Monday, 29 February 2016

Wegener or Hastings?



This spotlight display that was photographed in Rovaniemi on the night of 5/6 January rises the question of whether it is Wegener or Hastings arc or both together that is seen here. From the outset, as there is helic arc but no subhelic arc one could argue that it is rather the uppervex Hastings arc.




When comparing with simulations (below) it does not look that obvious anymore. The curvature and extent of the arc looks more akin to Wegener than Hastings (the side view image). And it is possible to simulate Wegener without subhelic arc by using thin plates in column orientation. Indeed, to have best agreement with the images, in all simulations thin plates were used both in Parry and column orientation.
In the image on the left the crystal h/d is 0.1 for all simulations. In the one on the right it is h/d 0.03 for rotations of 0.5 and 5 degrees, for full rotation simulation h/d 0.1 was used. Crystals are triangular, oriented in Parry orientation so that the tip is pointing up.
Well, none of these simulations come out satisfying. The light source elevation for simulations is -5 degrees.
Marko Riikonen / Nicolas Lefaudeux
– added one more image with simulation containing both Wegener and Hastings



Wegener or Hastings?



This spotlight display that was photographed in Rovaniemi on the night of 5/6 January rises the question of whether it is Wegener or Hastings arc or both together that is seen here. From the outset, as there is helic arc but no subhelic arc one could argue that it is rather the uppervex Hastings arc.




When comparing with simulations (below) it does not look that obvious anymore. The curvature and extent of the arc looks more akin to Wegener than Hastings (the side view image). And it is possible to simulate Wegener without subhelic arc by using thin plates in column orientation. Indeed, to have best agreement with the images, in all simulations thin plates were used both in Parry and column orientation.
In the image on the left the crystal h/d is 0.1 for all simulations. In the one on the right it is h/d 0.03 for rotations of 0.5 and 5 degrees, for full rotation simulation h/d 0.1 was used. Crystals are triangular, oriented in Parry orientation so that the tip is pointing up.
Well, none of these simulations come out satisfying. The light source elevation for simulations is -5 degrees.
Marko Riikonen / Nicolas Lefaudeux
– added one more image with simulation containing both Wegener and Hastings



Friday, 26 February 2016

Diamond dust halos on the night of 12/13 January, part III



Has anyone seen subparhelic circle inside subparhelia? Neither have I. Not before stacking the photos from the latter part of the 12/13 January night. In one of the stacks this missing segment is seen faintly inside subparhelia. The version of the image above, made by Nicolas Lefaudeux, was done with emphasis on getting the segment stand out as clearly as possible.

Below is a more conventional looking version of the image and simulation. The plate oriented crystals in the simulation were fully triangular. Regular hexagons do not make the missing segment, except at very high light source elevations.





What else? The arcs that in Finland these days are called the Schulthess arcs are quite well defined, especially in the last photo set of the night (below). One sees both concave and convex components extending from subparhelia to parhelia, though the latter component seems to have a gap around the horizon, not reproduced by the simulation. I have never seen in sun display photos the concave component between parhelia and horizon. It is always just the convex component that is visible.

Marko Riikonen




Diamond dust halos on the night of 12/13 January, part III



Has anyone seen subparhelic circle inside subparhelia? Neither have I. Not before stacking the photos from the latter part of the 12/13 January night. In one of the stacks this missing segment is seen faintly inside subparhelia. The version of the image above, made by Nicolas Lefaudeux, was done with emphasis on getting the segment stand out as clearly as possible.

Below is a more conventional looking version of the image and simulation. The plate oriented crystals in the simulation were fully triangular. Regular hexagons do not make the missing segment, except at very high light source elevations.





What else? The arcs that in Finland these days are called the Schulthess arcs are quite well defined, especially in the last photo set of the night (below). One sees both concave and convex components extending from subparhelia to parhelia, though the latter component seems to have a gap around the horizon, not reproduced by the simulation. I have never seen in sun display photos the concave component between parhelia and horizon. It is always just the convex component that is visible.

Marko Riikonen




Thursday, 25 February 2016

Diamond dust halos on the night of 12/13 January, part II



More photos from the 12/13 January night. The image above is a view opposite to the spotlight. Seen is blue circle, diffuse, Wegener and subhelic arcs. Below are two more images, the blue-minus-red image shows the “column 351/361”, which is the Kern arc equivalent of 46° supralateral arc. The lamp is about 5 degrees below the camera.

Marko Riikonen





Diamond dust halos on the night of 12/13 January, part II



More photos from the 12/13 January night. The image above is a view opposite to the spotlight. Seen is blue circle, diffuse, Wegener and subhelic arcs. Below are two more images, the blue-minus-red image shows the “column 351/361”, which is the Kern arc equivalent of 46° supralateral arc. The lamp is about 5 degrees below the camera.

Marko Riikonen





Wednesday, 24 February 2016

Diamond dust halos on the night of 12/13 January, part I



With this post starts the presentation of January 12/13 night results in Rovaniemi. All photos are in spotlight beam. The image above is a singe frame with 30s exposure. Just basic halos there. In the rest of the images emphasis is on the crystal glitter formation that makes an intensity threshold at the zenith. The ray-path for this effect, which is quite common in spotlight displays, is 3-5-7 and here it is from column oriented crystals. What’s peculiar is the psychedelic colours of individual crystals, as shown by the left side image below that is a maximum stack of four frames. In the 20 frame average stack next to it the colours are washed away. The lamp is the usual 5 degrees below horizon. In the images below are two single frames which display the coloured glitter even more clearly and another maximum stack of 14 frames. Individual frames are all 30s exposures.


Marko Riikonen







Diamond dust halos on the night of 12/13 January, part I



With this post starts the presentation of January 12/13 night results in Rovaniemi. All photos are in spotlight beam. The image above is a singe frame with 30s exposure. Just basic halos there. In the rest of the images emphasis is on the crystal glitter formation that makes an intensity threshold at the zenith. The ray-path for this effect, which is quite common in spotlight displays, is 3-5-7 and here it is from column oriented crystals. What’s peculiar is the psychedelic colours of individual crystals, as shown by the left side image below that is a maximum stack of four frames. In the 20 frame average stack next to it the colours are washed away. The lamp is the usual 5 degrees below horizon. In the images below are two single frames which display the coloured glitter even more clearly and another maximum stack of 14 frames. Individual frames are all 30s exposures.


Marko Riikonen







Lunar odd radius display in diamond dust



On four out of last five winters Tapio Koski has photographed lunar diamond dust odd radius halos in the Rovaniemi area. These one-per-winter occurrences are almost solely responsible for lunar diamond dust odd radius displays photographed in Finland during those years. This winter we wanted take part in the tradition. Yet despite numerous odd radius displays we had harvested in the beam, those by the moon – or sun for that matter – were simply not coming.

Except on the night of 20/21 January, which was the month’s last diamond dust night in Rovaniemi. During the day, when driving in the city, we paid attention to Fairbanksian amber, a beautiful yellow glow in the sun direction that can be seen in cold weather and with which we became familiar on the successful halo expedition to Fairbanks in January 1996. This gave us an omen of foreboding that a night of big odd radii diamond dust was finally on the cards for Rovaniemi. Weather forecast was with us too, as the temperature was expected to drop to -33° C – the magic number that Walt Tape has given as being in the centre of the temperature range favourable for odd radii.

The display appeared as some thin water cloud that had momentarily overtaken the sky cleared away. The first halo visible was upper 23° plate arc, many others soon followed the suit. In the beam only a crappy plate dominated display was visible – the pyramid stuff was higher up.

Jarmo Moilanen / Marko Riikonen








Lunar odd radius display in diamond dust



On four out of last five winters Tapio Koski has photographed lunar diamond dust odd radius halos in the Rovaniemi area. These one-per-winter occurrences are almost solely responsible for lunar diamond dust odd radius displays photographed in Finland during those years. This winter we wanted take part in the tradition. Yet despite numerous odd radius displays we had harvested in the beam, those by the moon – or sun for that matter – were simply not coming.

Except on the night of 20/21 January, which was the month’s last diamond dust night in Rovaniemi. During the day, when driving in the city, we paid attention to Fairbanksian amber, a beautiful yellow glow in the sun direction that can be seen in cold weather and with which we became familiar on the successful halo expedition to Fairbanks in January 1996. This gave us an omen of foreboding that a night of big odd radii diamond dust was finally on the cards for Rovaniemi. Weather forecast was with us too, as the temperature was expected to drop to -33° C – the magic number that Walt Tape has given as being in the centre of the temperature range favourable for odd radii.

The display appeared as some thin water cloud that had momentarily overtaken the sky cleared away. The first halo visible was upper 23° plate arc, many others soon followed the suit. In the beam only a crappy plate dominated display was visible – the pyramid stuff was higher up.

Jarmo Moilanen / Marko Riikonen