By Nigel Pankhurst
Dense fog has brought misery to thousands of travellers after causing major disruption to flights in the run-up to Christmas.
But why do such weather conditions result in chaos at airports such as Heathrow?
Fog has left planes grounded at Heathrow airport
Flying an airliner is widely seen as a hi-tech affair, involving aircraft that are bristling with navigational and safety devices to assist the pilots.
So how can weather conditions such as the fog that has descended over much of the country over the past few days bring so much chaos to airports?
The root of the answer, says David Learmount of Flight International magazine, is that pilots still rely a surprising amount on a very simple piece of technology - their eyes.
"When visibility is good an airport like Heathrow will always operate at about 98% capacity. It assumes you have a certain minimum amount of visibility, which is about five miles," he said.
"In good visibility the aeroplanes on their approach can be put close together because if anything goes wrong ahead the pilot can say 'I don't like this' and take action.
"The pilot can make the decision very easily if the visibility is very good ahead. He doesn't have to wait to be told by traffic control."
It is not just as an aircraft comes in to land, he explains, that pilots use their eyesight as a safety tool. Visibility is vital as planes move about on the ground. Very similar, in fact, to driving a car on the road.
"Planes have the ability to fly blind. The only trouble then is if you have really thick fog you can't see to taxi so the airport has to close. The visibility at the moment is good enough for taxiing on the ground," he said.
"It might, however, mean you have to taxi slower, which then might slow the whole process down. On the ground you use the same technology as a driver on the road uses for avoiding hitting something - your eyeballs."
So with pilots less able use their eyesight to pick out potential dangers, traffic controllers increase the airspace between planes so there is more time to react if anything goes wrong. This means less aircraft can use the airport. The result? Cancellations. Then, poor taxiing conditions can further clog things up.
HOW AUTOPILOT WORKS
Steering commands come from a computerised guidance program or radio receiver
Information is provided by motion and position sensors
A computer compares a guidance program to the aircraft's actual position
Motors alter engines and control surfaces when changes are needed
On runway approach the plane is guided by microwave beams from the ground
"The spacing of aircraft is at the discretion of traffic control, depending on how bad the weather is. In good weather it would be three nautical miles. In bad weather, frankly, you have to double that" says David Learmount.
"The thing that screws the whole thing up is that you have to space them further apart in case something goes wrong, such as a plane takes a long time to leave the runway or there has been an accident, so the pilot can see.
"People tend to think aviation is all technology, but no. Pilots rely far more than most people realise on their eyes.
"If you can't see the aeroplane ahead you can't slow down so you have to rely on traffic control so the decision time is slower."
But why has Heathrow seemingly been affected more severely than other airports? The airport, says travel expert Simon Calder, is simply full to the brim.
"It exposes just how Heathrow is at full stretch. The headlines could apply to the disruption we saw in August with the new security measures, or the summer before that or the summer before that," he said.
A crash in fog at Milan's Linate airport in 2001 killed 118 people
"It seems that Heathrow and its biggest customer, British Airways, simply are very exposed to the possibility of a little bit of disruption, or in this case having to reduce the flow rates - the number of flights allowed in and out of Heathrow - by 40%.
"To the extent that you are having 40,000 people a day whose Christmases are just being placed into jeopardy. These are people who have booked trips maybe last January.
"They wanted to make sure they would get their holidays or see their loved ones. I'm afraid they're now staring, they would say, into the abyss," said Mr Calder.
The last major aircraft accident involving fog happened on the ground at Milan's Linate airport in October 2001, causing the deaths of 118 people.
A Scandinavian SAS passenger jet speeding down the runway on take-off in fog collided with a light aircraft which crossed its path, killing all on both planes as well as four baggage handlers.
As a result, four employees of Italy's air traffic control authority were found guilty of manslaughter and each jailed for between three and four years.
As David Learmount puts it: "It doesn't matter what the conditions are, it's still invaluable to see out of the window."