clear crystals through a vertical side face and exit through their flat bases. So long as the crystals don’t wobble too much in the wind, the 90˚ inclination of these two sides results in pure rainbow-like colours. When the crystal alignment is just right, the entire cirrus cloud shines like a flaming rainbow (hence the ‘rainbow’ name, even though it is not, of course, a rainbow at all).

While most halos can be observed over any parts of the planet, this one is generally found only in those countries that lie below 55° latitude. North and south of this, the sun never rises above 57.8° in the sky. (That said, you might still be able to see the summertime halo in higher-latitude countries so long as you manage to climb a high mountain just as all the other requirements happened to coincide.)

Another example of the summertime halo was spotted by 11-year-old cloudspotter, Hugo, and can be seen in the gallery pages here.

Not only does this rare phenomenon have more than its fair share of names, it is also very beautiful. So while we are at it, why don’t we also start calling it the ‘beautiful arc’, the ‘arc of lovely colours’ and the ‘really rather delightful-looking halo’?

 

Current Cloud of the Month:
July 2010

Previous Clouds of the Month:
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
Cloud Reflections (December 09)
Numbers in the Clouds (November 09)
Sun Pillar (October 09)
Convection Clouds (September 09)
‘Pile d’Assiettes’ (August 09)
Cumulus congestus (July 09)
‘Asperatus’ (June 09)
Clouds at Night (May 09)
Sundogs (April 09)
Diamond Dust (March 09)
Cloud Streets (February 09)
Crepuscular Rays (Jan 09)
Valley Fog (December 08)
Cloud Shadows (November 08)
Contrails (October 08)
Mamma (September 08)
Kármán Vortex (August 08)
The Summertime Halo (July 08)
The Nor’west Arch (June 08)
Microbursts (May 08)
Irridescent Clouds (April 08)
Northern Lights – Aurora Borealis (March 08)
Ice halos (February 08)
Lightning (January 08)
Roll Cloud (December 07)
Banner Cloud (November 07)
Stratocumulus (October 07)
The Unclassified Cloud (September 07)
Alexander’s Dark Band (August 07)
Fumulus Snail (July 07)
Distrail (June 07)
Altocumulus undulatus (May 07)
Cumulonimbus capillatus (April 07)
Lacunosus (March 07)
Horseshoe Vortex Cloud (February 07)
Jet-Stream Cirrus (Janurary 07)
Altostratus/Altocumulus/Altowhateveritis (December 06)
Anti-Crepuscular Rays (November 06)
Stratocumulus (October 06)
Altocumulus (September ’06)
The Kelvin-Helmholtz Wave Cloud (August ’06)
The ‘Brocken Spectre’ (July ’06)
‘Whale’s Mouth’ (June ’06)
Noctilucent (May ’06)
Cirrus (April ’06)
Cap Cloud (March ’06)
Fallstreak Holes (February ’06)
Nacreous (January ’06)
Cirrostratus (December ’05)
Tuba (November ’05)
Virga (October ’05)
Cirrocumulus (September ’05)
Altostratus (August ’05)
Cumulus (July ’05)
Mamma (June ’05)
Pileus (May ’05)
Lenticularis (April ’05)
Stratus (March ’05)
Cumulonimbus (February ’05)
Contrails (January ’05)


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