Starship Asterisk*

Contrail Shadow X

Explanation: What created this giant X in the clouds? It was the shadow of contrails illuminated from below. When airplanes fly, humid engine exhaust may form water droplets that might freeze in Earth's cold upper atmosphere. These persistent streams of water and ice scatter light from the Sun above and so appear bright from below. On rare occasions, though, when the Sun is near the horizon, contrails can be lit from below. These contrails cast long shadows upwards, shadows that usually go unseen unless there is a high cloud deck. But that was just the case over Istanbul, Türkiye, earlier this month. Contrails occur all over planet Earth and, generally, warm the Earth when the trap infrared light but cool the Earth when they efficiently reflect sunlight. The image was taken by a surprised photographer in the morning on the way to work.

<< Previous APOD

This Day in APOD

Next APOD >>

First thing I thought of upon seeing this Scotland!

If it were illuminated from below, we would see the contrail in front of the cloud deck. We don't.

I suspect that the clouds at the bottom of the picture are very high - higher than the contrails. I suspect that the clouds with the shadow are lower than the contrails.

Nice picture though.

This is a classical optical illusion: the cloud cover is lower than the contrails and thus the shadows are below. The low standing sun deceive the observer, because the observer places the plane in between the low standing sun and the shadows overhead (according to the relative altitudes above the horizon, the shadows should be cast upwards). But one must just realize: seen from the plane, the sun has the same altitude above the horizon than for the observer on the surface: the sun is still higher (and over the plane) and the shadows are cast down and thus below the contrails.
See: https://atoptics.wordpress.com/2013/01/30/where-is-the-shadow/

The typical cruising height of a jet is about 12km and the diameter of the earth is 12,000km so even if the sun is setting on the horizon a shadow would only be cast effectively horizontally (maybe upward at 1 or 2 degrees). But great picture!

The contrails are ABOVE a thin layer of cirrus. the contrail shadow is cast downwards onto the cloud layer. Take a look at the diagram here:
https://old.atoptics.co.uk/atoptics/contr2.htm

Elon left his mark.

ChristophHD wrote: ↑Tue Apr 23, 2024 7:29 am This is a classical optical illusion: the cloud cover is lower than the contrails and thus the shadows are below. The low standing sun deceive the observer, because the observer places the plane in between the low standing sun and the shadows overhead (according to the relative altitudes above the horizon, the shadows should be cast upwards). But one must just realize: seen from the plane, the sun has the same altitude above the horizon than for the observer on the surface: the sun is still higher (and over the plane) and the shadows are cast down and thus below the contrails.
See: https://atoptics.wordpress.com/2013/01/30/where-is-the-shadow/

well if a lower level thin layer of cirrus haze you refer covers the field of view we can't tell if the trail is in front of it or behind. And a trail is too thin to tell if it shines being illuminated by a front-light or a backlight; it's only certain when Sun (or Moon, as in the reference) is high above the horizon (as it is in the reference)

I agree with others here that we are seeing the shadows of the contrails cast down onto the clouds, not up. (I'm not convinced it's even possible to have a contrail shadow cast up onto a cloud... the Sun would have to be on or slightly below the horizon, and I think the directly illuminated cloud would overwhelm any shadow.) In this image, it appears that the Sun is just behind the tall building in the center... not close to the horizon at all. No way that geometry allows for upward directed rays that could cast a shadow from beneath!

Chris Peterson wrote: ↑Tue Apr 23, 2024 12:53 pm I agree with others here that we are seeing the shadows of the contrails cast down onto the clouds, not up. (I'm not convinced it's even possible to have a contrail shadow cast up onto a cloud... the Sun would have to be on or slightly below the horizon, and I think the directly illuminated cloud would overwhelm any shadow.) In this image, it appears that the Sun is just behind the tall building in the center... not close to the horizon at all. No way that geometry allows for upward directed rays that could cast a shadow from beneath!

from beneath

VictorBorun wrote: ↑Tue Apr 23, 2024 2:03 pm

Chris Peterson wrote: ↑Tue Apr 23, 2024 12:53 pm I agree with others here that we are seeing the shadows of the contrails cast down onto the clouds, not up. (I'm not convinced it's even possible to have a contrail shadow cast up onto a cloud... the Sun would have to be on or slightly below the horizon, and I think the directly illuminated cloud would overwhelm any shadow.) In this image, it appears that the Sun is just behind the tall building in the center... not close to the horizon at all. No way that geometry allows for upward directed rays that could cast a shadow from beneath!

from beneath

Right... but relevance to my comments?

Chris Peterson wrote: ↑Tue Apr 23, 2024 2:27 pm

VictorBorun wrote: ↑Tue Apr 23, 2024 2:03 pm

Chris Peterson wrote: ↑Tue Apr 23, 2024 12:53 pm I agree with others here that we are seeing the shadows of the contrails cast down onto the clouds, not up. (I'm not convinced it's even possible to have a contrail shadow cast up onto a cloud... the Sun would have to be on or slightly below the horizon, and I think the directly illuminated cloud would overwhelm any shadow.) In this image, it appears that the Sun is just behind the tall building in the center... not close to the horizon at all. No way that geometry allows for upward directed rays that could cast a shadow from beneath!

from beneath

Right... but relevance to my comments?

ok I am fitting the shadow, the trails and Sun… and you are right, Sun is high in the sky

VictorBorun wrote: ↑Tue Apr 23, 2024 3:49 pm

Chris Peterson wrote: ↑Tue Apr 23, 2024 2:27 pm

VictorBorun wrote: ↑Tue Apr 23, 2024 2:03 pm
from beneath

Right... but relevance to my comments?

ok I am fitting the shadow, the trails and Sun… and you are right, Sun is high in the sky
Contrail Shadow X (2024 Apr 23)-.jpg

I'm not sure what the lines in your image are intended to demonstrate. To me, they don't make it any clearer that the contrail shadows are being cast down onto the clouds, not upwards.

ayankforyes wrote: ↑Tue Apr 23, 2024 5:18 am First thing I thought of upon seeing this Scotland!

Nice. A prefect match. I guessed you must mean the flag, and I was right:

Chris Peterson wrote: ↑Tue Apr 23, 2024 3:52 pm

VictorBorun wrote: ↑Tue Apr 23, 2024 3:49 pm Contrail Shadow X (2024 Apr 23)-.jpg

I'm not sure what the lines in your image are intended to demonstrate. To me, they don't make it any clearer that the contrail shadows are being cast down onto the clouds, not upwards.

only that the sun is well above the horizon in this APOD; but that's enough to exclude illumination from beneath

These are not 'contrails'.

P. Mitchell wrote: ↑Tue Apr 23, 2024 6:08 pm These are not 'contrails'.

They are most certainly the condensation trails produced by passing jet airplanes. Contrails.

Interesting discussion on an APOD I was expecting to see none on! And I took the text at it's word, but it's now (pretty) clear to me that the shadow is cast downward, not up. Thanks!

The sun is too high for the shadow being cast from below. The thin cloud layer makes the picture dubious, but the sun elevation (and tint) leaves no doubt. Please, review this.

Thanks, everyone. I (now) agree that it is most likely that the X is caused by the contrails being illuminated from above, not below. I have now updated the text on the main NASA APOD to indicate this. I apologize for the mistake.

- RJN