What is so special about Bouguer's halo?
Marko Riikonen writes: “My experience from Finland is that you either have fog or diamond dust. On the two occasions I have seen a display where fogs and diamond dusts passed and the moments when halos and fogbow were seen at the same time lasted just seconds. In photos these mostly show up together only because the time exposure had captured both fog and diamond dust stages on film.
Summit Station on the Greenland Ice Cap is getting a reputation for rich displays of atmospheric optics. Here we have “Bouguer’s Halo” the very rare simultaneous appearance of a fogbow and a halo. Mouse over the image for an enhanced view. More about why this ‘halo’ is exceptional below.
Summit Station has a webcam installed by David Noone on a 50m tower and recently it was modified at the suggestion of Jari Luomanen (optics site) to better show atmospheric phenomena. Jari watches it assiduously from Finland and at ~10am Greenland time on October 14th he spotted the Bouguer's halo and right away e-mailed veteran optics observer and photographer Ed Stockard (photostream) at the Station. Ed was just about to go outside to work rather than halo watch. On Jari's alert he grabbed a camera and emerged into the -33 Celsius air. The sun was shining through a thin fog with blue sky still visible. Ed took images sunwards (the 22° halo), opposite the sun (fogbow) and the fisheye view at left.