deltahotel - What I meant by unplanned runway closure was an aircraft emergency/disabled aircraft or perhaps runway surface breakout. It would be extremely unusual for both 27L and R to be out due fog. All LHR runways are CAT III equipped in respect of ILS and lighting, however the IRVR (instrumented runway visual range) for each runway can differ greatly so ATC would likely adjust the landing and departure runway to suit. The downside with fog is runway capacity is reduced due to the need to increase the arrival spacing on approach so ILS signal integrity is not affected. The reduced landing rate figures based on the IRVR value are published in the UK AIP. This results in ATC flow control measures which can result in flight cancellations, mainly short haul, particularly flights where there is more than one daily frequency to/from a particular destination.