Flight cancellations through fog.

[scotbill]

Most modern aircraft have the ability to land in fog provided the crew have completed the extra training.
The problem is that ATC have to use much greater separation between landing aircraft in very low visibility so that the preceding aircraft is well clear of the runway and the Instrument Landing System transmitters before the next lands.
Really thick fog is best for the best equipped airlines as many of the lesser machines divert!
Paradoxically, once the vis improves the delays worsen as all the diverted aircraft try to get back in.