toptics
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Highlights

The not so ordinary sundog

Timothy Heitman captured this evocative scene between the Florida Keys and Florida mainland.

Sundogs (22° parhelia) flank the sun. Sun and sundog glitter paths reflect from the glassy sea.

Image ©Timothy Heitman, shown with permission





 


Sundogs (and the 22° halo) are the most often seen optical effects in the atmosphere. They are visible on average more than once a week - far more often than a rainbow. Common they may be, simple they are not.

Their colours come from two refractions through the side faces of plate aspect hexagonal ice crystals. Aerodynamic forces align their large hexagonal faces within a degree or so of horizontal.

Ray paths through the crystal are not simple. Only when the sun is near the horizon do rays have an easy passage. Otherwise they skew and bounce up and down between the upper and lower horizontal faces.

Skew rays from a sun away from the horizon have another consequence. The "22 degree" parhelion moves further and further from the sun as it climbs. Confusingly, the 22 degree label refers only to the angle of minimum deviation through the crystal.

There is a further complication. Each sundog is double!

Ice is birefringent and rays entering crystals split into two polarised components. They generate two slightly displaced dogs. Flip or quickly rotate a polarising filter and you might see a sundog jumping back and forth as the two components are seen. Try it.

Ice birefringence                                                                  Glitter paths

Sundog formation                    minimum deviation

Sundogs not 22° from sun