London Paralympics Halos

Katie Mortlock caught this bright sundog (left), parhelic circle and 120° parhelion at the London Paralympics 31st Aug '12. The red panels belong to the Beat Box, an interactive sound pavilion. More images below.

Images ©Katie Mortlock

The parhelic circle passed through the sun. The fragment of circumscribed halo above the sun shows that horizontal column crystals were present as well as plates.
The absence of colour makes the 120° parhelion difficult to spot against cloud although both it and the parhelic circle can be edged by faint corona type diffraction colours . The 120° parhelion is not common because thick or non regular hexagonal plates are needed to give bright ones.

Prismatic sundog colours

The parhelic circle is produced by hexagonal plate and column crystals via a great many ray paths.

Its intensity suddenly lessens a few degrees farther from the sun than the 120° parhelion. Many rays rely on total internal reflection inside the ice crystals and this abruptly ceases to be possible beyond a critical angle.
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Ice crystals in the high cirrus produced the halos. Cirrus is composed of ice crystals at all seasons and places on the planet.

The colourful sundog was made by horizontal plate crystals. Sun rays enter a near vertical side face and exit through another inclined 60° to the first.

The same crystals generated the colourless parhelion 120° around the parhelic circle from the sun. In the ray path shown, rays enter a top face then reflect twice internally before leaving through the bottom face. The entrance and exit angles are equal and opposite so there is no net dispersion into colours - take away the internal reflections and the path is equivalent to looking through a plate glass window.