Monday, 24 January 2011

The Saskatoon halo display

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A handful of halo displays have been of such an impact that they have become known by the location of occurrence or by the observer’s name. The display in Canada’s Saskatoon on 3 December 1970 belongs in that group – it is simply called ‘the Saskatoon display’. This halo complex was of remarkable intensity but its main merit is that it provided the first photographic documentation of parhelia at about 46 degrees distance from the sun. More precisely, they are 44° parhelia, which are the parhelia of exceedingly bright ordinary 22° parhelia. “Secondary halos” and “multiple scattering halos” are the terms that have been used to describe halos formed in such a way.

Thus far, only one photo of the display’s 44° parhelia has been published, printed in black and white in the June 1972 issue of Weather magazine in an article “Unusual arcs in the Saskatoon halo display” by W. F. J. Evans and R. A. R. Tricker. The cover of that issue is shown above, with a photo of the circumzenithal arc from the display. The cover was originally in color but I have only this black and white scan. Shown also is a drawing from another Weather article “Photometeors at Saskatoon on 3 December 1970″ in the 1971 issue 26 by Earle Ripley and Bernard Saugier.

Of the few people who photographed the Saskatoon display, Earle Ripley was watching it at the University of Saskatchewan campus. Above are two photos of the 44° parhelia that Earle send me a couple of days ago after I had contacted him. He gave permission to go ahead with publication on the internet. The photos were taken about 10:30 am, at which time the sun elevation was about 9 degrees. The intensity of the 44° parhelia in those photos is unparalleled. This is made even more remarkable because of the high sun elevation. No other 44° parhelia photos exist for this high a solar altitude. The higher the sun is, the more extensive the diamond dust cloud has to be vertically in order to have enough crystal mass for the formation of the 44° parhelia. The horizontal extent of the diamond dust in 44° parhelia displays is probably always of the order of kilometres.

After Saskatoon, 30 years passed until the next photos of 44° parhelia were taken. Now it appears that we get one photographed case every year. This year's display came from Sweden, last year from Czech Republic. None have been as great as the Saskatoon, but displays of similar magnitude and style certainly recur - at least in the vast uninhabited expanses of polar areas - and one day someone is bound to be around when that happens, hopefully with a camera.

[25 January: The black and white Weather cover was replaced with colored one, thanks for Walt Tape for sending it]

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