Is the sky flickering in Rise of the Triad 2013 normal?? VRR, V-sync, all settings...nothing work.
Or are those supposed to be flashes of lightning during a thunderstorm, but continuous?

Oy, this game is so jank at times. This *was* a known issue back in 2013, they supposedly fixed it, did another update and it came back, and were supposed to fix it again. But, clearly, they did not.

Anyways, I just mucked around, and V-Sync is the answer it seems.
In the launcher go to to Avanced Options, which does NOT let you in until you start the game at least once after first install.
Scroll down, bottom left is VSync. Turn that on.
IMPORTANT, hit ACCEPT SETTINGS. Play game. May cause the usual "mouse feels laggy" some FPS have with VSync.

If the game expects a more normal 60Hz refresh rate it may not behave well with VRR though. Can also try turning VRR off and/or limiting to 60Hz. I do not have a monitor faster than that to test. These would have to be done in the NVidia/AMD control panel, as they don't exist in the game config.

Thank you for your answer.
But I have already tried all these settings. But the flickering in the sky remains.
I've watched Youtube. And in every let's play of the game the sky is flickering. Perhaps that's how it's supposed to be, and it's lightning during a thunderstorm?
You see the flickering sky at the beginning in the menu and later in the game:
https://www.youtube.com/watch?v=8FhyhwhJZgQ&list=PLKt_el6xVhJASb0s6E5TOrYOfydlEVmWd

That video shows the regular flashing of lightning working as intended. The VSync fix is for when those flashes show up on top of the sky (and not melded into it) as white boxes that look similar to screen tearing.

I understand. Thank you very much for your help.