Foggy day
I worked at Guys Hospital in London yea many years ago. In medical imaging and (then) high bandwidth networking.
We had a research group from the medical school decanted into offices in a tower block acrosss the street (Weston Street).
A laser link pointing out a window was set up to connect their LAN.
One London day was foggy.... sure enough we watched as the fog grew in depth , gradually creeping up the building.. till it reached the laser. No more connection!