Odd shadow? NW-Germany, 31.5.2014 [Volumetric Contrail Shadows]

xari

Closed Account
this trail was one of a lot others in the sky, but only this one had this grey line in front of it.
P1540911.JPG
 
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Mick West

Administrator
Staff member
First of all, that appears to be a contrail, it looks just like a contrail, so I don't see any reason to label it a "chemtrail". I've edited the post accordingly. If you'd like to discuss the difference between contrails and "chemtrails", please start another thread.

But, very nice shot. It seems like what you have captured is an unusual combination of two types of contrail shadow - a parallel shadow cast on a layer of cloud beneath the trail, and an "edge" shadow caused by the contrail being lined up with the Sun (from your point of view), like this:



It's also possible that the second shadow is being cast on a distinct layer of cloud below the first layer. Do you have any more shots showing the shadow, preferable with a wider angle?

See:
http://contrailscience.com/contrails-dark-lines-chemtrails/
 
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Jason

Senior Member
First of all, that appears to be a contrail, it looks just like a contrail, so I don't see any reason to label it a "chemtrail"

But, very nice shot. It seems like what you have captured is an unusual combination of two types of contrail shadow - a parallel shadow cast on a layer of cloud beneath the trail, and an "edge" shadow caused by the contrail being lined up with the Sun (from your point of view), like this:



It's also possible that the second shadow is being cast on a distinct layer of cloud below the first layer. Do you have any more shots showing the shadow, preferable with a wider angle?

See:
http://contrailscience.com/contrails-dark-lines-chemtrails/
How exactly does it make a shadow appear on the cloud level below it Mick? Wouldn't clouds below a contrail block you from seeing the contrail to begin with? Or are these clouds of the haze or fog variety? And I'm guessing its an illusion too because the shadows appear to be "above" the contrail. Could it be the sun is low in the horizon, casting a reflection above the contrail on the cloud cover above it?
 

mrfintoil

Senior Member.
Could it be the sun is low in the horizon, casting a reflection above the contrail on the cloud cover above it?

Nope, I am quite sure that this is a case of a shadow cast through a volume of fog, like Mick explained.
The reason why I am quite sure is because I work with volumetric effects due to my profession as a graphics artist, and I often simulate various atmospheric phenomena in my work. This photo looks exactly like a volumetric shadow.

The reason why we see the shadow is because of the light fog surrounding (or below) the contrail. When light hits the fog, the small aerosols that makes out the fog (water droplets or ice crystals) are lit up as they direct light from the sun towards the viewer. The aerosol particles that are positioned in the path of the shadow of the contrail are not lit up, and we perceive this as a shadow.

The reason why we say "volumetric shadow" is because the shadow is not projected and perceived on a flat surface, but through a volume of particles.
 

Mick West

Administrator
Staff member
How exactly does it make a shadow appear on the cloud level below it Mick? Wouldn't clouds below a contrail block you from seeing the contrail to begin with?

No, it's a thin layer of fairly transparent cloud. The cloud is being lit from above by the sun, the contrail blocks the sunlight falling on that layer of cloud, and so the segment of the cloud layer that is shaded is dark, all the way through.

We are accustomed to seeing a shadow on a surface, but if a shadow is cast on something that is translucent, then the shadow goes through it.

Here, I'm casting a shadow on a translucent white plastic cutting board:


And here is what it looks like from underneath:


Notice that because of scattering, the shadow is much sharper on the top surface than it is viewed from underneath.
 
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Mick West

Administrator
Staff member
And part of the illusion here is that the contrail is so much brighter than the layer of cloud, so it looks like it is in front of it (i.e. lower altitude).

The illusion comes because you expect a cloud layer to block light. But because it's just a very sparse collection of tiny water droplets, most of the light is either passed through, or scattered into white. The white light coming directly from the contrail ADDS to the light from the cloud layer.

It's a bit unintuitive.
 
I have seen a volumetric shadow in a thick layer of haze gradually appear as I drove towards where the contrail crossed the sun. When I stopped, it gradually disappeared as the contrail drifted away. (Another reason for getting a dashboard camera.)
 

xari

Closed Account
hahaha, what kind of site is this?? It´s NOT a contrail, it´s a CHEMTRAIL and no further discussion with me!
 

Mightyhoof

New Member
hahaha, what kind of site is this?? It´s NOT a contrail, it´s a CHEMTRAIL and no further discussion with me!

WoW really?? You are going with that response? So even when someone explains to you why and how this shadow happened you are going with that answer? Really immature...And to answer your question this site is about scientific proof which are provided to you by some really intelligent/skillful people who go out of their way to explain things to a person like you who than responds with the most immature answer EVER. Seems to me you came here expecting how you wont get an adequate answer but when you did you didn't know what else to say. So better luck next time :)
 

mrfintoil

Senior Member.
hahaha, what kind of site is this?? It´s NOT a contrail, it´s a CHEMTRAIL and no further discussion with me!

Someone came here with an already made up mind I see. But since you wondered about the "grey line", to better understand how such shadow works I made this simulated image showing how a volumetric shadow looks like from different angles:

contrail_shadow.gif

Light is made up of photons travelling in straight lines, unless the direction is bent by the interaction with reflective and refractive materials (or gravity if we are geeky). But you can think of light as a huge number of rays travelling from a light source, through space to finally reach your eyes (or the camera) where it becomes interpreted as light.

As you can see in my simulated version, the more parallel to the trail path you get the more visible the shadow becomes. This is because there are less lit atmospheric particles in the path of the shadow below the trail, and if you view the shadow perpendicular to the direction of the trail you will have less unlit particles in your field of view. Or a more accurate description, a perpendicular view has more lit particles, because a shadow is the absence of light and light is not the absence of shadows. A shadow is not something, it is the effect of the lack of something.

Sounds tricky? But the principle is quite easy to understand, volumetric shadows becomes noticeable depending on how much shadow is accumulating in your field of vision, which is affected by things like your viewing angle and the angle of the light. As you can see in your photo, it was taken quite parallel to the direction of the trail, so the shadow became quite noticeable.
 

Mick West

Administrator
Staff member
Also...Where is the plane?

Another optical illusion. Because no light passes through the plane, it's a lot dimmer than the contrail, so the haze and the cloud layer renders it invisible. The contrail is brightly lit because the light from above passes through it and is scattered to white.

You can actually see a bit of a shadow of the fuselage in the first shadow.
 

Jason

Senior Member
It is backed like a 747, they have quite characteristic curve out and around of the contrails.


The A380 has much straighter contrails.
OT: This might seem like a stupid question, but for a layman what is the difference between white contrails and grey or dark grey contrails?
 

Ross Marsden

Senior Member.
Actually, comparing with a genuine B747 trail, it's not the same at all. It is probably distinctive of a certain aircraft, though.
It would be interesting to build up a catalog of trail vs aircraft type.
 

Mick West

Administrator
Staff member
OT: This might seem like a stupid question, but for a layman what is the difference between white contrails and grey or dark grey contrails?

Mostly lighting - although a more dense contrail is more likely to look dark if it's lit from behind. Like with clouds. All clouds are made of the exact same thing - water (in the form of ice or liquid drops), so they are all the same "color". But the "color" you see is a combination of:
  • Transmitted light - light that goes straight through, like the blue sky behind the contrail showing through, or the sun being visible through the cloud.
  • Scattered light - light that bounces around inside the cloud before you see it. Making it white.
The amount of white depends on the angle of the sun, and the density of the cloud, and the effects fo shadows. Look at this cloud:


It's all the same color. But some of it seems dark grey.

The more behind the cloud (or contrail) the sun is, then the darker it will seem - providing it is dense enough. The thin edges here are white, as they are not dense.


It also depends what is behind the cloud/contrail, and how well lit that it. Sometimes a contrail can appear dark just because it's against a hazy sky, and the contrail pointing towards the sun, so is not getting much light. Like:
 

Trailspotter

Senior Member.
Not from Germany, but definitely odd and probably volumetric contrail shadow that I spotted in my recent photos (taken June 11, 2014 at 13:02 UTC with a 6 second interval):
IMG_6224.JPG
IMG_6225.JPG
I zoomed on the plane that was heading toward the Sun, hoping to snap the moment when it would be passing across the Sun disc. However, the plane "missed" and passed above the Sun position. The short darker lines are probable volumetric shadows pointing toward it (the Sun position is outside the frame under its bottom edge and sightly to the left). There is a large angle between the contrail and the shadow, like the @mrfintoil' simulation above (#14), which initially led me to think that it was the contrail shadow.
BTW, the plane was a Thomson Airways B757:
Screen shot 2015-06-13 at 09.07.09.png

Edit: As @Spectrar Ghost pointed out below, the volumetric shadow probably comes from the plane itself and not from its contrail.
 
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