The earth is round, and the curvature limits visibility distance. Smoke in the distance can only be observed under ideal conditions.
I suspect seeing smoke at 18 or 20 Km would be a very rare event.
Radar, of course, changed everything; even then, the horizon and the three mile limit (aprox 5Km) limited detection.
I use the 8 Km as I suspect it is more realistic.
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Irish1958
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