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Comment by ridaj | Hacker News Reader

I sort of freaked out the first time I saw something like this.

Then Wikipedia held my hand and said everything was OK and this even has a name:

https://en.wikipedia.org/wiki/Fata_Morgana_(mirage)

"...The optical phenomenon occurs because rays of light are bent when they pass through air layers of different temperatures in a steep thermal inversion where an atmospheric duct has formed..."

To me that's the most Men In Black explanation I've ever heard.

Definitely confirms this is aliens.

Or proof the earth is flat.

Nah, they're just sailing to Valinor.

Basically the layers of air above the water have slight variations (temperature, humidity, pressure) between them in such a way that they each have a small change in refractive index that over a distance bends light upwards.

Then you have another group of layers of air further up in the 'marine boundary layer' that bends the light back towards the waters surface.

This is called an 'atmospheric duct' and is somewhat similar in the effect to a fibre optic cable.

Poor mans source: I wrote my thesis 5+ years ago on refractive effects in the maritime boundary layer [1]

[1] https://ceed.wa.edu.au/wp-content/uploads/2017/02/DSTO-Refra...

Agent D, you're making it worse...

You can confirm it at home

https://www.youtube.com/watch?v=968gVUAY9Mg

Here is a lay explanation.

Light travels at different speeds in different substances. But it always takes a path that is locally fastest - meaning that any nearby path would be slower. (This is called the Fermat principle.)

You can see this principle at work when you put a stick into water. Because light travels more slowly in water, the light first heads mostly straight up, then bends when it hits the air. The result is that light does not travel a straight path to your eye. Which means that the part of the stick in the water looks like it is where the light comes out of the water, rather than where the stick is. As a result you can see the stick visibly bend.

Now what is happening here is that you have a layer of warm air over cold air. Light travels faster in warm air. (That is because as air warms it expands, making it less dense. Less dense means that there is less getting in the way of the light and it can move faster.) Therefore that fastest path is for the light to go up into the warm air, go along the warm air, and then dive back down to your eyes.

In many places you can see the reverse of this on hot days where hot ground makes for a hot air layer next to the ground. When the conditions are right the light from the sky can reach your eyes by skimming along the ground, and you get blue patches in the ground. In a desert this can look like water in the distance.

OK so, that's how the air ripples work for heat... because of the density of the air.

It makes me wonder, if you had a 10 inch globe of vacuum suspended in air, how different looking thru it would appear vs the air. It seems like you might be able to detect it via sight alone.

Yeah, it'd definitely be visible. I imagine it'd be like looking at an air bubble while underwater, in terms of refraction, except that air doesn't have surface tension so the boundary wouldn't look quite the same.

That's what a visible shockwave from an explosion is. Light bending through sharp pressure discontinuities.

https://i.redd.it/jbx7x4a66em21.jpg

https://i.ytimg.com/vi/_31RYF3p42E/maxresdefault.jpg

http://waitbutwhy.com/wp-content/uploads/2014/10/explosion-i...

The vapour cloud which often forms occurs in saturated air as a partial vacuum forms behind the shockwave:

https://frontlinevideos.com/wp-content/uploads/2018/05/massi...

https://media.tenor.com/images/a6572b4115f67f2aeea90c529d598...

There's a spot where I walk my dog near my house. It follows a power line trail up a hill, and you can turn around and see Denali. For whatever reason, on warm spring days Fata Morgana distortion is fairly common from this spot. I've seen it a few times. At it's most obvious, Denali will appear to be sitting on a pedestal of sheer cliffs, thousands of feet high, surrounded by non existent tabletop mountains and improbable, fantastically shaped spires that would be 10,000 feet ASL if they were real. My cellphone camera doesn't do it justice to though.

Have you uploaded pictures anywhere?

No, I just tried posting to imgur and it didn't go so well. It is a white mountain on the horizon, taken with a cell phone. Honestly the pictures don't convey much. If you zoom in, you can see the distortion, though. I'm sure a decent lens could capture it.

This is not a Fata Morgana, it's a false horizon.

https://www.youtube.com/watch?v=er1mh90wN-k

This new image needs to go in the wikipedia article. The example images are kinda bleh. But this one really illustrates the concept.

A fata morgana is a special type of superior mirage, where the object is heavily distorted, often to the point of being unrecognizable. The ship in the article kept its shape and is very much recognizable.

I've seen it once and I haven't seen an explanation until now. My wife and I were kayaking on a lake in Northern Sweden and far far away we see a boat that is just moving slowly, lifted up in the air. It was completely calm and we could not hear it (very far away), it was ghostly.

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