Thomas said:
But in the same video clip, they are actually looking at the objects slightly above the clouds. Meanwhile they, themselves, are slightly above the clouds:
The objects certainly appear to be above horizon level in the beginning of this clip. Take a look.
It looks at first as if that's a blank sky, or even a blank ground or sea the objects have as a background in that footage. But it can't be as the camera elevation and the plane's direction of flight show that both the camera and the plane are fairly level.
So where's the horizon? It should go righty across the middle of the footage.
I believe the answer is that it's invisible.
The infrared cameras seem to be blind to features like the sky, the clouds and the ground if they are more than maybe, at a guess, 10 km.
You can see an example of this at 18 seconds in the 19 megabyte footage. The ground just disappears into blackness beyond what I'd guess are ground features maybe 10km distant.
Only really hot objects show up at long distance, like the oil-tanker at 50km that you mention.
I'd expect the tanker to appear as a bright spot on a blank background, and the sea to be invisible that far away.
I believe the horizon is at the same place or even slightly above for example, the "headlight" objects we see in the footage.