Fog poses a unique challenge for pilots in that you can't really see in fog. You would think that would make landing aircraft impossible but there's no fuss over fog for pilots flying in and out of YVR, which utilizes an Instrument Landing System (ILS) to guide pilots down to the ground safely when fog is at its thickest.
The ILS provides critical navigational information to aircraft on approach to assist with landings. Routine maintenance and flight checks of the ILS are a regulatory requirement to ensure the system meets stringent certification standards prescribed by Transport Canada. Flight checks involve a specially equipped Canadian Regional Jet (CRJ-200), seen above, that carries navigational equipment necessary to test and certify the ILS equipment is accurately calibrated, functioning properly and safe for use.
The tests are carried out by NAV CANADA, the agency responsible for the safe co-ordination and the efficient movement of aircraft in Canadian domestic airspace and international airspace assigned to Canadian control.
Some smaller regional airports are not equipped with ILS equipment, which is why you still see a few disruptions during low-visibility days, but YVR runs near normal even in fog thanks to the specialized equipment implemented and maintained at YVR.