"Icebound" wrote in message

Now, in general, when you had manned systems, airports attempted to have
visibility markers out to *at least* 15 statute miles, and a report of 15
was the norm for "unrestricted" visibility.

When I learned to fly in the Denver area, I got accustomed to wx reports
that included "clear and 70". The guys in the tower at Stapleton would just
look south. If they could see Pike's Peak, the vis was 70. Easy.
Infallible. No gizmo to break.