What is freezing fog?

By Renee Duff, AccuWeather meteorologist

Freezing fog can pose just as many hazards as snow and ice during the winter months, but what exactly is this weather phenomenon?

Before diving into the basics of freezing fog, it is first important to understand the definition of fog.

In its simplest sense, fog is a cloud near the ground that consists of tiny liquid water droplets.

Fog can form when moisture is added into the air or the air cools to a point that it becomes saturated.

The main hazard from fog is reduced visibility that can lead to airline delays and a heightened risk of vehicle accidents on the roadways.

Fog AP

Dozens of cars and trucks crashed on the A19 highway in Zonnebeke, western Belgium, amid dense fog on Tuesday, Dec. 3, 2013. (AP Photo/Yves Logghe)

Fog is considered dense when visibility is reduced to a quarter of a mile or less.

Freezing fog is fog that forms when the temperature at the surface is at or below the freezing mark (32 degrees Fahrenheit).

“In addition to the hazard of reduced visibility that comes with fog, freezing fog brings additional dangers,” AccuWeather Meteorologist Ryan Adamson said.

(AP Photo/Robert F. Bukaty)

Freezing fog rises from the Androscoggin River as dump truck driver Jimmy Figer crosses the Memorial Bridge during snow removal duty in Rumford, Maine, in February 2018.

(AP Photo/Julio Cortez)

A jogger trots at Pier A Park as a wall of fog blankets the Hudson River behind it, Friday, Jan. 12, 2018, in Hoboken, New Jersey.

(AP Photo/Kirsty Wigglesworth)

Pedestrians walk over the Millennium Bridge as freezing fog shrouds St Paul's Cathedral in London, Monday, Jan. 23, 2017.

(AP Photo/Robert F. Bukaty)

Rime ice extends several feet horizontally from a sign marking the summit of 5,774-foot Mount Adams, the second-highest mountain in New England, on Tuesday, Nov. 17, 2015, in northern New Hampshire.

(AP Photo/Robert F. Bukaty)

In this Tuesday, March 10, 2015, photo, rime ice extends horizontally from a metal pole at the summit of Mount Washington in New Hampshire. Rime ice forms in the direction of the wind-driven fog that often blows across the summit at hurricane force.

(AP Photo/Jim Cole)

The top of Mount Washington in New Hampshire is coated in a rime ice, freezing moisture in the air on Thursday, Feb. 1, 2007.

(AP Photo/Charles Rex Arbogast)

The Chicago Skyline sits as a backdrop as fog drifts across Monroe Harbor with temperatures well below zero on Monday, Jan. 6, 2014.

(AP Photo/Alastair Grant)

The London Eye ferris wheel is photographed by tourists who stand on Westminster Bridge on a foggy day in London, Monday, Jan. 23, 2017. Freezing fog covered the capital as cold weather conditions continued.

“When freezing fog forms, any moisture deposited by the fog onto roadways can also freeze,” Adamson explained. “This can cause icy spots, especially on bridges and overpasses, so motorists need to exercise extra caution when there is fog with temperatures below freezing.”

Untreated sidewalks can also develop a slippery glaze of ice during freezing fog events, putting unsuspecting pedestrians at risk for falls.

At the airports, freezing fog events can lead to flight delays due to deicing operations and even cancellations.

If the freezing fog is very thick and/or persists for several hours or even days, ice can accumulate on trees, bushes, signs, power lines and other exposed surfaces.

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“While the ice can look very pretty, the extra weight of the ice on trees and power lines can cause them to come down, which can lead to power outages,” Adamson said.

Days of freezing fog are not uncommon across the valleys of the interior western United States, as cold, moist air gets trapped in the lower elevations during the winter months. Such a setup can occur underneath a persistent area of high pressure.

Under the right conditions, freezing fog can create a rare spectacle in the sky, known as light pillars.

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