Athens & Halos

Greek Halos - High over Athens Saturday 16th May and imaged by toomanytribbles. Image �toomanytribbles, shown with permission.

The bright inner halo is a high-sun (68° altitude) circumscribed halo produced by horizontal column crystals in the Athenian cirrus. This halo is almost circular at that solar altitude and often mistaken for the common 22 degree halo. Circumscribed halos, as here, have sharper more intense colours.

To the right in the top image (below the sun!) is an arc with widely spread colours, the result of refraction and dispersion through crystal faces at 90� to one another. The crystals making it are less certain however. They could be more column crystals giving an infralateral arc. They could be horizontal plate crystals producing a circumhorizon arc which is not particularly rare at the 38� latitude of Athens. Both arcs might be present.

The two arcs are compared in the HaloSim ray tracing at page bottom. The simulation used only a mixture of column and plate crystals. The arcs in the sky are most easily distinguished by direct visual observation. The circumhorizon is parallel to the horizon whilst the infralateral arc curves upwards from the horizon at its ends.

Image (2) shows another manifestation of column crystals - a high-sun parhelic circle tiny in comparison to its vast extent when the sun is low. Light reflects off column end faces or plate side faces to make the parhelic circle. Many other internal ray paths also contribute.

The crystals were real, the halos themselves had no real corporeal existence instead being only bundles of light rays reaching the eye and camera.

It was right here in Athens and Greece that Anaxagoras (c500-428BC), Aristotle (384-322BC) and others were the first to realise this and attempt to explain halos and rainbows as scattering and reflection by objects in the atmosphere.