High Sun Halos, Athens
A notable display of rare and some less rare arcs imaged by Kosmas Gazeas at the University of Athens Observatory on May 18, '14.
All images ©Kosmas Gazeas, shown with permission

Above the sun and the inner 22 degree and circumscribed halos is a ghostly and small parhelic circle.

The sun was 68° high making the circle just 22° radius - coincidentally the same as the 22° halo.

The width of the parhelic circle suggests that the column crystals forming it had wobbles of 1° or so about their horizontal axes.

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Note the almost circular circumscribed halo. A great many of them are mistaken for a 22° halo.

They deserve better recognition. They have more saturated colours and are sharper.



A mix of (1) hexagonal ice columns drifting with their long axes nearly horizontal, (2) randomly oriented columns of some kind and (3) a few hexagonal plates formed this display.

Nearest the sun is the common 22° halo from the randomly oriented crystals. Touching it above and below the sun is the circumscribed halo from the horizontal columns. At the sun’s high altitude of 68 degrees this halo has lost its low sun ‘droopy’ shape and is instead a tight oval.

Twice as far from the sun there are two more halos. One is just visible next to the Greek flag in the top image. They can be better seen in the two images at lower right one of which is enhanced by colour subtraction. There is a section of a strong almost circular arc and tangent to it a much weaker arc extending further from the sun. The stronger arc might be interpreted as a 46 degree halo. However, there is no sign of it above the sun and the 22 degree halo formed from the same crystals is comparatively weak. An infralateral arc formed by rays passing between the side and end faces of horizontal columns fits the images. It also appears naturally in the HaloSim ray tracing at right using just enough horizontal columns of medium length to make the circumscribed halo and parhelic circle. The weaker arc touching the infralateral (it appears on two images but we cannot be wholly certain of its existence) is the usually more frequent circumhorizon arc made by horizontal plates.

Plate crystals of course give sundogs or 22° parhelia but they did not appear over Athens because the sun was too high to allow them to form.

A star of the show is the small parhelic circle seen best in the image below. It was mostly formed by reflections from the end faces of the plentiful horizontal columns. The weakness of the circumscribed halo suggests a scarcity of plates, the more usual major source of the parhelic circle.

HaloSim shows a weak Wegener arc. Careful enhancement fails to show it reliably in the images. We could invoke imperfections on the Athenian column end faces (but we see an infralateral arc!) or bubble inclusions. We shall never know.