Rare High Sun Halos, Laredo, Texas
Pictures by Araceli Cadena and Lizett Montiel of halos in miles high icy cirrus on May 29, 2014. Thanks to Richard Berler “Heatwave” Berler, Chief Meteorologist at KGNS TV who alerted OPOD.     Copyright of all images is with the photographers
Parry-Lowitz orientation

Column crystals nod
about a horizontal axis passing through opposite prism edges. A subset of the traditional Lowitz model.

Images at KGNS-TV by Lizett Montiel.

The outer arc (Parry or Parry-Lowitz) is immensely bright. A faint parhelic circle passes through the sun. Near the building we see the infralateral arc with widely spread colours.

(6) The existence of Lowitz arcs was until comparatively recently hotly debated. Now cameras capture them many times each year.

Lowitz halos were traditionally explained by plate crystals taking all rotational positions about a horizontal ‘Lowitz’ axis. Now, with more sightings, many displays are somewhat better accounted for by allowing Parry crystals to rock or nod from horizontal in 'Parry-Lowitz' mode.

At right the ray tracing uses only Parry-Lowitz columns, ordinary columns and some plate crystals. The Lowitz crystals generate both halos X and Y (the latter is a 46° Lowitz arc).  But for the possible problem of the weak infralateral arc compared to images, the plates could be tossed away leaving only two types of crystals to account for the display, we might even relax the restrictions on the Lowitz orientations.

A more economical explanation and hard to distinguish from (5).

The choice is yours!  Economy and exotic or more complexity and more commonplace.

Plate crystal orientation
(4) Arc Y just above the infralateral arc remains unsimulated.

Add some randomly oriented crystals.

They produce a 22° halo but at this high sun it is almost indistinguishable from the circumscribed halo.

More interestingly, a weak 46° circular halo appears.

Its distance above the infralateral arc is very sensitive to the sun's altitude. At an 80° high sun it is 3.8° which compares to the 4.3-4.7° estimated from the images.
Parry oriented crystals have their long axes horizontal and their upper and lower side faces also horizontal. 4,5,6 is a helic arc from the Parry crystals.
Colour subtraction versions of Araceli Cadena's images.

The lower arc measures 27.6° from the inner halo (prediction 28° for infralateral or circumhorizon arc).

At left Halo Y is 4.3° above it. At right (43s later) the separation is 4.7°.

What are the halos?

(1) Let's throw some crystals into HaloSim and attempt to ray trace the display. First of all some horizontal hexagonal columns.

The columns generate the 'circular' halo as a high sun circumscribed halo. Reflections from the column end faces produce a tiny parhelic circle.

Below the sun is a faint infralateral arc. Ray tracing could make it more intense by shortening the column crystals. However, that would brighten the parhelic circle too much and give a brighter Wegener arc that is not seen in the photographs.

Halo X is not from horizontal column crystals. Could it be a high sun Parry crystal arc?

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

(5) A remaining issue is the weakness of the infralateral arc. Perhaps some plate crystals were also in the Texan skies to produce a circumhorizon arc? Below the sun the infralateral and circumhorizon arcs overlap.

At left we have ray tracings from a mixture of (a) columns, (b) Parry columns, (c) randomly oriented crystals and (d) plates.

The ray tracing reproduces the features of this rare display.

Is this the only explanation?

(2) Add some Parry oriented columns to the mix and Halo X appears. Rays forming it enter an upward facing prism side and leave through the lower horizontal face (path 4,6 lower Parry arc - see face number scheme).

Problem!  A second Parry arc appears in the ray tracing that is not in the photographs. Its rays pass through the same faces but reflect internally from face 5 (path 4,5,6). This is a high sun version of a helic arc.

At top left the 77° high sun is surrounded by a bright ‘circular’ halo. A smaller and fainter circle passes through the sun and extends just beyond the bright halo. This is undoubtedly a high sun parhelic circle and far different from its usual huge appearance near the horizon when the sun is low.

The arc touching the 'circular' halo and extending out beyond it at top [call it Halo X] is peculiar and very rare..

Below the sun and pictured at top right (80° sun) there is a large and colourful arc. There is a faint arc [Halo Y] slightly above it. The enhanced views at right show it clearly.
"Singly oriented columns" drift with their long axes nearly horizontal. They take up all rotational positions about the axis.

An infralateral arc forms from rays entering an end face and leaving through a lower side face.

(3) We can blur out the unwanted arc by tuning the Parry crystals. The right hand simulation has Parry crystals with an increased wobble (standard deviation 1° instead of the 0.3° in the left hand ray tracing). The main Parry arc is now also less unnaturally sharp and more like that in the images.

Some halo ray paths are more sensitive than others to small crystal misalignments.