Odd or even?
Whether a sun ray forms a sundog or subparhelion depends on a count of internal reflections.
Sundogs or 22 degree parhelia are said to arise from rays entering a side face of a horizontal plate ice crystal in cirrus cloud and leaving through a second side face inclined 60 degrees to the first. That is, like the curate's egg, true in parts. Unless the sun is low on the horizon or the plate is unusually thick events are not quite that straightforward.
Instead, once inside the ice, most rays intercept the lower crystal face and internally reflect. They can reflect up and down between the upper and lower large faces several times before eventually leaving through a side face.
An even number of internal reflection gives a sundog above the horizon. An odd number produces a subparhelion.
Subhorizon halos are, like their above horizon counterparts, produced by each sun ray interacting only once with a crystal. They are not halos produced from the light of other halos. |