Saturday, 30 January 2010

A complex halo display on car windshield

Photos and videos of the windshield halos seen on the night of 26/27 January in Tampere are now available in two galleries ( 1 - 2 ). The display was dominated by plate and Parry oriented halos. Circumzenith and circumhorizon arcs were strong as were the upper and lower suncave Parry arcs. Parhelic circle and 120° parhelia were also well developed. Less conspicuous were the subhelic arc ( 3 ), upper sunvex Parry, 46° Parry and the blue spot. Additionally, two unidentified features were seen.

We did not photograph the crystals, but we think the display was formed in frost crystals that have their basal or prism faces parallel to the windscreen. Both populations could be separate crystals, but also 90 degree crossed crystals could be a consideration. Visually the frost seemed rather grainy, which suggests compound crystals.

The halos were observed with a LED lamp pointed through the windshield as well as with reflected light (the lamp on the same side of the windshield as the observer). By moving the lamp we were able to observe in real time the transformations in halos that occur as the light source elevation changes (4). Videos show the shift between circumzenith and circumhorizon arc. There are also peculiarities. One is that the parhelia does not seem to move much further from 22 halo as the lamp elevation rises.

Both two unidentified features were documented. The other one was found from the video: in the video, far below parhelic circle at around 120 azimuth, appears a vertical pillar (5). The other effect was a diffuse arc that was seen when we lit up the windscreen from the front of the car and watched it from behind the lamp. This elusive effect was documented in a video (6).

After hours of documenting we ended the night by taking a ride and videoed the halos as they swept the windshield. As we neared a streetlamp, first one to sweep the screen upwards was the lower suncave Parry followed by circumhorizon arc. Then, as the lamp was overhead and behind, parhelic circle swept the screen downwards. If the lamps were a bit further on the side relative to our driving line, then also 120 parhelion was seen on parhelic circle.

It was a great night. In those six hours that we spent worshipping this complex display, we learned all the time more on what we were seeing and created new tricks to take photos. Certainly this winter’s top halo experience for both of us.

Marko Riikonen, Jari Luomanen

Friday, 29 January 2010

Possible explanation of Moilanen Arc

I would like to present a possible explanation for the Moilanen arc. I have read about the existence of this unexplained arc a few weeks ago and tried to figure out possible explanations. After getting the reflected lower sunvex parry and some pyramidal arcs that were also looking like the Moilanen arc, I think I finally have a crystal that makes a very good candidate for the explanation of Moilanen arc.

Responsible crystal
The responsible crystal would be a crystal made of a two hexagonal usual hexagonal crystals stuck together. The simplest configuration would be the one of a plate crystal (called the main plate) with another plate or a column stuck on one of its base (called the stuck crystal).
Figure 1.

Condition of creation of Moilanen arc
The conditions for the creation of a Moilanen arc from this configuration are the following:

1: The main plate must be horizontal

2: The stuck crystal is on the inferior base of the plate

3: The bases of the stuck crystal must be perpendicular to two side faces of the main plate

1 and 2: this configuration is aerodynamically stable when stuck crystal is centered on the inferior base plate. To figure this, one can think of a paratrooper, the parachute being the plate crystal and stuck plate or column being the charge. So naturally the main plate will rotate so the stuck crystal is on the inferior base. Now why would the stuck crystal be centered on the main plate? I think that for this stuck crystal configuration to happen, the crystal density must be very high. So in the case of crystal swarms created by snow machine, the crystals would be stuck when they are small, just after their creation. So the growth of the crystal, will happen after. The growth being isotropic, the stuck crystal will naturally be centered on the main crystal. And the aerodynamically stable condition is the main plate being horizontal with stuck crystal on the inferior base.

3: I have to admit that the perpendicularity of the bases of the stuck crystal and two sides of the main plate is the most challenging point. My best explanation on this is that as the two components are crystals, there are definitely orientations of stucking that will be more stable. I hope that crystal experts will be able to help on this point to validate that this orientation is more likely to happen than any other. Crystal samples obtained during the display of 14th of December 2009 by Mika Aho, Ágnes Kiricsi and Marko Riikonen show some crystal looking very much alike the one described in the proposed explanation
Figure 2 and 3.

Ray path responsible of Moilanen arc
The ray path responsible of the Moilanen arc is the rays entering a vertical side face of the main plate and exiting the upper lateral side face of the stuck crystal. The prism angle in this configuration is 30° but with the entrance face vertical.
Figure 4.

Usually associated halos
This configuration of crystal is so still capable of producing all the plate arcs (parhelia, parhelic circle, circumzenithal arc, pillars, etc…) and also the Parry arcs (sunvex through the stuck crystal and suncave through the upper base of the plate and the stuck crystal. So this configuration seems to be well compatible with the halos usually observed with the Moilanen arc.

Spurious arcs
I have been able to check it thoroughly but there does not seem to be any spurious arcs from this configuration. Spurious arcs would be created by rays entering side faces of the main plate and side faces of the stuck crystal. I will try to check it, but for the most simple other rays, it does not seem to exist direct ray path, at least if the stuck crystal is a regular hexagonal crystal.

Other rays would just create already known halos. Rays entering by the base of main plate and exiting by the sides of stuck crystal would create parry arcs, and by the base of main plate and exiting by the base of stuck crystal would create circumzenithal and supralateral arcs.

Distance from the sun
The distance between the apex of the Moilanen arc and the sun can be easily calculated using the 30° prism with vertical entrance face description. At low sun elevation, the distance is about 11°. It increases slowly to 13° at 10° solar elevation, 15° at 15° solar elevation, 18° at 20° solar elevation. As the sun elevation increases, the rays path becomes less likely because of the stucking area and also of the increased incidence on the exit face. The total internal reflection condition is met when the solar elevation is higher than 26°. This is in agreement with the fact that Moilanen arc are usually observed at low sun elevations.
Figure 5.

Shape evolution
I have made a simple ray tracing program to simulate the Moilanen arc shape. This simulation tool does not take into account the real crystal shape, only the relative orientation of the faces. The shape are represented for 0°, 3°, 6°, 9°, 12°, 15° and 18° solar elevation. The simulated shape matches very well the photographs of the arc, with a reduced curvature at higher solar elevation. The simulation shows the arc being more extended on the sides as I don’t take into account the real 3D shape of the crystal, but just the relative face orientation.
Figures 6, 7, 8.

The proposed explanation is based on a 30° prism which entrance face is vertical. In this explanation, the responsible is a horizontal plate on which is stuck a plate or column crystal. I think this type of crystal explains very well most (if not all) of the observed properties of Moilanen arc. More simulations to check that no spurious arcs arc created would help confirm this, as well as some insights of crystals experts on the probability of getting such configuration.

Nicolas Lefaudeux, January 29th 2010

Thursday, 28 January 2010

Display from Poland

On 15 January Marcin Matusinski from Poland observed one of the best Polish halo displays. It was visible on a ski slope on the Male Skrzyczne Mountain in the Beskidy Mountains (elevation about 950 m). The crystals originated from the snowguns. The display lasted almost the whole day. The conditions were unstable but they were the best about 10:20 a.m. when the Sun altitude was 13,5 degrees. In the morning the Moilanen arc could be observed ( 1 ). Later, in addition to the more usual phenomena, there were parhelic circle, Parry arc and infralateral arc ( 2 - 3 ). The most interesting was the anthelic with Tricker and diffuse arcs ( 4 ).

Jakub Marchewka

Wednesday, 27 January 2010

Parry and helic arc

The photo of this display is from László Dubek. It was shot on 28 December 2008 in the skiing cente of St. Corona, Austria. For further photos of the display, you can visit László's gallery. ( 1 )

Windshield halos

Last night we were observing a halo display on a car windshield. We will return to this later when all photo and video material is processed.

Marko Riikonen, Jari Luomanen

Forked Pillars and Helic Arcs

I expanded the "Forked Pillars and Helic Arcs" article. As before, it is on my page ( 1 ). For observers probably the most intriguing addition to the article is the suggestion to watch for a zenith ring. With hindsight the idea of a zenith ring is painfully obvious.

Walt Tape

Tuesday, 26 January 2010

Blue spot with green

This blue spot, that shows also green coloring, was observed by Patrik Trncak in the Czech Republic on 25 January. For further photos of this high-cloud display, click the following links. ( 1 - 2 )

Monday, 25 January 2010

Pillars pillars pillars!

January 2010 in Tampere, Finland has seen an incredible onslaught of pillars. Starting from New Year’s Eve cold weather (which recently has been fed by Siberian high) has settled over Finland. The temperatures have been around -20 °C. Pillar forests have reigned night after night, as the conditions have been mostly favourable to crystals that only produce pillars.

Snow making in the ski centres of Tampere area has stopped about two weeks ago. Despite this, diamond dust has been a nightly occurrence. Sufficient humidity and probably urban emissions have contributed to crystal precipitation.

We have hunted halos practically every night, driving around Tampere to find the crystal swarms. To our surprise these swarms - usually a few kilometers across - remained in one place throughout the night. Our impression is that local topography affects this behaviour.

More images and some further observations are available in separate galleries (1 - 2)

Marko Riikonen and Jari Luomanen

Tuesday, 19 January 2010

Lighting up a crystal glitter subsun

A subsun that glitters in separate crystals against the ground can look pretty nice, but in the photos it as a rule turns out a disappointment: only a few anemic glints are captured. But there is a recipe to make the image match the visual perception or go even above it. That is to stack a series of photos using the lighten method (in Photoshop).

In the image above, there is a comparison of a single photo and a lighten stack of 99 photos. The camera was on tripod and the photos were shot fast with series mode. During shooting a car was moving in the parking lot (a ghostly image of it is seen on the right in the stacked image). These frames were omitted from the series because sun's reflections from the car might have been disturbingly imprinted in the stack as the lighten mode takes always the brightest pixel of the stacked images.

Marko Riikonen

Monday, 18 January 2010

Subanthelic halo

This white pillar rising from the subanthelic point was captured by the Swiss panorama webcam of Davos Rinerhorn (2281m) a bit more than a year ago on 14 November 2008. The picture was submitted by Bertram Radelow. By clicking on it, you get to Bertram's homepage, where you will see the full-size version of three images from the period between 8:30 and 10:00 local time.

Sunday, 17 January 2010

46 degree halo from Davos

Bertram Radelow was the observer of this halo display on 14 December 2009. The temperature was -15 degrees that morning in Davos, Switzerland, and the crystals originated from the nearby snowguns. Further images of the display are available on Bertram's site ( 1 - 2 - 3 - 4 ).

Saturday, 16 January 2010

New halo in elliptical halo / Bottlinger family Part II

A video and two new images of the 09-10 January 2010 Display in Kangasala, Finland are now available for viewing (1 - 2 - 3). As you follow the first link please note that the video is displayed below the still image.

The video consists of 79 consecutive still images, each shot with a 6s exposure time. The camera shot a new frame automatically as the last one finished so the exposures lasted for 479 seconds (just under 8 minutes). Based on the EXIF data recorded by the camera the 79 frame sequence lasted for 585 seconds (so 106 seconds of the time were used by the camera's internal electronics and the operation of the shutter mechanism between frames). This time frame of 585 seconds is compressed into 20 seconds on the video.

The width of the halo changes abruptly between frames in the video. At one point (about three seconds into the video) the shift is particularly dramatic. Either the change is due to sudden changes in crystal orientations or the apex angle of the halo forming wedge in the crystals was changing (probably in steps, not in smooth continuity).

Part I of this post can be found here: (4)

Edit: new animated images are available (5 - 6). See comments for more information.

Wednesday, 13 January 2010

Odd radius snow surface display in Tampere

Last saturday I went to lake Pyhäjärvi ice to see diamond dust that was there. It turned out to be water fog, but there was moderate snow surface 22 and 46° halos. The 22° halo was broad, unlike any other seen this winter, and I thought of seeing a ghost of 24° halo outside it. So it was time to take photos.

Stacking revealed the odd radius halos better. The one above - a stack of 138 photos - is made with lighten mode in Photoshop, and shows 9, 20, 24 and 35° odd radius halos. The 24° one is not well separated from the 22° halo and it shows better in 293 frame stack by average mode using Registax ( 1 ).

The lighten method (introduced by Jari Piikki for snow surface halos), which chooses the brightest pixel of the stacked photos, provides more realistic looks with the crystal glitter and here the realism is somewhat enhanced due to high number stack. It has the downside is losing colors: colored pixel is replaced if it overlaps with a white one. But you can still see plenty colored crystal glitter in the 46° halo in the larger image that opens by clicking the photo.

For the lighten method stack photos are taken low down where most of the halo is formed of separate crystals. From standing height the halo becomes more solid, especially near the horizon and then average stacking mode works better.

Having seen a couple of snow surface odd radius displays now it seems they can be expected when the snow surface has been fed with water fog. The odd radius halos rarely show up clearly visually, but a broad 22° halo, that is not well defined from its inner edge, seems to be a sure sign.

Marko Riikonen

The brightest halo in a display: the Moilanen arc

Here is an example of a display where Moilanen arc intensity exceeds other halos. The arc is also sharp. If crystal sampling Moilanen arc displays has begun to feel frustrating, we still should carry the microscope with us: these kinds of displays are certainly worth a look.

The display was photographed by Harri Mänty on January 9 in Vantaa, Finland. The temperature was about -15 to -16° C. Click the image to see the rest of Mänty's photos.

Tuesday, 12 January 2010

44° parhelia from the Czech Republic

The display with 44 degree parhelia was observed by Roman Szpuk in the morning of 5 January at the meteorological station in Churanov, Sumava Mountains. The temperature was -11 °C, and the crystals may have originated from the nearby ski slope's snow guns. Besides some more usual halo forms ( 1 ), the display also showed 120° parhelia and faint reflected Lowitz-arcs ( 2 - 3 ).

Sunday, 10 January 2010

New halo in elliptical halo / Bottlinger family

On the night of 9-10 January in Kangasala we observed light pillars that were accompanied on both sides by steeply rising arcs. These arcs were seen both with floodlights (two rightmost halo photos in the collage above) as well as with narrow beam spotlight (the leftmost photo above). In floodlights the arcs were somewhat straight, while in the spotlight beam they tapered towards each other, perhaps forming a loop around the pillar.

The display was visible for almost four hours in a very localized pocket of the Kangasala industrial area of 500 meters across at most. Atmospheric humidity was high, the area was surrounded by water fog. Probably emissions in the industrial area initiated nucleation of this fog to ice crystals. The temperature was -16 degrees Celsius.

This halo probably arises from a simple reflection from nearly horizontal crystal faces. The crystals we used in the simulation (see above image; software used: HaloPoint 2.0 beta by Jukka Ruoskanen) are similar to those used in simulating elliptical halos and Bottlinger's rings. The raypath for the halo is same as for Bottlinger's rings, except that it occurs on the lower pyramidal face. The atmospheric conditions (temperature, high humidity) support the idea of such a crystals since Bottlinger's rings and elliptical halos are mostly observed in altocumulus virga, which form in similar conditions.

Due to the halo forming raypath, this halo could be called as the super-Bottlinger if we were to adapt the naming system suggested by Walter Tape or the supra-Bottlinger in case Gunther Können’s naming system was used.

The series of images captured in the spotlight beam show variation in the angle of these new arcs. Presumably this happened because the angles of the reflecting faces changed. It had been long suspected that elliptical halo sizes can change during the display - our observation provides now supporting evidence. However, we did not observe any elliptical halos. At times an elliptical glow was suspected around streetlights but these were never confirmed by photographs.

Crystals were sampled and photographed at the same time as the display was photographed in the spotlight beam. The crystals have dendritic growth and it is difficult to see the structure responsible for the new halo. Possibly the tips of the branches are sloping. Such feature has been earlier pointed out by Walt Tape in a snow crystal photo. From the brightness variation between the pillar and the new halo in different stages of the display, we may assume that the extent of the sloping area in the crystal varies.

Additionally, the display contained diffuse helic arc like extensions from the lamp, as seen in the middle photograph. These were also observed when only pillar was visible. Possibly the abundant crossed plates were behind their formation.

See more photos from this night of halo hunting ( 1 - 2 ).

Jari Luomanen, Marko Riikonen

Tuesday, 5 January 2010

Lunar odd radius display in Fairbanks

Fairbanks was treated to a nice odd radius lunar display on the morning of December 31, 2009. The image above consists of four of my frames stacked by Jari Luomanen. I hope to have more photos and information on the display available on my site ( 1 ).

Walt Tape

Sunday, 3 January 2010

Moon elliptical halo in Kangasala, Finland

Eetu Saarti photographed this double ellipse in the new year's night. The phenomenon lasted for 15 minutes and was followed immediately by 22° halo. Click the image to see a gallery with two more photos.