The Summertime Halo
The circumhorizon arc is an optical effect caused by the sunlight shining through the ice crystals of high clouds. It is one of the many rings, arcs and points of light known as ‘halo phenomena’, which can appear when ice crystals in a high cloud take the form of regular hexagonal plates or columns that reflect and refract the sunlight like tiny prisms.
The circumhorizon arc appears as a broad, flat line of bright colours, parallel to the horizon and positioned well below the Sun. It is sometimes misleadingly referred to as a ‘fire rainbow’ even though it has nothing to do either with fire or with rain. This optical effect appears when sunlight passes in through side faces of tiny hexagonal-plate ice crystals and comes out from their bases. Since the crystals are horizontally aligned because they are falling like autumn leaves, the Sun needs to be is high in the sky for the effect to appear. To be precise, it needs to be at an angle above horizontal that is greater than 58 degrees. Only then, can the light pass through the crystals at the correct angles for it to cause the horizontal band of colours.
The rarity of a circumhorizon arc depends on where you’re based. The lower the latitude, the greater your chance of spotting one when Cirrus or Cirrostratus clouds are in the sky. Les Cowley, Member (0014), reports in his Atmospheric Optics site that from most locations in the US they can be observed about five times a year, but from locations in northern Europe you might see them only once or twice in a year. Likewise, they’re more commonly seen in Australia than in New Zealand. You’ll never see a circumhorizon arc, however, from latitudes above 56 degrees – in the Northern Hemisphere, that’s anywhere north of Copenhagen, Denmark – since the Sun never climbs high enough in the sky.
Nor is it possible, unless you’re near the equator, to see a circumhorizon arc throughout the year. For most of us, the dependance of this vibrant optical effect on a such high Sun means that its horizontal streak of pure, spectral colour will only ever grace our skies during the summertime.
A circumhorizon arc produced by Cirrus homogenitus, an ice-crystal cloud that developed out of an aircraft condensation trail, spotted by Christa Harbig over Pendleton County, West Virginia, US.