Temperatures dipped to nearly -20 degrees Fahrenheit overnight.

London fog airplane
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A wave of freezing fog swept over southern England on Monday, shrouding landmarks and causing flight delays and cancelations at several airports.

The weather phenomenon created a mysterious-looking veil across vast swaths of London, the Midlands, and southeast Wales. The Met Office issued a severe weather warning, as a spokesperson from Heathrow Airport urged travelers to check with their carriers before attempting to arrive at the airport as dozens of flights were canceled, The Guardian reported.

London fog
The city of London is shrouded in fog Jan. 23, 2017.
| Credit: Photo by Tolga Akmen/Anadolu Agency/Getty Images

“With Heathrow operating at more than 99 percent capacity, there are no gaps in the schedule that can be used for delayed flights, and as a result some passengers may experience disruption to their journeys,” the spokesman said, according to The Guardian.

Freezing fog occurs when droplets of water that comprise fog are “super-cooled,” in below-freezing temperatures. When these droplets come in contact with a solid object, they freeze, creating a coating of ice, Scott A. Mandia, the assistant chair for physical sciences at Suffolk County Community College in New York, told Travel + Leisure.

West Sussex fog
West Sussex is covered in freezing fog.
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When the weather is below freezing—London’s temperature plunged to nearly -20 degrees Fahrenheit Monday morning—the fog freezes into ice on any solid object it touches, such as a sidewalk or a plane. Freezing fog can cause dangerous road conditions, as it essentially coats everything in a sheet of ice, including airport runways.

“As far as I know, the ice itself does not damage a plane or a monument but it certainly makes air travel more dangerous as icing on an aircraft impairs its ability to fly or cars the ability to navigate slippery roads safely," said Mandia.