A British Airways plane is seen through dense fog on the tarmac at Heathrow Airport, London, Monday, Jan. 23, 2017. Thick fog has caused numerous flight delays and cancellations at London Heathrow and other area airports. The Met Office forecasting service said visibility at Heathrow had been reduced to roughly 100 meters (yards), leading to an overall slowdown in operations. (Steve Parsons/PA via AP)