BA nightstop many of their aircraft in Europe in order to provide early flights back to the UK. Easyjet keep their aircraft at their UK bases overnight. The result of this is that when the fog arrives, Easy have a full fleet of aircraft at home base ready to shoot off to destination. Many of BAs aircraft are stuck downroute with big air traffic control slot delays and can't get back into London on time.

Fog always causes this much chaos at LHR simply because the whole place is absolutely chock full to capacity. Normal landing rates are of the order of 60 aircraft per hour. As soon as the fog arrives, ATC drop this to thirty. The holding stacks around LHR then fill up with aircraft with +40 mins airborne holding delays. Aircraft in the holds begin to divert, and aircraft yet to depart for LHR are hit with enormous slot delays (4 hrs is not uncommon) as ATC try to clear the backlog. Flight crews start to run out of duty hours leaving diverted aircraft stranded and later flights cancelled due to lack of crews. Meanwhile at LHR, ATC have started landing aircraft on both runways instead of just one to try to clear the holds. This impacts upon the departure rates, leading to long queues at the holding points and airfield departure delays. Aircraft are held on stand awaiting start clearance and so all the stands get filled up and there's nowhere for the inbounds to park causing more delays. And don't get me started on de-icing! Aaah the joy of Heathrow.