Click on image for corresponding simulation
This is a record of several halo 'displays' I have either photographed myself (on medium-format colour transparencies or with a digital camera) or received from other people. Halos are produced by refraction and reflection from hexagonal ice crystals floating in the atmosphere with a range of random and quasi-stable orientations. The mechanisms of formation of the various coloured (refraction) and uncoloured (reflection) arcs have been quite well understood by the use of computer simulations which, using Monte-Carlo techniques, trace large numbers of rays through distributions of crystal types. I have used the computer program HALO by Les Cowley and Michael Schroeder to simulate parts of these displays. A highlight is a clear photograph (above), taken at Landshut in Bavaria on 25 February 1996 of the rarely photographed Lowitz arcs. In addition to the observations, I have performed some experiments with a hexagonal quartz crystal which mimics some of the ice crystal properties.
I should like to thank Les Cowley and Michael Schroeder for their energetic reaction to my emailed suggestion that I might have photographed the Lowitz arcs. Their encouragement stimulated me to create these web pages. They are still developing the HALO simulator, which is now able to compute halos in colour.
Any use of these images or simulations for publication should be with permission.
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