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This past weekend, Thanksgiving in Canada, I had the
good fortune of being able to camp in the Rocky Mountains of
Alberta. Our camp area is around an abandoned grass airstrip.
On both Saturday and Sunday mornings it was cool; the
temperature was -3° and -1°C respectively. The grassy airstrip
was very frosty. As the sun came up over the mountains in a
perfectly clear sky, the frost crystals on the uniform grassy
field produced a lovely optical effect.
Looking towards the sun, one could see a parabolic arc
openning towards the sun in the frost on the ground. This arc
was coloured red to violet from the inside out. I believe the
horizontal arc was a 22° halo normally seen as circular around
the sun in the air. This halo was the conic section of the
horizontal ground intersecting the cone of vision around the
sun. The sun elevation at the time was approximately <10°.
There were no ice crystals or fog in the air, thus no "normal"
halo effects.
I have noticed this effect several times on uniformly
frosted large surfaces, but not so distinctly. I have also
seen this more distinctly on a smooth parking lot after a very
light and uniform snowfall of about 1 cm.
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