Following the comments about London smog I received this from one my
colleagues :-

"When I was there in the 70s, the obs books from when it opened were still
there. It was interesting that during the smog events, the obs had something
like this each hour
2 yards
3 yards
1 yards
2 yards
1 yards
4 yards
2 yards

I suspect that they changed the viz out of boredom. I know that at some
stations (though whether at LAP I don't know), string was strung (!) between
the obs office door and the enclosure so that people didn't get lost. I was
at Kew Observatory in the mid 60s - and sometimes you had to know your way
round in the fog - but that was because it was in the middle of a golf
course and near the river, and fog drifted off the river."

Jon.