As all aircrafts are faced with delays because of the foggy conditions (holdings, ATC slots, diversions) these delays pile up throughout the day and are difficult to catch up. The big lakes around London Heathrow are the big cause of these foggy mornings.
I took an evening flight so we were delayed by at least 90 minutes. Once on-board the captain was very informative and told us even though planes like the A321 can easily fly in and out of aiports during foggy conditions, there are other security requirements in place by ATC.
They need to be able to visually double-check what they see on the ground radar to avoid any potential taxiing colisions. This may take a few extra minutes but as luchtzak said it all add's up.
I travelled via LHR from Geneva to Vancouver today (yesterday). The 1500 flight GVA-LHR was cancelled, but as I checked in early they put me on the 1335 - which actually departed GVA at 1510. The Captain said that the delay was due to one runway at LHR being closed in the morning. The Vancouver flight was also delayed, about an hour - but we made that up and arrived on time.
the actual reason for all of these delays yesterday at LHR was due to one runway being closed. During the night they had done some maintenance of the runway, but it had rained on the new surface that had been layed rendering it useless to use.............
but for today 28/7 fog and low viz is causing 60min+ delays
and if you're wondering why all LHR flights were delayed on thu 28& fri 29/07, here's the explenation.
In the morning, a message had arrived at the FedPol offices that there was a possible bomb threat! Eurostar (Waterloo st.), BA and BD were at risk that day on LHR routes. Extra security was put at C/I rows, border control became more strictly, all unattended luggage was reported,... Panic was created at first, but after a while everything settled down a bit. Even the lounges had recieved regular visit of the police 8O
I was amazed that nothing came on the news that day, coasing panic seems to be the hype nowadays...