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October 14, 2017

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Into the white storm

July 15, 2016

I was talking a bit earlier about Denmark's capricious weather. Well it definitely isn't summer, except maybe for a day or two in between. As a matter of fact I used all my energy on these few beautiful days, staying awake all night, and all the day after, and the night after again... From July 12th to 14th (so 48 hours if you do the math correctly), I have only slept ten hours. Certainly not healthy, my mom would say, but what I saw these two nights was definitely worth not being healthy.

 


July 12th, 7:00 pm. The weather forecast announces good weather for the night but I look at the sky and ask myself if the people behind the app have taken illegal substances. It's all overcast, except for an opening in the west that allows me to take a time-lapse of the sunset. After the sunset, big ugly clouds cover the twilight sky again. I force myself to stay longer and decided to give the forecast a chance to redeem itself. Around midnight, the night sky magically clears out, underlying a higher pressure system passing over us. And sure enough half an hour later, I can see the sky carving softly into shiny waves and bands: NOCTILUCENT CLOUDS IN SIGHT! I take advantage of the long but faint show to take some real-time footage trials with my Canon camera, which is after all not made for shooting in the dark, given its disappointing low-light ISO performances. The highlights of the night were two-fold.

The first one was the nice waves and shapes NLC took, even if they were kind of faint (equals a 2-3 on a brightness scale of 5). The second one was a much brighter (5) system that was lower on the horizon. It is unclear to me whether they are brighter because they are lower on the horizon, thus compacted from an human eye point of view. Or maybe it is because, against my former beliefs, NLC are not just 'one sheet' of ice crystals, and can build up in several layers or in a system.

 

July 13th, 2016. The whole day has been beautiful and a clear night had been promised. I take my full gear in the hope of getting some 500mm shots as well if there were going to be NLC again. At 11.15 pm I stick my nose out the window and I could already see some faint NLC in the E-NE. I am heading out with no hesitation to Esterhøj where I could have a 360 degree vista. I did not get disappointed! An hour later, I thought the NLC were fading away, but they were intensifying! here is how they evolved during the whole night (The gallery will be in respective and consecutive order):

 

1) 11:30 pm to 00:30 am - Faint NLC to the E-NE moving slightly to the N and intensifying as the night falls. Bands, waves and net-like structures were visible. NLC between 15 and 20 degrees azimuth.
2) 00:30 am to 2:00 am - NLC spreads all the way to N and also developing at the NE. I can see a whole system building up in the mesosphere in the NE, with some NLC colors I had never witnessed before at the canopy (read further below). System in the N shining very bright. Azimuth between 20 and 30 degrees.

3) 2:00 am -2:30 am - system in the N started fading. System in the NE intensifies and spreads outwards, gains azimuth (35 degrees)

4) 2:30 am - 4:15 am - the whole system gains in azimuth from 35 till 120 degrees (well past overhead) and at 4:15 am, when they faded away, they spread from W to E! It is unclear to me whether the expansion is due to the sun rising closer to the horizon, thus illuminating further overhead, or if it is purely because the clouds actually expand at the same time from a 'storm-like' system. I know one cannot really talk about storm, but they really look like so at some point.

 

As mentioned before, I noticed something really strange. On the third image, the canopy layer in the upper right corner appears greyish or faded. On some close-up, they actually appear reddish, which is really unusual and never mentioned in literature. As studied and seen, NLCs are take and reflect the color of the sky in the background. From bottom to top, it's red, orange,  yellow, greenish, blue, dark blue. If you look closely, the NLC below are blue, as they are supposed to be. These are pink, red and brown. Why is that? My one hypothesis was that NLCs had actually a way more complicated structure than I had understood. It is obviously not just one roughly flat sheet! I am not sure how thick NLCs can be, but the only explanation was that this layer was overshadowed by the thicker layer passing below, reveling a real 3D structure, that is so hard to identify. My second theory is that they happen because of (or in addition to) another unknown phenomenon (the layer below is also in the shadow and is usual blue). Could it be because of the refraction of another element than water or ice crystal? Or maybe another optical refraction phenomenon I do not know of.