Monday, 28 February 2011

Fake sun again

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A little longer gap since the last post. The upcoming halo book occupied me during the last days as I made last adjustments before submitting it last night to the publisher for layout.

Anyway, the issue here is fake sun again. The photos above were taken by Olli Sälevä in Rovaniemi. The first two are fresh, from 22 February. After I had written about the fake sun, Olli started to look for it and took these photos. He saw a bright yellow fake sun that day near the sunset, but parking the car and getting the camera ready took 2-3 minutes during which time the real sun already appeared and the fake sun waned. But you still can get the idea here. A bit later the sun got again behind cloud and a spot of light was seen above sun.

The two last Olli's photos are older, from 25 April 2008, showing a beautiful sky with Strato/Altocumulus and pillar. Again here is a situation where fake sun might appear.

Now that I have written already four posts about the fake sun and seen some photos of candidates, it is time to say what I think about the issue. To save some dignity with the phenomenon - so that we would not start reporting every brightening in sun pillar as fake sun - we should only report as fake sun those that have the potential to be taken as real sun. This also takes into an account cases where the real sun is seen, but in which the fake sun is so bright that it could be taken as real sun if the real sun was behind the cloud. By this criteria, I think, the fake sun is a rare phenomenon. And one needs an account from the observer of the sighting, determining it alone from the photos is not enough. The photos by Julie Broson show a true fake sun, because at first for a moment Julie did not know which one was the real sun. Neither was, as the real sun was behind the cloud.

Friday, 25 February 2011

More elliptical halos

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Ari Laine photographed this elliptical halo on 24 February in Lahti. It was already third occurrence within two weeks in Finland. This time the ellipse is more like an elliptical glow rather than well defined ring, but outside the glow above the sun there might be a faint ring visible (one has to be cautious about jpg artefacts).  During the last two weeks the conditions have been favourable for ellipses in Finland as it has been clear skies and around -15° C temperatures during the daytime, which is the optimal temperature for the elliptical halo crystal formation. In this case the temperature on the ground was -16° C, but the ellipse formed in an ice crystal layer that slightly above the ground. In Tampere similar crystal layer situation was observed as well, however, the halos were the usual stuff.

Thursday, 24 February 2011

Moon diamond dust odd radius halos in Finland

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On the evening of 17 February Tapio Koski went out on the river ice in Muurola, Rovaniemi, to photograph northern lights. They were not good and he took photos of the moon halos that appeared in the sky. It was an odd radius display and it contained two extremely rare halos: the 20° and 35° lower plate arcs, of which the lower 35° plate arc is even more difficult to catch, as it is never seen much above the horizon. Everything went smoothly for the documentation of the display. The location was optimal with low horizon and Tapio was using wideangle which allowed to capture the whole display and he took series of photos on a tripod for stacking.

First two of Tapio's photos above are single frames with only slight unsharp masking. The next three images are four and five frame stacks and have stronger usm to enhance the lower 35° plate arc at the horizon.  In the fifth image the different radii plate arcs are identified. Moon elevation for the photos is 31-32 degrees. More Tapio's photos are on his site.

Odd radius diamond dust displays are rare in Finland. All together about half a dozen cases have been reported since 1983. The only other lunar odd radius display in Finland was captured by the Finnish meteorological institute automatic aurora borealis camera in Kevo, Northern Finland, on 8 January 1985. Above is a drawing of the display by Jarmo Moilanen.

The odd radius diamond dust displays are formed in urban areas from the power plant and factory plumes at low temperatures. While rare in Finland, in Alaska's Fairbanks they are seen regularly every winter due to the cold weather.

Wednesday, 23 February 2011

Old Dutch halo observations in Onweders, Optische verschijnselen

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The Dutch weather amateurs journal Onweders, Optische verschijnselen, that was published between 1880-1961 contains plenty of observations of atmospheric optical phenomena. Two days ago I visited the Repository library in Kuopio to mark the relevant pages (mainly halos) from the publications for scanning. Pasi Juttula from the library scanned everything yesterday and I have put them here for download. Not all years are there, issues before 1896 are missing from the library collection as well as year 1933. Also I did not mark some of the last issues for scanning, because there seemed to be nothing interesting. Above is a selection of halo drawings from the journals, as well as photos from my visit to the library.

Monday, 21 February 2011

More fake sun photos and drawings

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Fake sun observations are stacking up! There is much more than I thought in the beginning. First, Mike Hollingshead sent some of his old photos which look much like a fake sun. In the first two photos the fake sun is above the sun and in the third photo, where real sun is completely hidden, below. And today I found out, while visiting the repository Library in Kuopio to check the Dutch Weather amateur's journals Onweders, optische verschijnelselen, that there is a regular column devoted to observations of double suns and moons, before rainbows and halos. Plenty of sightings are listed from about two decades starting from the end of 19th century, with two drawing published from years 1897 and 1899, shown in the last two images. Yesterday also one fresh appearance was reported in Finland.

The effect is not really as forgotten as I thought, because it is mentioned in Minnaert's Light and color in the open air, which is not that old book (my Minnaet has been lost for many years). Mike suggests that the effect can't be at all rare. This maybe true. Possibly people just don't look for it because one has to look near the glare of the sun, or even when they look, they don't think it is anything special. And if one lives in a place where low sun can not be seen, this limits the oppotunities to observe the phenomenon.

Some additional issues are worth emphasizing about the fake sun. First, the sun need not be at all hidden for it to be seen. This is seen nicely in Johannes Hevel's draving from 5 February 1674, the fourth image above. Also my observation from 10 May 1987 (fifth image) is a situation where sun is shining unobstructed. The note for the observation says that the fake sun above the real sun was only SLIGHTLY fainter than the real sun. Second point that emerges from many images above is that the fake sun can appear some distance apart from the sun. And finally, not every pillar has a fake sun; there is my other observation (sixth image), from April 1987, where the situation would look fruitful for the fake sun, but only pillar was reported. For me fake sun is something that is or can be potentially mistaken for a real sun. Since I started to write about this, some people have been dubious about fake sun in general or not really understanding how it should look. I can only say that when one sees a true fake sun, there is no need to question anymore.

Two names have been used for this effect, fake sun and double sun. The French use the former (faux soleil) and the Dutch use the latter (dubbele zon). Both have their advantages and disadvantages depending on the situation. The Dutch have also observations of fake moons. I wonder if there are any sightings of fake planets or stars.

Saturday, 19 February 2011

Diffuse arcs in the spotlight display on November 2007 in Rovaniemi

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These photos have been around and some are still in some blog probably, but I have not made simulations. This was seen in the beginning of November 2007 in Rovaniemi, my second experiment to make halos with portable spotlight. And it was good! Even in the middle of this all light pollution near the center of the city the display was strong.

Different views of the display are given above but the attention here is towards the anthelic arcs in the third and fourth image. There is a combination of diffuse and Tricker anthelic arcs, which together with the subhelic arc make some people see a ruminant skull in the third image. Now a good Tricker arc like this in spotlight displays is quite rare, usually it is the diffuse arcs that dominate and Tricker is almost non-existent. The simulations of this display were made with regular hexagons (with little variation allowed) that had on average aspect ratio 0.9, which is already more plate than column crystal (last image). This created the Tricker and diffuse arc combination as close to the photo as possible. It does not stand close scrutiny, but that's the best I got. Simulations are made with Jukka Ruoskanen's HaloPoint software.

The diffuse arc is mainly composed of two components, diffuse-A and B, which can be called also as Tränkle and Greenler, who found these halos in the computer simulations. But it is known that there is more diffuse arcs components, in sixth image I have dissected them into four components, A, B, C and D. Thusfar we have been only able to distinquish A and B in photos of the spotlight halo displays, the C and D are weak and get mixed with the main components. But perhaps here one can see the influence of diffuse C in the photo. The seventh image gives a comparison with simulations where are all four components and the C component removed. The difference between the simulations is subtle, but maybe there is a slight indication that diffuse C may play here a little part in strenghtening the diffuse A.

Friday, 18 February 2011

Is here another fake sun?

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A couple of days ago I made a post about the fake sun, showing the excellent photos of Julie Bronson and drawing by Giovanni Cassini. This reminded Mika Aho of his photos of similar looking effect that he took on 19 May 2008 in Korpilahti, Finland. The two photos are shown above and they were shot with an old cell phone camera, which explains the slightly less than optimal quality. The sun is behind the edge of dark Stratocumulus cloud and the fake sun candidate is seen below the sun, with short and fainter extension of pillar further down. The crystal precipitation from the cloud is not visible.

Now it looks like there may be a circular fake sun image of the sun under the cloud edge, but it is not very bright. The cloud edge is actually much brighter.  By strict definition one could say that a genuine fake sun should be so bright that one would take it for a real sun, but of course there is a continuum of intensities, and putting an arbitrary line somewhere along the range is not very fruitful. So, with some reservations due to the images quality, I might say that probably we are looking here at a faint version of a fake sun. In any case it is good that people have started looking for this, be it old photos or fresh displays in the sky.

Wednesday, 16 February 2011

Rare 44° parhelion in Stockholm

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David Åsbrink took these photos of a diamond dust halo display in Stockholm on 8 December 2010. The phenomenon was noticed by many people. In addition to being a bright and beautiful display this one contains also a rarity: inside the 46° lateral arc there is a weak colored spot, the 44° parhelion. It is marked in the third image with an arrow. 44° parhelia are the parhelia of very bright normal parhelia and they were photographed first in 1970 in Canada's Saskatoon. Since then about 13 more photographed cases have accumulated. These days it seems like we are getting at least one photographed case every winter. The solar elevation for these photos was 8 degrees.

Tuesday, 15 February 2011

Another elliptical halo

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Two posts and and three days back there was an elliptical halo in the sky and today ellipse appeared again. It had been a cold night with down to -30° C, and during the daytime as the thin cloud cleared and sun started shining more strongly, also crystal glitter in the air increased. Couple of times I looked out and saw only pillar and poorly defined subsun, but last check came also with faint elliptical halo. Above are different versions of the photos, stacked 13 frame average and lighten images and two single images. Heavily usmed average stack seems to indicate two slightly separate elliptical halos.

This formed in crystals precipitating from very low, almost invisible cloud layer. Temperature at Tampere stations ranged from -15 to -20° C at the time of the observation, the coldest being at Siilinkari on the lake Näsijärvi. I am not sure if heat plant plumes had anything to do with this. The connection was not obvious, at least.

Edit 15 February: Eetu Saarti saw much better ellipse in Kangasala the same day.

Sunday, 13 February 2011

Fake sun

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Julie Bronson's photos above show a largely forgotten effect of a sun pillar, the fake sun. It was photographed on 19 January 2009 in South Central Minnesota, USA.

Sometimes, when low sun is just behind an edge of an opaque cloud that is precipitating ice crystals, an image of the sun can been seen below or above the cloud in the icy virga. The effect is so perfect that one is easily fooled to think it is real sun. It is blindingly bright, circular and about the size of the sun. Only when the true sun comes up from behind the cloud, the cheat is revealed.

Today this optical effect of sun pillar is almost unknown, but in the past sky watchers have made detailed notes on it. Auguste Bravais listed in 1845 five persons who have observed the fake sun, including Johannes Hevel, Pierre Bouguer and Giovanni Cassini. Above is shown the observation by Cassini on 18 January 1693. Unfortunately it is a bit cropped from the left.

Julie Bronson's images of the fake sun may be the only ones existing. Her documentation of the phenomenon is commendably extensive and comparison with Cassini's 300 years old observation is good. I have seen fake sun a couple of times during 25 years of sky watching. Appearances of this effect are short and camera must be handy when the time comes.

Saturday, 12 February 2011

Possible elliptical halo

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Half an hour ago I was talking with Jari in the phone and monitored the sky at the same time. There was Stratocumulus drifting fast and it was turning into ice. I saw an elliptical halo and eventually got some photos, but I am not so sure about this anymore. Maybe it is an ellipse, maybe not.

Update: the sky got almost completely overcast for a while, but then Stratocumulus receded and I was ready with camera waiting for the ellipse. Which appeared and also shows up clearly in the photos (the second collase). All together this was a beautiful case. Fast moving low clouds, crystals in the air, iridescence... I have seen in similar conditions an elliptical halo in Kilpisjärvi while walking up the Saana mountain. Possibly they are not that rare there in early spring around March.

Update II: it turned out many people saw this in Tampere, Helsinki and Lahti. Best photos took Olli Leivo and Jari Luomanen. If my memory is correct, the temperatures in Tampere stations were between -10 and -14° C.

Sunday, 6 February 2011

Halo observations by J. R. Blake in the Antarctic (part II)

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Completing the first post of Roger Blake's halo observations in the Antarctic, here are the rest of his halo drawings together with the written descriptions. The display at midnight between 2 and 3 December 1958 comes as two dravings in Blake's book. It was remarkalbly accurate observation of a complex diplay with all four kaleidoscopic arcs, especially when one takes in to account that it was observed from a moving dog sledge. The display had also its contribution to halo science: both Tricker and Greenler used the observation to support the theoretical considerations of the kaleidoscopic arcs. In the drawing these arcs are 1) helic arc; 2) subhelic arc; 3) subanthelic arc and 4) Tricker arc. The observational account in the above image is not given full, but only from the part concerning these arcs.

Friday, 4 February 2011

Diamond dust halos at -2° C from Italy

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Diamond dusts close to freezing point are not common. This display, photographed by Francesco Cadini on 3 February 2011 a few kilometers outside Milano, occurred when Francesco's car thermometer showed values between -1.5 and -2° C. Taking into account a possible inaccuracy, he says that it could not have been colder than -3° C.

The display formed from thick water fog. Cadini tells that in the area where the display was seen there is a phenomenon called the "chemical snow",  which occurs in high pressure situation with fog and clear sky above the fog. The abundant pollutans in the air are supposed to initiate this chemical snowing and in one case 2 cm of snow accumulation has been recorded. There is an Italian language description of the phenomenon.

Most likely the pollutants were also responsible for the transformation of the water fog to observed diamond dust. A quick look at some papers reveals that if certain substance acts as an immersion freezing nuclei and contact freezing nuclei, the latter works at much higher temperatures. So probably the formation of this diamond dust was initiated by pollutant particles colliding with water droplets. Once the process starts, sudden freezing of droplets may produce splinters that further freeze other droplets and chain reaction is created. The water droplets also evaporate and the released water vapor deposits on ice particles, and through this growth we get the crystals with proper faces for halo making.

Supercooled water fog in general has a tendecy to turn into diamond dust locally. If you drive around the city when it is below freezing you are likely to encounter spots of diamond dusts.  Usually only the lower layer of the fog turns into diamond dust and thus you can see halos only at night in the light of streelamps. Sometimes, though, as in Cadini's case, a whole fog column turns into ice crystals and sun or moon halos can be observed. The process punches a hole in the fog and in the photos above the edge of the foggy wall is seen clearly.

Cadini's display has also weak Moilanen arc, which shows best in heavily usmed image. He suspects that this might be the first Moilanen arc photographed in Italy.

Wednesday, 2 February 2011

Old photos of halos from anthelic and subanthelic region

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Alan Clark sent these scans of his old slides. The anthelic arcs in the first two photos were photographed in 1980 in Canada, while the two latter images with subparhelic circle, subanthelion and possible faint arcs upwards from the subanthelion were photographed in 1978 on a flight across the US, en route to NASA Ames. Halos suffer markedly when scanned from slides but it is always nice to see any documents of these rare halos.

Tuesday, 1 February 2011

Halo observations by J. R. Blake in the Antarctic

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There are two books devoted to halo observations in the Antarctic in 1950's. One is by the Swedish meteorologist Göstä Liljequist and another by the Australian glasiologist Roger Blake. The images above are from Blake's book "Solar Halos in Antarctica", which I found for sale in an Australian antikvariat a few years ago. Blake was in a project called the "Souther Seismic Traverse", using the Australian research station Mawson as a base for the trips further south. The time he spent in the Antarctic spanned from 30 September 1958 to 17 January 1959 and halos were a sideline research for him, clearly inspired by the Liljequists's work in the early 50's. Above are shown three displays from Blake's book.

The first display, observed from a field trip at the location 70.1° S and 62.1° E on 21 November 1958, contains 9° halo. Blake wrote the following notes on it: "The parhelic circle extended only about 10° either side of the sun, being rather faint. It formed a closed semicircle with a very faint Hall's halo, the radius of that halo being approximately half that of the 22° halo, but observations being greatly impeded by the sun's brightness."

The second display was seen near the first display's location, on 28 November 1958. It has sunvex Parry arc and halos at the anthelic region: "Through the anthelic point was a bright, white vertical pillar, reaching the ground. Also visible was part of the parhelic circle, the intersection of the two arcs being a 'spot' of greater intensity. The parhelic circle did not exists elsewhere."

The third observation on 29 November 1958 was made in the same area as two previous ones. It has several interesting features: "The 22° and 46° halos were both very brightly coloured, and both 22° and 46° parhelia were present, both pair being very bright. The vertical pillar extended from the horizon to the top of the 22° halo, which was brilliant. From this point extended the upper contact arc which merged smoothly with Parry's Arc; both these arcs were coloured and very bright. [... ] The circumzenithal arc was also present, being very brightly coloured. A fairly bright, coloured arc, passing through or close to the zenith and concave to the sun was visible, the colours being very pure and distinct. Because of its position, it was difficult to obtain any estimates; however, it appeared to have approximately the same curvature as the 22° halo. The full 180° of the parhelic circle was visible on either side of the sun, being intersected approximately 90° from the sun by a pair of white pillars extending from the horizon slightly above the parhelic ring. The points of intersection resembled mock-suns, though white. Also at 180° there existed a somewhat fainter, white pillar extending to just above the horizon, the point of intersection with the parhelic circle again being brighter. The display disappeared as the cloud cover increased."

Now I could go on discussing what Blake really saw in 29th November display, but that would not lead anywhere. Suffice to say: a photograph would have been nice.