In the
frigid grasp of
winter, when your
nares are briefly sticking shut with each inhalation, and you’ve dispensed with
gloves in favor of the huddled warmth of
mittens, there is a visual redemption for the coldness endured. This is the weather for
sundogs, pale,
wraith-like images escorting the sun across the brilliant
blue sky.
If you haven’t seen them, they are short arcs of light flanking the sun, often appearing as part of a faint halo totally encircling the sun.
The most striking ones occur when the sun is near the horizon and the air is loaded with ice crystals, specifically tiny (less than 0.5 micrometers in diameter) hexagonal ice crystals suspended in thin, nearly invisible cirrostratus clouds. These ice crystals refract the sunlight at an angle of about 158º, resulting in the appearance of sundogs about 22º from the sun.* The amount of refraction is dependant on size of the crystals, so instead of a precise, neat rainbow, you get stretched pale images.
Sundogs are seen in short arcs in a horizontal plane with the sun because the pencil-like ice crystals predominantly tumble through the sky with their long axis oriented horizontally. The faint halo seen with the sundogs is from the crystals that are at the correct orientation for their location to refract light to your eye.
So there you are, a technical explanation for a simple apparition.
Have fun looking, but remember, don’t stare at the sun.
*The math: Line of sight is 180º ; subtract the 158º refraction and you get the image at 22º.