Why does fog cause a problem for flights?

There has been a lot of press today particular about the impact of fog on flights in London, but this made us wonder what affect does it have and why.

Todays weather has been dense fog across large parts of the UK and particularly London and the South East.  London City Airport had to cancel all flights and one point and 10% of flights into and out of Heathrow were cancelled, why?

UK airports have procedures that they follow when visibility is reduced to under 600 metres.

Passenger aircraft are are generally fitted with an Instrument Landing System (ILS), which allows them to be guided towards the airport by a radio signal when there is reduced visibility and it can also direct them during take-off.  This is all well and good, except, runways have to be completely clear when an aircraft is landing or taking off following this procedure and this in itself leads to more space being required between flights.

Airports such as Heathrow which are already at near capacity then have to delay and/or cancel flights as there simply isn’t enough spare time between normal flight operations to follow this procedure.

Below are some of the affected airports and links to their latest travel advice and information:

London City Airport
Manchester Airport
Southampton Airport
Gatwick Airport
George Best Belfast City Airport
Cardiff Airport