Monday, 31 October 2016

Diamond dust halos in spotlight beam in the evening of December 2, 2015

By Marko Riikonen and Olli Sälevä

Here are shown the rest of the photos from the night that yielded the second capture of the anomalous Wegener/Hastings. From the golf course parking lot, where we took those photos, we walked into the golf course, and were able to place the lamp even lower down.

The display was no more as good, but in the photo above and two below there is nevertheless again visible a short patch of Wegener/Hastings on top of the 22° halo. Whether this one has an anomalous curvature, is hard to say. Judging from the 46° stuff that is seen against the forest, the crystal orientation would rather be column than Parry, because it has the looks of a 46° infralateral arc, not Tape arc, which would be more spotlike.

The swarm shifted and we followed it to a field over the river. There we continued taking photos, as shown below, but that was cut short when the spotlight started flashing at 10 pm. The 60Ah car battery had run out of power and it was game over for the night.

It was painful because the conditions seemed to continue at least for the next five hours and probably even amping up as the temperature rose from -11 to -5 °C. Under clear skies such a warming would have killed the diamond dust, but now it was overcast and the clouds kept hanging low, meaning they were likely nucleated to ice by the snow guns.

From this on the winter’s halo hunt continued with two batteries, but on one cold night in January even that was not enough. That time, however, a helping hand was extended by the Ounasvaara ski center folks, who borrowed one of their batteries.

Sunday, 30 October 2016

Another occurrence of an anomalous Hastings/Wegener

By Aleksi Velhola and Marko Riikonen

In an earlier post we showed a photo of a weird downward curving patch of Wegener/Hastings. Here is another case that occurred 9 days later on December 2, 2015. This time we also got a nice comparison to normal “Wegstings” by superposing photos taken from the same camera position.

The anomalous Wegstings appeared on two occasions and was well visible to the eye. At both times the display was less brilliant than at peak stages, during which normal Wegstings was seen. Same was true of the earlier display on 23 November.

A closeup of normal and anomalous Wegstings appearances superposed to one image.

The crystal sample is interesting. Crystals range between plates and equidimensionals, which one would not expect from the strong column orientation halos in the display. It just adds to the ever increasing file of cases where the samples are not what would be expected. The dish was out long time, something like half an hour, so it represents all different stages of the display put together. However, column orientation halos were all the time present.

Saturday, 29 October 2016

Spotlight display with a strong Moilanen arc (and a crystal sample)

By Marko Mikkilä, Jarmo Moilanen and Marko Riikonen

After having been hunting diamond dust halos since darkfall, at 1am we withdrew along with disappearing crystals back to the apartment to sleep. But this did not mean calling it quits. The forecast was for the conditions getting better, so every one hour each of us woke up in their turn to check the situation.

At 4 am it was time to go – there were streelight pillars over the city. In 20 minutes we were sitting in car and five minutes later arrived to the golf course parking lot next to the ski center. From there we walked to the golf field and away from ski slope lights and stayed there until 7 am.

The display was not dazzling, but it had a Moilanen arc that was strong in comparison to other halos. Consequently, in the crystal sample we expected an abundance of crossed long plates, which are thought to be responsible for Moilanen arc, but they were very few. Perhaps they were small and did no fall in the dish in representative numbers. In general, not much crystals settled on the dish.

Another thing about the crystal sample was that it contained some pyramids, and indeed a short lived 9° halo was seen in the beam when our cameras were not yet ready. Pyramids are often found in samples, but unless they are numerous, they don’t translate to actual odd radius halos in the sky.

There was some variation in the topography at the location, and so for a change, we had the lamp above the camera for all photos. The date is 23 November 2015.

Friday, 28 October 2016

The "leftovers" from the 23 November 2015 display in Rovaniemi

By Jarmo Moilanen, Marko Mikkilä and Marko Riikonen

In three previous posts we have dealt with different stages of the 23 November display that had some interesting features, like an anomalous Hastings/Wegener arc and a possible 4th Tape arc. Here we show the rest of the material we got that night.

After the lunar display was gone, wind direction changed and we followed the diamond dust to a new location. As usual, there is only one place to choose from when temperatures are not very low and the new field we found ourselves in was less optimal than the earlier one, having a rather confined space and yellow streetlights nearby. But you photograph where you can.

The image above and two below show that it was overcast again, moon glowing weakly through the cloud. In the beam, the display was not nearly as good as it had been before, yet it was by no means poor – it is only because we had been just a bit earlier spoiled by a monstrous halo complex we didn’t feel that inspired anymore. Add to that the ever more demanding calls of the land of slumber (we had been up most of the previous night and not sleeping in the daytime either because halos wouldn’t let us), we packed it all up and headed for the caressing warmth of the indoors even though the display was still kicking on.

                                                     Lamp elevation is around 0 degrees in the three photos above.

Who knows what there would have been on the offing after we gave up, there was still plenty of night left. As were about to doze off, the stratus clouds started breaking and moon shining bright again. Any change in conditions means also change in halos, and this one may have been for the better, judging from earlier breaking of clouds which produced a major spectacle.

In the image gallery at the bottom there are also photos from the first action of the night, soon after it was dark. Crystals were collected and there were plenty of plates with internal structures.

Wednesday, 26 October 2016

Lunar all-sky diamond dust display on 23 November 2015 in Rovaniemi

By Jarmo Moilanen, Marko Mikkilä and Marko Riikonen

This night conditions were overcast and we were watching halos in spotlight beam. But there was a moment when clouds all turned into ice crystals, allowing moon shine bright and make a passable display. We switched off the spotlight and managed to take some photos before it got cloudy again. Camera lenses were frosted, which caused some blurred areas and bright dots in the images.

Tuesday, 25 October 2016

A major spotlight display with possible 4th Tape arc component

By Jarmo Moilanen, Marko Mikkilä and Marko Riikonen

On 23 November 2015, we were watching diamond dust halos develop under overcast skies in Rovaniemi. As we stood on a rectangular field a couple of hundred meters across, we followed halos slowly gather momentum in the spotlight beam, reaching climax when clouds were cleared away for a short while – and revealing at the same time also a lunar display. Here is an excerpt from Marko’s observation log written the next day:

“The display just adds gear. We are looking at beautiful subanthelic stuff, subanthelic arc, diffuse arc… It becomes monstrous when the cloud almost disappears. That is when we get also a moon display with full parhelic circle. No one seems to be in a rush to photograph the moon display. The beam display is sheer grotesquery. The laser scapel sharp, 100% pure glitter of the tangent arc and uppervex Parry.”

In the photos the lamp is 5-6 degrees below the horizon. In the image above, the most noteworthy feature perhaps is the slight intensity enhancement in the 46° infralateral arc, marked with arrows in the simulation below. These could be sings of the fourth, as of yet unobserved Tape arc component (raypath 14), which is highly unlikely to be ever seen in solar displays as it appears in simulations only when light is several degrees below the horizon. Although it is probably here, we would still hope for a better example from lower light elevations that would make it easier to identify.

Another feature of interest is the “Ounasvaara arc”, which in the photos looks like an odd continuation of circumnadir arc. This Parry orientation halo is visible particularly well in the two photos below, that, unlike the photo above, were taken off-beam, from underneath. Although off-beam images are not visually as appealing for their non-uniform lighting, the halos in the brightest part of the beam are enhanced in them.

We regret of not getting a visual of the Ounasvaara arc – judging from its intensity, it should have been easily visible to the eye. Well, something for the next time.

As usual, we also had a gasoline filled petri dish out to collect the crystals. No serious end cavities this time, which is in accordance with the observation log’s note of “100% pure glitter”.

At last we switched off the lamp to give the moon display the stage. However, by the time we got to it, the lunar halos were already starting to decline. In our next post we will show nevertheless the few photos that were taken.

Sunday, 23 October 2016

An occurrence of anomalous Hastings/Wegener

By Jarmo Moilanen, Marko Mikkilä and Marko Riikonen

The image above shows what looks like a patch of Wegener or Hastings on top of the 22° halo. But instead of having the usual horizontal curvature, it is bending slightly downwards. Because of the view angle, though, the effect is not as evident as it could be. Anyway, if it were standard Hastings or Wegener, it would curve steeper up in the photo.

We have no idea how it formed, our attempts at simulating have come up empty-handed. The display was seen in Rovaniemi on 23 November, 2015, and the arc appeared at a stage when the display was still progressing to reach its peak.

Nine days later, in the beginning of December, we got another, better sighting, suggesting it is not exceedingly rare. In a similar manner, it did not occur when the display was at its best, but when the display was undergoing a momentary low. We will post about this later.

Saturday, 22 October 2016

Did near ground turbulence cause the 22° and 46° halos in this spotlight display?

By Marko Riikonen

Crystal samples should be enlightening, but all too often they just make you confused. The observation I made on 22 November 2015 in Rovaniemi is a case on point, although an observation recently published by Alexander Haußmann may now provide a solution.

The issue is about the halos from poor crystal orientations – the strongly developed 22 and 46° halos. One explanation for such halos is that they are born from conglomerates of crystals, which, because of their irregular shape, tumble instead of falling stably.

Yet here the crystal sample did not reveal any compound crystals. Sampling bias is pretty much out of question as such crystals, due to their large size, would be expected to fall faster than single crystals and thus should not be able to avoid the collecting dish.

Another often-heard explanation is equidimensional crystals. They can’t decide which orientation they want to be in, so they tumble too. Looking at the crystals of this display, such an explanation feels tempting.

But then we have other displays where the crystals are pretty equidimensional and yet there is no strong random halo component. Moreover, evidence has been building up that equidimensional (or even slightly plate like) crystals of triangular habit can take Parry orientations. So what’s going on?

A diamond dust display photographed by Alexander Haußmann last winter at Mt. Klínovec in the Czech Republic suggests an answer. In the beam of car headlight there was visible Moilanen arc, but photographic stacking of the video frames revealed additionally a bit of a circular “Moilanen halo”. No such thing has been known before, which talks of the high stability of the Moilanen arc crystals. That it was seen nevertheless, Haußmann thinks, was due to strong wind, which near the ground became turbulent and destabilised the crystals so much that even Moilanen arc crystals were thrown off-balance.

As the wind was exceptionally strong also in Rovaniemi display, it could in a similar manner answer for the strong 22 and 46° halos seen in the photo above. Just like in the Czech display, the light beam was close to the ground, so turbulence had a chance to randomize the crystal orientations. And because the crystals were equidimensional and quite large, that would have served to kick them out of balance with ease.