Wide Moilanen Arc captured 3rd December 2010 at Davos, Switzerland by Bertram Radelow (Halo Site).   The as yet not fully understood halo is here very wide. Below Bertram describes its development from snow gun nucleated crystals. ©Bertram Radelow, shown with permission.

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed

"Today I "caught" a nice and prominent Moilanen arc, produced by snow guns.  

At the time I shot the pics the usual icy [radiation] fog which forms during December nights had disappeared completely an hour since.  The icy fog in the pictures is a 100% product of an ensemble of six snow guns in downtown Davos.   The ice fog then was transported by a very gentle wind and had time to "ripen".

Picture 1 [left] is about 1.3 miles away from the snow gun, time or the drift are 20 minutes, roughly estimated.  The pic shows that the special Moilanen crystals are the first to form out of the unstructured grains produced by the gun.

Pictures 2 and 3 are about 1.8 miles away (only five minutes later) and show the further metamorphosis within short time.  The "Minuteman rockets" surrounding the small frozen lake are part of a Christmas decoration - illuminated by night in various colors they look pretty nice!"

Snow guns eject small seed nuclei into the air on which halo forming ice crystals subsequently grow from condensation of atmospheric moisture. The growth occurs over hundreds of metres to several kilometres downwind. Snow guns that make the best halos use enzymes as crystal growth nuclei.   The 'artificial' crystals have the same structure, forms and habits as naturally occurring ones. However, for reasons not yet understood (type of nucleate, atmospheric conditions), they can be near optically perfect and generate rare halos.

The rare Moilanen arc, named after halo expert Jarmo Moilanen of Finland, is characteristically ‘V’ shaped and appears above the sun between it and upper rim of the 22° halo.   In images (2) and (3) a weak and diffuse 22° halo and upper tangent arc frame the Moilanen arc.    Diffuse halos can be an indication that they are produced with very small crystals – consistent with Bertram’s comments that halo forming crystals were just starting to form out of the artificial ice fog.  The Moilanen arc is however already relatively sharp.  Whatever crystals formed it were already of some size and development.

The Moilanen arc is coloured – a red lower edge is visible in the top image – showing that rays forming it have undergone refraction.    Its distance from the sun and shape can be simulated by tracing rays through an ice prism with two faces inclined at an angle of 34°.  The prism edges must be nearly horizontal with the narrowest part of the 34° wedge uppermost.  

The wedge angle does not correspond to that from any facets physically reasonable in ice crystals.    One possibility is that the 34° wedge is present in some clustered or twinned crystals.