Atmospheric
Optics
Moilanen Arc & Ski-Slopes
A beautiful diamond dust halo display captured by Heather Kluesner (recipes) near Elizabeth, Illinois, USA.
The first V-shaped halo directly above the sun is the rare Moilanen arc named after its Finland discoverer, Jarmo Moilanen.
The bright 'V' above that and touching the 22° halo is the common upper tangent arc.
Faint sundogs lie on a parhelic circle passing horizontally through the low sun.
Images ©Heather Kluesner, shown with permission
There is uncertainty about the ice crystals and their orientation that generate the Moilanen arc.
From its shape and angular distance from the sun we know that
it is formed by rays refracted through two ice facets inclined 34° to each other. The intersection of the two facet planes is horizontal.
In spite of the availability of diamond dust for crystal sampling and much hard work by observers, the actual crystals are yet to be identified.
In the last few weeks there has been a spate of sightings of the Moilanen (M) arc. In all cases they were in low level diamond dust and within a few km of a ski slope. Heather was 16-19 km from a ski resort. There is a definite association between the operation of ski resort snow making guns and M arc sightings nearby.
I asked halo expert Marko Riikonen whether he was aware of any M arc sightings not near ski resorts or in high cloud rather than diamond dust:
“In Resolute I recall seeing an M arc in diamond dust with the wind coming from the sea. But in between the sea and our location on top of a hill there was the Resolute settlement, so it is not out of question that smoke from the houses spawned the M arc.
That's the only case I know. It is hard to tell about other people’s experiences. The diamond dust from guns and other plumes can go far, like 10 km, some say even 50 km, but I am not so sure about that. Anyway, if those cases you mention were within a few km, that definitely is within the range of the guns.
And besides, when the conditions are good, all kinds of smokes make their own dusts too.
I am not aware of any M arcs in higher levels ice clouds.”
Most snow making guns now emit small active proteins that act as nuclei for snowflake formation.
The nuclei can also help form smaller exquisite optical quality plate and column crystals kilometres away from the ski resort. These in turn can give outstanding halo displays with many rare halos.
