What is causing this?

[Genady]

This phenomenon can be observed on the island where I live at sunset. From time to time, not often, maybe a few times a year. This image holds for a couple of minutes and then fades and dissipates.

I think I know the answer, but I might be wrong.

Pix from my porch:

 

 

Edited December 25, 2022 by Genady

[mistermack]

Off the top of my head, the prevailing wind is coming from the direction the camera is pointing in. There is a land feature away in the distance that interrupts the flow of the lower more moist air, forcing it left and right. Colder drier air comes down to take it's place, and as it holds less moisture, the sky is showing through it.

Complete guess of course. 

[Genady]

1 minute ago, mistermack said:

Off the top of my head, the prevailing wind is coming from the direction the camera is pointing in. There is a land feature away in the distance that interrupts the flow of the lower more moist air, forcing it left and right. Colder drier air comes down to take it's place, and as it holds less moisture, the sky is showing through it.

Complete guess of course. 

Could be, especially since there is in fact another island in that direction, not far behind the horizon, about 50 miles away.

However, the data don't support it. The camera is pointing to the West, the sunset. The prevailing wind is from the North-East, the trade wind. The wind blows from the West only during one short period in September called, wind reversal. I didn't observe this phenomenon during a wind reversal.

[mistermack]

Ok, so a jet passed that way some time ago and left a vapour trail. The vapour trail seeded the existing moist air causing it to condense and fall, leaving a trail of drier air behind. 

[Genady]

2 minutes ago, mistermack said:

Ok, so a jet passed that way some time ago and left a vapour trail. The vapour trail seeded the existing moist air causing it to condense and fall, leaving a trail of drier air behind. 

We don't have much air traffic here. Only about 2 jets a day. I am sure there was no jet on that line that day. I certainly would've noticed otherwise.

Moreover, the "trail" stays in place without going anywhere for a couple of minutes while the wind blows, and the clouds move.

[Genady]

23 hours ago, Genady said:

the "trail" stays in place without going anywhere for a couple of minutes while the wind blows, and the clouds move

Because of this observation, I thought that the phenomenon is rather optical than atmospheric.

[mistermack]

Are you sure that there aren't winds at a higher altitude flowing at 90 degrees to the lower ones? I've seen that many times, with high cloud at odds to the lower ones. And the coriolis effect causes that too. 

[Genady]

No, I'm not sure in this.

[sethoflagos]

On 12/25/2022 at 8:08 PM, Genady said:

Could be, especially since there is in fact another island in that direction, not far behind the horizon, about 50 miles away.

If this is occurring just after sunset, could it not be a shadow cast by that island from below your horizon. This should cut out the red end of the spectrum leaving just a blue backfill from Rayleigh scattering.

[Genady]

3 minutes ago, sethoflagos said:

If this is occurring just after sunset, could it not be a shadow cast by that island from below your horizon. This should cut out the red end of the spectrum leaving just a blue backfill from Rayleigh scattering.

This is what I think, too. Here is a little calculation supporting it.

The Earth circumference is about 25,000 mi. The Sun makes it in 24 hours. Thus, it moves about 17 miles a minute along the Earth surface in our area, i.e., near the equator. That is, about 40-50 miles in 2-3 minutes. Thus, after our sunset, the Sun is still for 2-3 minutes above the horizon for that neighboring island. What we see then is its long shadow in the sky.

[sethoflagos]

3 minutes ago, Genady said:

This is what I think, too. Here is a little calculation supporting it.

The Earth circumference is about 25,000 mi. The Sun makes it in 24 hours. Thus, it moves about 17 miles a minute along the Earth surface in our area, i.e., near the equator. That is, about 40-50 miles in 2-3 minutes. Thus, after our sunset, the Sun is still for 2-3 minutes above the horizon for that neighboring island. What we see then is its long shadow in the sky.

And what you see depends quite sensitively on that island lying between you and the sun's disc, hence much of the year someone else gets to see it.

[Genady]

4 minutes ago, sethoflagos said:

And what you see depends quite sensitively on that island lying between you and the sun's disc, hence much of the year someone else gets to see it.

Right. Moreover, even when the position is right, the atmospheric conditions don't always cooperate. Then, nobody gets to see it.

[sethoflagos]

2 minutes ago, Genady said:

Right. Moreover, even when the position is right, the atmospheric conditions don't always cooperate. Then, nobody gets to see it.

Just sneaked a look at your profile to find out where you're based and it doesn't say. 

Lagos is similarly placed in that 'dusk' is very short as the sun drops vertically.

[Genady]

10 minutes ago, sethoflagos said:

Just sneaked a look at your profile to find out where you're based and it doesn't say. 

Lagos is similarly placed in that 'dusk' is very short as the sun drops vertically.

Bonaire

[sethoflagos]

28 minutes ago, Genady said:

Bonaire

Lucky you!

[Genady]

26 minutes ago, sethoflagos said:

Lucky you!

Thank you! No complaints  

Edited December 26, 2022 by Genady
typo

[mistermack]

2 hours ago, sethoflagos said:

could it not be a shadow cast by that island from below your horizon.

That's a good call. The only thing that bothers me is that it's not perfectly straight, but there might be a reason for that, to do with variations in the moisture content of the air in between the island and the camera. 

[Genady]

1 hour ago, mistermack said:

it's not perfectly straight

I think it might have to do with the island's shape and orientation, so that the sunset time and the sun's angle are not the same in different parts:

Edited December 26, 2022 by Genady

[mistermack]

It might be complicated, with some of the light in the sky caused by reflection off the sea surface, and the island blocking some of that.

[swansont]

It becomes more apparent when you can see the feature causing it

https://twistedsifter.com/2014/01/the-sky-shadow/

[Genady]

This is wonderful.

5 minutes ago, swansont said:

It becomes more apparent when you can see the feature causing it

But then it's not a puzzle.

[TheVat]

Bon aire, indeed.  

[Genady]

10 hours ago, TheVat said:

Bon aire, indeed.  

I agree, but the etymology is not clear:

Quote

The name 'Bonaire' is thought to be derived from the Caquetio word 'Bonay', meaning 'low country'. The early Spanish and Dutch modified its spelling to Bojnaj and also Bonaire. French influence, while present at various times, was never strong enough to make the assumption that the name means 'good air'. According to another theory, the name might be derived from the Spanish phrase "buen aire", which does mean 'good air', as the Spanish were the first Europeans to colonise the island.

 (Bonaire - Wikipedia)

[mistermack]

Old French debonaire, from the phrase de bon aire (“of good stock, noble”).

[Genady]

Or Portuguese, bom ar, bons ares...

I think that one of the points in support of the Caquetio origin, Bonay, is the fact that it is name of a place and there are many local places - as well as local plants and fishes - that still retain their Caquetio names in the local language, Papiamentu. In which the island is called, Boneiru.