I experienced a long delay on an easyjet flight just over a week ago on the day of the bad fog.
The reason for the delay was initially due to the fog and air traffic control but then they needed to get a new crew due to the crew not having enough hours left in their shift to fly, this pushed the delay out to nearly 5 hours. We then got on the plane and they discovered a fault with some of the communication equipment so eventually by the time this was fixed our total delay was 6 hours.
Under the EU rules you are automatically entitled to compensation after a 5 hour delay but not if due to weather.
Given this was due to multiple factors will compensation be due in these circumstances?