Freezing fog: What is it and how dangerous is it as dozens of flights cancelled
London was faced with disruption on Tuesday morning after a blanket of freezing fog descended on parts of the capital, prompting dozens of flight cancellations.
At least 23 departures and arrivals were cancelled at London City Airport, including inbound British Airways flights from Amsterdam, Berlin, Dublin, Dusseldorf and Rotterdam.
Meanwhile, flight BA 8731 was redirected to Gatwick Airport, after holding for 30 minutes off the southeast coast.
Planes at Gatwick were "de-iced" this morning as a precautionary measure, but there appeared to be no significant disruption to flights.
In a warning to travellers, police urged drivers to ensure their car windows are ice free before heading to the airport, and to be mindful of icy roads and foggy conditions.
Tuesday's disruption demonstrates that freezing fog should be taken seriously, but what is it, and why is it so dangerous?
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What is freezing fog?
Freezing fog forms when land cools overnight underneath clear skies – the same as with regular fog.
With no clouds to trap it, the heat absorbed by the planet's surface during the day radiates back towards space, resulting to cooler temperatures on Earth, the Met Office says.
As air cools down, it is less able to hold moisture without it condensing, eventually leading to the formation of fog, which is essentially like a cloud that touches the ground.
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When fog forms in temperatures that are below freezing, the tiny water droplets in the air become "supercooled", but still remain in liquid form, because liquid needs a surface to freeze upon.
This is why we see white deposits of feathery ice crystals, known as "rime", forming on surfaces when freezing fog hits, particularly on vertical surfaces exposed to the wind.
Normally this phenomenon is more common on mountains and higher ground, but this week it has been seen in towns and cities.
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Why is freezing fog dangerous?
Freezing fog is dangerous because of it leads to very poor visibility and ice forming on roads and other surfaces.
This makes it difficult to drive, fly, or even walk outside, which is why it caused problems on airport runways and motorways this week.
Urging motorists to remain extra vigilant on the roads, the Met Office advises people to drive very slowly and with their headlights dipped.
This is because putting them on full-beam would cause the flight to reflect off the fog, resulting in a "white wall" effect that dazzles oncoming drivers.
