Subhorizon plate crystal halos

On the night of 7/8 December in Rovaniemi water fog was nucleated to diamond dust by snow machines, and a major display was observed in a spotlight beam for several hours. The photo above is from the end stage when plates dominated.

The lamp is downhill below the horizon (about 11.5 degrees), which shifts subhorizon halos above the horizon. Halos are named in the simulation which is made with Jukka Ruoskanen's program. Sub-Kern and sub-120° parhelia have not been documented before. By other means these are difficult to see. One needs to sit in an airplane which happens to pass through a high quality crystal cloud and sub-Kern is also located so low that one probably is not able be view it from small airplane window.

Crystals used in the simulation were plates which prism faces were allowed to variate freely in size within some limits. This produced the rather uniform stretch of subparhelic circle where sub-Liljequist parhelia are becoming hard to distinguish. Had there been solely regular hexagons in the air, the sub-Liljequist parhelia would have been better defined.

There was also circumnadir arc, which is another new feature. Les Cowley ( 1 ) has more on this and other subhorizon halos in the display.