Ground Halos ~ A frozen Finland lake glitters as if gemstone strewn. More at home in the skies, a 22° and rare 46° halo are formed from the glints of ice crystal gems on the lake surface. Jari Luomanen stacked 69 images to capture the visual impression.

“It was an insane day. A stacked image is the only way to approach and even surpass the visual experience. We humans have two eyes and we also tend to move around: our vision is constantly fed with myriad glints as we move by the surface crystals. The camera on the other hand only has one eye and is stationary during the exposure. Hence, it is necessary to combat this limitation by capturing numerous photos that are then stacked together as a sum image. Here the approach was taken quite far and the result is a dense field of crystals where the halos are seen very clearly. I especially like the way the photo reveals the way light is being scattered outside the halo rings.

The horizon and the sky were layered into the stack from the last shot of the series that was exposed without the sun blocking filter that is necessary to capture these kinds of photos.”

The effects of thermal cycling, sublimation, recrystallization and extra crystallization from humidity supplied by water on top of the lake ice convert fallen snow crystals into larger and simpler forms of good optical quality. These hexagonal prisms lie tumbled more or less randomly, ideally oriented to form the circular 22° and 46° halos. At left we see regular prisms, those on the ground are no doubt less regular but they still have the same interfacial angles.

Rays passing between side faces inclined 60° form the 22° halo. Rays traversing a side face and an hexagonal end face – refraction through a 90° prism – make the 46° radius halo.

To the eye or camera the halos always look circular. However, the track of the glinting crystals along the ground is a hyperbola. The glints show nicely that these halos are not 'rings'. 22° and 46° are only mark the inner edges of glows that extend far beyond.
The scene by moonlight. A faint moon dog tries to compete with the 22° ground halo.
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Marko Riikonen also captured the daylight halos with a 143 image 'peak hold' stack. The camera was closer to the ground.

Directly below the sun are perhaps stronger hints of a diffuse pillar, indicating that the crystal orientations were not all random.