I remember Gatwick's RVRs sensors being very close to the ground and reporting very low RVRs when the fog only reached half way up the aircraft legs!!
The other side of the coin is that airfields that have very tall control towers often have to go into LVPs because ATC are in the clouds whilst everyone at ground level can see quite clearly. Seen it happen at AMS a few times, perhaps time to draw a halt to the "mines taller than yours" competition that seems to be going on at the moment before they build the new one at LHR.