Monday, February 21, 2011

Circumzenithal and supralateral arcs -- learning to look, learning to see

Imagining image labels is one thing; having facts about photos is another.

I decided to ask an expert, Les Cowley of Atmospheric Optics (, about the ice halo pictures I took this weekend in Regina, Saskatchewan, Canada.


The first image in the post, Sundogs, tangent arcs and double solar halos shows the edge of a second halo. What is this likely to be?
Les Cowley, Atmospheric Optics: Firstly, congratulations on capturing a beautiful ice halo display. Saskatchewan is known for its spectacular halo displays and one seen over Saskatoon in 1970 is world famous.
Photo © Shelley Banks
The inner circular halo of your display is a 22° halo formed by randomly oriented hexagonal ice crystals in the cold air. The brightening at its side is a trace of a sundog made by hexagonal plate ice crystals drifting with their large faces horizontal. Another type of crystal, a long hexagonal column, made the arc at the top of the 22° halo – an upper tangent arc.
At a distance twice as far from the sun is a halo with more widely spread colours because the sun’s rays were refracted through crystal faces angled at 90° rather than the 60° that formed the inner halos. It is probably a 46° halo but it could also be a supralateral arc. These two halos are often hard to tell apart.
What is the best way to capture huge displays like this?
LC/AO: These halos are HUGE. Ordinary camera lenses cannot get them wholly in their frames. However, a fish-eye shrinks the immensity of the sky into a few tiny pixels. Do we want that? The 46° halo is a rare event and it is immense, so large that we have to swivel our heads to span it. An ordinary lens, by showing just a section of the halo, preserves the sense of its immensity and our eye view of it. So take lots of photos (frostbite permitting!) with an ordinary lens and savour the collection.
Photo: Shelley Banks
The post Rainbow arc and solar halo and video show a faint rainbow, with a curve that follows the line of the solar halo. What is this likely to be?
LC/AO: That is part of a circumzenithal arc and when seen in its entirety the most beautiful halo of them all. There is a supratlateral arc (or 46° halo!) touching it.
Solar halos and sundogs are fairly common in winter; how common are these other two displays?
LC/AO: These big halo displays are not uncommon where it is so cold that ice crystals – diamond dust – drift nearby in the air. Sundogs are very frequent and the more spectacular displays occur maybe two, three or even more times each winter. The secret is to make a habit of searching for them whenever outdoors.


Learning to look, learning to see.

Les Cowley quoted with permission, and thanks.


Update, April 16, 2011: My picture of  the sun with an ice halo partially obscured by branches is the Optics Picture of the Day at Cowley's Atmospheric Optics.


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