curve of the earth mostly. It's easy to see if they ran over a radar station, but something at 5000 feet is only visible for about 150 miles directly from ground level. That's why planes of a certain size are required to stay in the 10k+ height unless they're going for a landing
And that's assuming it's over the ocean. Once plants and ground gets in the way, visibility is much less.
curve of the earth mostly. It's easy to see if they ran over a radar station, but something at 5000 feet is only visible for about 150 miles directly from ground level. That's why planes of a certain size are required to stay in the 10k+ height unless they're going for a landing
And that's assuming it's over the ocean. Once plants and ground gets in the way, visibility is much less.