Light of a halo is polarized. This gives a one tool to make more reliable halo identifications. 

Polarization of the inner edge of the halo is depending on the main (or c) axis of the crystal. Inner edge polarization is perpendicular to direction of the c-axis. If c-axis is vertical (parhelia for example), then inner edge of the halo is made by horizontally polarized light and vice versa. So when looking halo through polarization filter, which blocks horizontally polarized component of the light, halo seems to move away from the sun.

23° plate arc problem

There is one quite common problem which can be solved with polarization: 23° upper plate arc can be misidentified with (22°) Parry arc or high sun 22° upper tangent arcs. It is known that 23° upper plate arc may be the only odd radius halos in the display. It may even be the only halo on the sky. It acts like Parry arc and it is located only a degree or so higher. 23° plate arc can be also misidentified with lonely high sun 22° tangent arc. Or Parry arc may be misidentified as 23° plate arc or high sun 22° upper tangent arc as 23° plate arc.

Polarization gives a tool to be sure if the halo is odd radius plate arc or not. Polarization of 23° plate arc is opposite to polarization of Parry or upper tangent arc. 

Here is two-frame animations made by using real halo photographs. V-frame show halo through polarization filter which let only vertically polarized light through. H-frame show halo through polarization filter which let only horisontally polarized light through. V and H frames chances every 0.5 second.

Polarization of 23° plate arc. 6 May 2006 in  Vaala, Finland.  Note that the arc is higher when V frame is shown. So the inner edge polarization is vertical as 23° should have. This case was sure since there were other odd radius halos too.

Polarization of high sun 22° upper tangent arc. 12 June 2006 in Vaala, Finland. A white dot is placed here to help notice the polarization shift of the halo. Notice that the arc is higher when H frame is shown. This shift was also observed visually, so it was know already while observing that the arc was a upper tangent arc. In this case lower tangent arc was visible only very faintly. In the end of the display it came brighter for a moment.

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© Jarmo Moilanen 2005