Today (yes I know its Apr 1, this is not a joke), I saw that wondrous phenomenom, a "shaft of sunlight". Of course, I didn't have my camera with me to capture it (ain't it always so - I always carry my camera with me except when I'm popping out to the corner shop...) Anyway, it was a couple of hours from sunset (6 PM BST here in southern UK) and there was a layer of broken cloud between me and the sun. Through one of the holes in the clouds there was a very distinct "ray of sunlight", which extended past the cloud into the clear sky beyond. I looked and marvelled for a while, then started wondering... Why does it show up in the clear sky? The "shaft" was distinct, a lighter, greyer colour than the sky. And yet, it was still the same, single light-source (the sun)travelling through the same atmosphere (after it got past the edge of the cloud). What caused the shaft? Anyone here know the answer? I'm baffled... Cheers, Diolma (I have a couple of theories, but they're just that, theories, and I don't want to distract/bias/start people down the wrong thought-line/show myself up as totally brainless, so I'll not state them. Yet.)

Attached Link: http://www.islandnet.com/~see/weather/elements/sunbeams.htm

Earth & Sky

Many thanks for the links, Quest, but they don't answer my problem. I was looking up at a sky (at approx 45 degrees(?) which showed a "sunbeam" against a part of the sky which had no clouds in it. It's difficult to describe, but part of the sky was (when looking up at it) clear(ish) blue. The sun was shining both on that part of the sky and through the cloud. Looking (at the clear part) of the sky, there was a very distinct "sunbeam" (IE, a change of colour/brightness, fanning out and fading with distance) from the hole in the cloud. I don't understand how this could occur. But I witnessed it... Sunbeams against occluded backgrounds I can understand. But how can physics explain a "shaft of light" from the same light source against the same background and through the same medium? My 1st theory is: The hole in the clouds wasn't 100%. Thus the light there got reduced. So the light in the sky was affected by that (in some way). But I'm in no way convinced that that's the whole answer. Still baffled... Cheers, Diolma

I'm baffled by what you're saying, but if you can see shafts of light in the atmosphere, I've always thought it was because of what's floating about in the atmosphere (rain/dust/smoke/fog etc...)

Other than that, I can't suggest anything :-/

Len.

pumeco: exactly. That's what I always thought. Yet I saw it. I swear this is not an Apr 1 joke! I really did see it! But it was the SAME light shining through the SAME atmosphere (albeit through a hole in the cloud). Had it been just in the area under the cloud, I'd be happy. But it wasn't. The effect showed up in the clear sky beyond the cloud (ie, closer and more overhead). Just in case it has any relevance, I was looking near due west at the time.. My thoughts are tending towards the light being partly attenuated by going through an (almost, but not quite) clear part of the cloud and thus affecting the visual result. But I still can't work out how a a "diluted" light (the shaft) could possibly affect the undiluted light of the clear sky... Damn! I WISH I'd had my camera with me so I could show you all the effect! Ahh, well, Cheers, Diolma

OK, bear with me. Im not a good writer at the best of times (otherwise Id have a proper job) and Im slightly pissed. Ill waffle with some background and then get to the specifics of your case.

Air is pretty transparent, so visible rays, shafts and beams are produced by light interacting with suspended particles (dust and tiny water droplets mostly henceforth referred to as stuff). Refraction, reflection and diffraction (of light by stuff) all play a part in the formation of visible rays. Instead of all the light racing off in one direction as it usually does (such as in a vacuum), some gets bounced and bent when it interacts with stuff. If, from the point of view of an observer, the bounced and bent light reaching the eye is brighter than the background a ray of light is seen.

There is much more stuff in the lower atmosphere to cause bouncing and bending than in the upper atmosphere thats why a clear blue sky is always brighter around the horizon than overhead. Except immediately around the position of the sun backlit stuff is the most visible of all.

When the sky is free of cloud, a low sun will illuminate the lower atmosphere stuff around it evenly, and all the bouncing and bending effects will be fairly uniform and not particularly noticeable. But if broken cloud intervenes between sun and observer, only some of the lower atmosphere stuff will be directly illuminated (the rest is in shadow) and that directly illuminated stuff will do its bouncing and bending thing.

If the bounced and bent light reaching the eye is brighter than the background, be it a dark hill, reverse side of cloud, or upper atmosphere, then a visible ray, beam or shaft will be seen.

Does that make sense? I think so, but then Ive had beer plus some Bruichladdich 10 (which I could only gain access to by removing the poxy Sainsburys security device, that some uninformed tilly left on my purchase, with a soldering iron).

All it takes is for the bent and bounced light (after interacting with stuff) to be brighter than the background. If the background upper atmosphere is dimmererer than a lit column of lower atmosphere stuff, then the latter will be visible as a ray/shaft/beam.

LOL PJF, yes - it almost makes sense. I certainly get your drift (and I'm having the same problem as you re: imbibed liquids..) I think I'll try and sleep on this one and try to come up (tomorrow) with either a rational explanation, or a diagram to explain what what I saw.. Cheers, Diolma

Yeah, that... what PJF said!
I knew that ;-)

Len.
(Sainsburys security device - tut tut tut)

Bravo & thanks! I've seen lots of those this year.

Shaft you, mate. ;-)

F*ck you, no need to get cocky 'cos you just earned 64 points on your Nectar card :-D

You should invite me round sometime Peter. I've nothing against a HobNob and a bit of Jam Roly Poly you know :-P

Len.
(LOL)

"LOL PJF, yes - it almost makes sense."

Frack me, that's more than I could've hoped for.

It's all a contrast effect, as with most of what our eye/brain mechanism sees.

If you look at the sun through a suitable filter, you'll see dark sunspots against the white background of the overall solar disk. This despite the fact that the light from the sunspots and the disk is all bright enough to blind you in an instant without the filter. That's contrast.

Likewise with seeing nebulosity in the night sky (naked eye or with telescope). The 'stuff' that bounces and bends (and absorbs and re-emits) light to make nebulae is less dense by a hundred times the best vacuum that mankind can manage on Earth. It's the contrast of this dim, dim light against the pure black of space that makes it visible.

"...no need to get cocky 'cos you just earned 64 points on your Nectar card"

Pah! Constast Len, constrast. Constrast your perception against observable reality - and you'll realise I'm not a complete lacky to corporate manipulation.

It's the biscuits and puddings that've made me a lardy boy, honest. The brew is entirely innocent in that regard.

However it did cost 3X's as much as the food. (Sigh) the price of sensation VS sustenance. ;)

The food will be gone by the end of tomorrow (maybe some hobnobs left...). The whisky will last for...

...longer.

I'm not that far gone, ICM, even though I've just rather sensationally dribbled some down my fleece. OMG, keep that candle away!

LOL,.... Yes, but which was referenced to sensation and which to sustenance? hmmmmmmm LOL ;) Cheers!

It's not that hard is it? that cloud layer has clouds in it, clouds cast shadows. no cloud means no shadow to cast. unshadowed area's of any object are receiving more light, thus looking brighter. Since the earth has an atmosphere the sky counts as an object too. Compare it to a volume atmosphere in bryce for instance. There doesn't need to be a cloud 'behind the stream of light' to see the extra brightness. Imagine you fly into space with a rocket. You look out of the window and the space u see is black. Space is black because it's a vacuum, pure vacuum has nothing in it to receive light, thus nothing to change the lights direction towards our eyes. If you're in space, in a vacuum, only the light that travels directly in our direction is seen (like the stars) around the stars is pure blackness. Now imagine your rocket that took u to space before is going back to earth and land you safely on the ground again. The lower you get in the atmosphere the brighter the sky looks. Why does it do so? Because it's not a vaccuum anymore. Particles of whatever, gasses, waterdrops, ('stuff' as PJF called it) they all can in one way or another change the direction in which the light travels, thus allowing some of it to bounce in the direction of your eyes which will receive the light, even if you're not looking directly at the light source. Just think of it this way, the more light passes through a certain area in the sky, the more light gets to be bounced into the direction of your eyes, and thus the brighter the area will look. Just like the streaming light rays in your brycean volume atmosphere scenes ;-)

I use a Leeds and Northrup Optical Pyrometer to measure temperature. It's basically a lens with a lightbulb filament across the center. You turn a dial on the side and it gives more power to the filament and makes it glow brighter. If I look at something thats 800C and set the filament at 600C the filament looks black. When I reach 800 on the dial it disappears because it's the same temperature as what I'm looking at. If I crank it up to 810 and keep going the filament gets brighter and brighter. Your shaft of sunlight looks bright because there is more energy in it than the surrounding air. If you could bottle the light from another section of the sky that doesn't have the beam and open it back up in the middle of the night You'd see a great whopping beam there. It makes a lot more sense now that I'm into my first Corona.

How far off am I on this simple reasoning? The shaft of light is emanating through the hole in the cloud from it's source, the sun. Very simply put, the shadow of the cloud is extending in exactly the same direction. The contrast between shadow and light is more pronounced and easily seen against darker background. As better explained above you are seeing light bounced off particles. In the case you observed there was enough difference in light illuminated particles (read sun coming through the hole) and un-illuminated particles (read portion of atmosphere in the shade of the cloud) to detect the difference with an un-aided eye even against the clear sky background. Willie, shade tree rocket scientist

Bugger!

Hey Peter, I'm so not sucked in by corporate manipulation, and I hate the way we're treated like halfwitted Cyborgs in this country. But, if I can find a way to save some dosh, I will.

It's better in my pocket than in theirs.

So, whenever I have to fill-up my Rover (very expensive to do) I bloody well make sure I get my Nectar points for it. In fact, I left my card at home once, so I actually drove out of the filling-station without filling her up, just so I could claim my Nectar next time.

Put it this way:

RoverSEI (multiplied by) 1 YEAR PETROL SUPPLY (=) LOADS OF NECTAR!

= One or two less "Great British Rip-Off Bills" to pay ;-)

Len.

"I'm so not sucked in by corporate manipulation...
But, if I can find a way to save some dosh, I will."

And the way you and many people have found is exactly the way a corporation wanted you to find - odd that. Rather than going to the effort of shopping around to force price competition amongst retailers, people prefer the convenience of feeling they're saving money via a retailer's baroque "loyalty" programme.

It's a classic "spend to save" scheme. In order to collect your "savings" points you keep going back to one retailer, and buying their offerings without seeing if the items are cheaper or better elsewhere. The more stuff you buy, the more lovely, scrumptious reward you accumulate. Overall you end up paying more - that's why they run the scheme. Plus they get to monitor and profile you as a consumer, enabling further manipulation.

"I left my card at home once, so I actually drove out of the filling-station without filling her up, just so I could claim my Nectar next time."

It's a very effective scheme, to be sure.

I totally agree with your view, seriously I do.

And if there's anyone who could groan a 'master groansman' under the table - it's me.

What I've found though, in England, it's a waste of time being the rebel where this sort of thing is concerned.

I could be stubborn (like you), but I'd be 'more' out of pocket because of it. We're bled to death here - no doubt about it. But, with the great British public being the sheep they are (most of 'em), the corporates will continue to do as they do (and they'll continue to get away with it).

AND,

they do as they do - because they can!

THEY CAN,

because the British don't rebel.
(If you can't beat 'em, F*ck 'em and leave 'em)

Len.
(Off to another country as soon as funds allow).

What you saw was likely due to a high altitude thunderstorm cell. The TS cell masks the sun at the horizon and what you see is a beam of light shining through clear sky. I live in Texas. We see this kind of thing often in our neck of the woods. Jim

LOL @ all! I love the way this thread has split into 2 - the sensible answers to my query and the background nonsense..:-)) The more I think about (which is not a lot, but I do think about it), I think "the sky being darkened by the shadow of the cloud (except for the bit with the hole)" is probably the reason. (Oh, and yes I do know all about particles in the atmosphere giving the sky substance and colour - I paid that much attention at school.) And that the fact that Sky is blue is 'cos his date stood him up..:-)) (Yeah, OK, it's really 'cos the other colours get absorbed more due to their longer wave lengths...) Cheers, Diolma

Attached Link: http://www.renderosity.com/viewed.ez?galleryid=995615&Start=1&Artist=TobinLam&ByArtist=Yes

Very similar, TobinLam:-)) The beam was a little greyer than in your photo, but that might have been due to almost anything, including just visual perception... Cheers, Diolma