Atmospheric
Optics
Subhorizon Halos ~ Above the horizon!!
A rare sighting in the Chilean Atacama Desert by Gill & Adrian Smith. The sun had set at ground level but still lit high clouds. The red ice halo glows at right and (far more rare) at left are normally seen below the horizon. Here they are in the sky. More below.
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Images ©Gill & Adrian Smith
Parhelia and subparhelia are both produced by sun rays passing through horizontally aligned ice plate crystals. The rays enter and leave through faces inclined at 60° to one another.
It is what happens inside the crystal that makes the difference. A subparhelion appears when rays reflect up and down between the large hexagonal faces an ODD number of times. We see a sundog when there are EVEN number of reflections.
The more familiar appearance of subhorizon halos. Here in glittering diamond dust are an intensely bright subsun/lower pillar with prismatic subparhelia on each side.
Imaged by Rob Noble
at Verbier, Switzerland.
Unusual apparitions are more worthy of investigation when they are seen visually and appear in the same position relative to the sun on at least two hand camera shots. This helps eliminate the possibility of camera and lens artefacts.
This image made four minutes earlier than the top one again shows at far left the red glow of the subparhelion. Image measurements give the glow 23.6° from the subsun in very reasonable agreement with predictions.
The red glow in the clouds at right is a lower sun pillar and subsun. They are directly above the sun which is 3 degrees below the horizon.
The glow at far left at the same height is something much more rare. Indeed it might only have been knowingly photographed a handful of times. It is a subparhelion or sub-sundog.
The subsun and subparhelion are below the horizon when the sun is in the sky. Here the geometry is reversed. The sun is below the horizon and these ice halos project in the sky.
At top the sun is 3° high. Directly below the sun and 3° below the horizon is a subsun and a lower sun pillar. The subsun is flanked by subparhelia.
Parhelia and subparhelia get closer as the sun sinks.
At sunset the above horizon and subhorizon halos merge.
As the sun sinks beneath the local horizon the 'subparhelia' climb into the twilight sky.
HaloSim ray tracings.
