Fog

Fog was a vapour. It proved troublesome to humans, whose vision it obscured.

In the 1900s, when a Rutan scout arrived on Fang Rock, fog pervaded the air. The Fourth Doctor hypothesised that the Rutan was using it to isolate the lighthouse keepers and their visitors. Reuben's assertion that, despite thirty years experience, he had not been able to see the onset of fog, gave credence to this. The Doctor had Leela sound the lighthouse's foghorn to warn any boats of the navigational hazards. (TV: Horror of Fang Rock)