# Above or Below: How to tell if a contrail is above or below a cloud layer

Discussion in 'Skydentify - What is that Thing in the Sky?' started by Mick West, Aug 28, 2014.

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Something that often happens when looking at contrails (or when people are looking for what they consider "chemtrails") is that a trail will pass though a thin region of cloud, and it's not clear if the trail is in front of the cloud (at a lower altitude), or if it is behind it (at a higher altitude). Very often it looks like the contrail is in front (lower). But how do you tell?

The short answer is that unless the cloud and/or contrail are thick enough that they block light, then it is impossible to tell. The scene will look the same (other than size) if the contrail is above the cloud or if it is below the cloud.

This seems a bit counterintuitive, however consider what a cloud (or contrail, a type of cloud) actually is. A cloud is made up of trillions of microscopic drops of water, or (if it's cold enough) trillions of microscopic ice crystals. Water and ice are both transparent. The cloud appears white because it's scattering sunlight light via refraction and reflection.

So the white shape you call a cloud is essentially made up of trillions of points of light. Contrails are the same. In the situations we are talking about, the contrail is usually denser than the cloud layer. This just means there are more points of light. in the contrail than in the cloud.

So if you put a thin cloud in front of a contrail, the cloud layer will not obscure the contrail in any way. It will simply add to it. If you put it behind the contrail, you get the exact same effect.

As an analogy, consider this firework display seen though a mesh of holiday lights strung over a street.

The mesh of lights represents the cloud layer, and the firework represents the contrail. Now we know that the firework is high in the sky, not in the middle of the street, but if we did not know that we might have a hard time telling if the mesh lights were visible through the firework, or if the firework was visible through the mesh lights. Because what we see is just the light from both of them added together, it does not actually make any difference what order the light is added together (although in this case there's the minor issue of the strings that the lights are on being visible).

To demonstrate this more, I created a Photoshop file with multiple layers representing:
• The blackness of space (what we would see if there was no atmosphere)
• Blue sky (from the atmosphere
• A plane, which is solid, and does block light
• A contrail, not solid enough to block any light, so shows up as white
• A thin cloud layer, also white, not blocking light, but more transparent.
These layers are arranged in the above order, with the cloud layer at the bottom (so it's "in front" of everything else), and then the contrail, then the plane. I've attached a small version of this file so you can try this yourself if you have Photoshop.

In the animation below, you see me move the plane from behind the cloud to in front of the cloud. You see it looks very different, and it's quite obvious when the plane is in front or behind.

I then move the contrail in front of the cloud layer. You see this makes no difference at all. The only thing you see change is the border of the image, which is just there to indicate where the layers are. I move the cloud layer back and forth, and as you can see the clouds and the contrail look exactly the same in both configurations.

So when the contrail is above a thin cloud layer it looks just like it is below it. And since our brains expect the contrail to be partially obscured if it was above, then they automatically default to it looking like it is below.

Not being able to tell does not mean there's a 50/50 change of it being above or below. In the majority of cases the contrail will be above, as contrail form at higher altitudes than most clouds. With a high layer of cirrus though, it might be impossible to tell.

#### Attached Files:

• ###### above and below cloud layer contrail bordered 3D Low Cloud Small.psd
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2. ### TrailspotterSenior Member

It could be difficult to tell from a static image if a contrail is above or below a cloud layer. It is much easier from a movie, especially, from a time-lapse movie:

3. ### LeiferSenior Member

Like when trail shadows are seen amongst other clouds.
It almost looks like the trail shadow is being lit from below, and the shadow is laid on the above cloud.

Not always so.
It's an illusion, based on the position of the sun.
There ARE layers......and the shadow basically dictates the layer heights, based on sun position.

In this pic, the sun is NOT at (or near) the low horizon to the left, to make the shadow above the trail.
(look at the shadows on the houses)
The sun is above, shining down. This means the trail is above the other clouds.

But the illusion is that the trail seems to be in front of (under) the other clouds, leading some people to think the trail is lower, just because it is "brighter".
It's brighter, only because it's more dense, with particles.

Last edited: Aug 30, 2014
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4. ### TrailblazerModeratorStaff Member

Here's an example of how height can be misleading. I took this photo last month.

The plane in shot is at 38,000ft. The older contrail it is crossing is at 37,000ft, i.e. BELOW this plane and its contrail. Yet to the naked eye, from the ground, it appears that the new contrail is in front of the old one, because it is brighter.

(Planes were identified using FlightRadar24: flight in shot is FR3186, older contrail is from EZY7209, evening of September 14.)

Plane in shot is FR3186: http://www.flightradar24.com/data/flights/fr3186/#453d171

Older contrail is from EZY7209 (aka U27209): http://www.flightradar24.com/data/flights/u27209/#453fee5

Note altitude readouts at the bottom. The photo was taken from a few miles east of the point where the trails cross.

Last edited: Oct 16, 2014
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5. ### TrailblazerModeratorStaff Member

I came across this video showing some "weird" cloud shadow effects:

Notice that there are actually two different sized shadows, suggesting there are two layers of cirrus, at different heights. Quite a bizarre effect.

Last edited by a moderator: Nov 11, 2014
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Amazing images, and yes, the shadow must be on a cloud layer that is much closer to the viewer. It's clearly being lit from behind. It's a similar kind of optical illusion. I'd love to see that.

There's a similar image (not as spectacular) here, with the same explanation.

7. ### JasonSenior Member

There's actually several different shadows in this cloud formation. It's an amazing pic by the way. I've never seen anything like this before. Just amazing. If you look closely, there is a shadow as represented by the red pen that almost mimics the larger shadow. But then if you look at the cloud itself, and at first I thought it was just the darker part of the cloud, you can the same outline of the larger shadow.... Or is this just my eye playing tricks on me...

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The red line is essentially the same shadow, cast on a medium distance layer. The larger shadow is on a closer layer.

The blue line is not a shadow on a cloud layer (it can't be, as the sun is behind the cloud, so its shadow edges can't be inside it), it's just a variation in the cloud itself.

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9. ### JasonSenior Member

But wouldn't the larger shadow be on the more distant cloud layer than the smaller shadow. I'm just thinking about a flashlight. The closer I hold an object to it the smaller the shadow.

The shadows are the same size, as the sun is so far away. The bigger shadow only looks bigger as it is closer to you.

Stand on some train tracks. Look one way - the track converge in the distance. Turn around, look the other way, they converge . Same thing.

While the rays look like they are diverging from a point light source, the sun is 93 million miles away, so the rays are parallel in the sky.

Something like this which seems to have diverging rays:

is actually parallel rays.

http://earthobservatory.nasa.gov/IOTD/view.php?id=76261

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12. ### TrailblazerModeratorStaff Member

The blue line can't be a shadow on a separate cloud layer, as you say, but I think it is a shadow of the top (more distant) part of the cumulus cloud, on the body of the cloud. Or, to put it another way, the outline of the portion of cold thick enough to stop the sunlight filtering through it.

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