Can you explain to me this shadow-line phenomenon in a photo?
January 26, 2013 10:16 AM   Subscribe

I feel like I understand a thing or two about light, but I can't figure out how/why this particular phenomenon happens. This photo I took shows a shadow line cast by my bank (on the right) that seems to just hang in the air.

I can never see this with my eye, but it shows up in my photos sometimes, so far only when I'm shooting straight up.

What causes this? I mean, I fully understand that the bank building is blocking sunlight that would otherwise hit the nearby buildings, those cables, and everything on the ground, but why do I see a shadow that's not really cast on anything? This baffles me, as the light source is not coming from the direction of me, there was no visible smog, and it was a cool, crisp, bright, sunshiny October day.

I haven't tried to recreate it with any of my film cameras or a better digital because I'm sort of in an exclusive relationship with my little point & shoot at the moment, but I can't imagine this phenomenon is peculiar to this camera.
posted by heyho to Science & Nature (16 answers total) 2 users marked this as a favorite
I'm assuming that it is something in the air that is reflecting sunlight; it doesn't have to be ordinarily visible to you.
posted by carter at 10:21 AM on January 26, 2013

The air is not perfectly clear. It has dust, fog, pollution, etc in it. You can see that it's a bit hazy that day. The haze, when lit by the sun, reflects light and looks brighter than unlit haze.

Basically, you have a thin cloud, half in shadow, half out, and the shadowed part is, expectedly, not as brightly lit.
posted by tylerkaraszewski at 10:22 AM on January 26, 2013 [2 favorites]

Yes. Smog. Just imagine if the air were filled with smoke from a fire. Those shadows would be even more noticeable. It's not an artifact of the camera. You can probably see it with your eyes if you squint. There is just too much light for your eyes to be that sensitive under sunny conditions.
posted by weapons-grade pandemonium at 10:31 AM on January 26, 2013 [1 favorite]

Can you explain this one that runs alongside and ahead of the plane/contrail? It's another line, but it doesn't seem to be a shadow cast by something, eh. It's so weird.

I seem to get lines similar to that one with more frequency; I shoot at the sky a lot when I see airplanes.
posted by heyho at 10:31 AM on January 26, 2013

That's the shadow of the contrail on the smoggy air. It just happens to align with the contrail somewhat. It runs ahead of the plane in the same way your shadow runs ahead of you when the sun is at your back. If the sun or the plane were in a different place, it might be more obvious as a shadow. Again, not a camera artifact. You can see these with the naked eye.
posted by weapons-grade pandemonium at 10:32 AM on January 26, 2013 [2 favorites]

I see this a lot - sometimes with my bare eyes, sometimes just in photos I've taken - here in San Francisco, where even if there isn't visible fog there's often a very thin and invisible-to-the-naked-eye marine layer. It's a cool thing.
posted by rtha at 10:41 AM on January 26, 2013 [1 favorite]

That's the same phenomenon as a "sunbeam" shooting through clouds.
posted by TheNewWazoo at 10:41 AM on January 26, 2013 [1 favorite]

Here's an analogy.
posted by weapons-grade pandemonium at 10:43 AM on January 26, 2013 [1 favorite]

Oh, for chrissake. Of course! I wish I'd have asked sooner; I always think this is just soooo mysterious and cool when I see it, but yeah, of course it's not strange at all; I just wasn't thinking about it with both lobes.

Thanks, guys! Now. Is 12:50pm too early to drink in an attempt to wash away the shame of having asked this publicly?
posted by heyho at 10:52 AM on January 26, 2013

It's a good question! And think about the mitzvah you've done for people who read this who didn't know the answer and were too shy to ask.

Also, it's always after 5 pm somewhere in the world. In any case, it's Saturday and it's after noon where you are, so go right ahead with that drink. Some people haven't even started their brunch cocktails yet!
posted by rtha at 10:56 AM on January 26, 2013 [1 favorite]

It's a good question. Also? Those are pretty photos.
posted by limeonaire at 11:00 AM on January 26, 2013 [2 favorites]

The reason you can only see it when looking straight up is that you are in line with the shadow's edge. The effect isn't very strong, and if you were trying to look at it from off to the side you wouldn't be able to see it, unless the smog were much, much thicker.
posted by Chocolate Pickle at 11:28 AM on January 26, 2013 [1 favorite]

Crepuscular rays might be the term you're looking for. In computer graphics, volumetric lighting is the term given to the techniques for modeling this sort of thing.
posted by smcameron at 5:00 PM on January 26, 2013

Also, one reason you're not seing it with the naked eye is that eyes and film (or ccd as the case may be) have very different responses to amount of is much more 1 to 1 linear and the eye much more logarithmic...also, the human eye, at much less than a single megapixel of resolution, is much more reliant on post-processing in the visual cortex...and we all know how reliable the human brain is, amirite?
posted by sexyrobot at 2:00 AM on January 27, 2013

The site Atmospheric Optics has photos of shadows of contrails, rocket plumes, mountains, the Earth, all cast on different sorts of haze in the atmosphere, as in your photos. There are some very counterintuitive effects. There's a rays and shadows section to the picture-of-the-day section, here are some more contrail shadows with clarifying diagrams.
posted by fantabulous timewaster at 11:50 PM on January 31, 2013 [1 favorite]

I stepped out on my back porch a little while ago and caught you a contrail shadow and a halo!
posted by rtha at 4:52 PM on February 1, 2013 [1 favorite]

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