Sorry I know I'm being incoherent Zilla...
Say I'm driving, see a 'bright sky' but no plume etc, no noise, just the sky brightening, how close would the probable detonation be?
Depends upon the yield of the weapon and time of day. Nevada night tests had the flash observed in Los Vegas. If it is noticable, go back to the 60's drill of drop immediately since you are probably close.
Count the seconds flash to bang. Close enough to five seconds per mile or three seconds per kilometer.
Just like lightning, only hopefully a longer count...
"Say I'm driving, see a 'bright sky' but no plume etc, no noise, just the sky brightening, how close would the probable detonation be?"(JP)
Depends on the actual type and size of the bomb and if it was detonated on, or in, the ground or in the air, as well as the terrain you are in. I'd say you were far enough away to safely turn the car around and hit the gas!
Small bomb maybe 5 miles? The biggest bomb, maybe 100 miles? I've heard the flash can be blinding if you are really close.