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Author Topic: Subsun: unusual halo on Whistler Blackcomb webcam  (Read 1352 times)
Amar Andalkar
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Subsun: unusual halo on Whistler Blackcomb webcam
« on: 02/03/14, 09:48 AM »

Looking through some of the various webcam images that I automatically archive at 10-minute intervals (mostly Cascade Volcanoes and WA ski areas, plus a few other volcanoes around the world), I noticed an interesting optical effect yesterday that I don't think I've ever seen on any webcam before: excellent clear images of a subsun, a glowing spot seen when looking down onto clouds, which looks like a bright reflection of the sun similar to that from a water surface. This occurs only when the clouds are composed of ice crystals, specifically hexagonal plates, which orient themselves flat as they fall and act as millions of tiny mirrors reflecting the sunlight up towards the observer from below (see AtOptics, Wikipedia, Google for more info and images).


(click for full-size 1920x1080 HD version)


This is not an uncommon optical effect, but it is highly unusual to see it on a webcam, and it made me start thinking why that is. The key issue is that only a very specific webcam placement is even capable of seeing this effect. It requires that the webcam be located on the edge of a high mountaintop or ridge above a steep-enough drop-off, that it looks in the direction of the winter sun (generally south, or at least between SE and SW, in the Northern Hemisphere), and that its field of view looks partially downward. Most mountain webcams fail all three of these criteria, especially the second and third, and so they have no chance of ever capturing this effect.

But the Whistler Blackcomb 7th Heaven lift line webcam installed in early 2013 near the top of the 7th Heaven Express lift on Blackcomb Mountain (just SE of Horstman Hut, true elevation about 2250 m / 7380 ft) meets all three of these requirements perfectly: it's located on the edge of a high ridge above a deep bowl, with a wide-angle view centered around SSW (200), and aimed slightly downward along the path of the chairlift. I really like this webcam because its view includes 2 of the Cascade Volcanoes of southwestern BC, Mount Garibaldi (2678 m / 8787 ft) 28 km / 18 miles south and The Black Tusk (2319 m / 7608 ft) 17 km / 11 miles southwest.

Here is a clear image from this morning, with prominent peaks labeled:


(click for full-size 1920x1080 HD version)


By the way, PeakFinder is great for figuring out what peaks are visible in any view (link to the view from this webcam: http://www.peakfinder.org/?lat=50.0915&lng=-122.884&off=40&ele=2199&zoom=5&dir=202). Their mobile apps work great too, even in the remote mountains since no cell service is required, only a clear view of the sky for the GPS to get your position -- it's by far the most expensive mobile app I've ever bought at $3.99, but well worth it!

And here is an image sequence from yesterday (February 2, 2014) showing the subsun over the several hour period that it was visible from the first faintest trace just after 11am until it faded away as the clouds cleared after 3pm:

(click any image for full-size 1920x1080 HD version)





















And a pretty sunset later that day:



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Jim Oker
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Re: Subsun: unusual halo on Whistler Blackcomb webcam
« Reply #1 on: 02/03/14, 12:13 PM »

Cool effect. You're certain that this is not just some sort of weird flare caused by the camera optics?
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Amar Andalkar
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Re: Subsun: unusual halo on Whistler Blackcomb webcam
« Reply #2 on: 02/03/14, 02:05 PM »

Cool effect. You're certain that this is not just some sort of weird flare caused by the camera optics?

I figured that might be one of the first questions. One advantage of having a large archive of saved images from the same webcam is that it's easy to rule that out. The lens flare on this webcam always looks the same on every sunny or mostly sunny day, a series of multiple spots in a line pointing from the center of the frame towards the sun's location. The spots are clustered in groups corresponding to scattering from multiple surfaces of various lens elements (or joined groups of elements), and include a large spectrum of colors on the inner spots due to dispersion (the variation of the index of refraction on a lens or coating as a function of wavelength) and the various anti-reflection coatings applied on different lens elements. Here are a couple of examples from early afternoon today:


(click for full-size 1920x1080 HD version)



(click for full-size 1920x1080 HD version)


Looking at multiple consecutive images on a sunny day makes the difference even more certain, because the line of lens flare radiating from the center of the frame follows the sun in a clockwise direction throughout the day as seen in these 2 images, while the subsun moves in a counterclockwise arc mirroring the sun's location in the sky above, not around the center of the image frame. The lens flare can be clearly seen on most of the subsun images from February 2 too. There is generally no flare at all on mostly cloudy days.

Took a quick look through my entire archive of saved images from this webcam, with automated 10-minute saves extending back to November 4, 2013, shortly after the camera was upgraded to a sharp 1920x1080 HD image (the initial installation in February 2013 provided a 1280x720 image which was not-very-sharp for some reason). There were no previous instances of a nice distinct subsun, but there was this lower sun pillar seen on December 2, 2013. This effect is very closely related to the subsun (see AtOptics), but caused by flat plate ice crystals which wobble more due to winds and turbulence, rather than floating nicely flat. The last two images likely include a combination of subsun and lower sun pillar. Note that a date-time stamp was not added to this webcam's images until December 13, 2013, so file modification times from the server are listed below.


Image modification time: 2013Dec02 13:55 (click for full-size 1920x1080 HD version)



Image modification time: 2013Dec02 14:35 (click for full-size 1920x1080 HD version)



Image modification time: 2013Dec02 15:05 (click for full-size 1920x1080 HD version)


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Jim Oker
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Re: Subsun: unusual halo on Whistler Blackcomb webcam
« Reply #3 on: 02/05/14, 04:17 PM »

Makes sense, and I did notice that it was quite different from the typical series of lens-element-induced flares - was pondering whether or not something like a glass/plexi sheet in front of the whole lens might do this, but I buy your take.

But now I'm left pondering what level of digital hoarding you might actually have gotten yourself up to!  Wink All in the name of science, I'm sure  Smiley
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wickstad
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Re: Subsun: unusual halo on Whistler Blackcomb webcam
« Reply #4 on: 02/05/14, 05:54 PM »

A clear day at Whistler!  That is something I have yet to experience!
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Amar Andalkar
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Re: Subsun: unusual halo on Whistler Blackcomb webcam
« Reply #5 on: 02/08/14, 07:58 AM »

Nothing but clear sunny days up there this week, along with a frigid Arctic air mass which is producing other colorful halo displays due to the occasional presence of thin ice-crystal clouds. Here is the fairly-common 22 halo (see AtOptics, Wikipedia for more info and images), along with the lower tangent arc (see AtOptics and effect of solar altitude, Wikipedia) superimposed on it:


(click for full-size 1920x1080 HD version)


The maximum solar altitude at Whistler on this date was about 23 with solar noon occurring at 12:26 PM (see my solar calculator page), which would put the 22 halo and the lower tangent arc just above the horizon -- which is exactly where they are seen in the webcam images!


Here is an image sequence from Tuesday (February 4, 2014) showing the 22 halo over the 2-3 hour period that it was visible from the first faintest trace just after 11am as some thin clouds started to drift by, until it faded away as the thin veil of clouds cleared around 2pm, with the lower tangent arc superimposed on it during the peak of the display:

(click any image for full-size 1920x1080 HD version)



















And another pretty sunset later that day:



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