By AccuWeather staff writer
On Thursday night, Dec. 10, the National Weather Service in Mount Holly, New Jersey, located about 20 miles east of Philadelphia, issued a freezing fog advisory for a portion of southeastern Pennsylvania and much of New Jersey.
The seldom-used advisory was met with befuddlement from a few social media users.
"Freezing fog"?! [sic] one curious Twitter user replied to the weather service on Twitter. "Is this just frost or is 2020 now effecting [sic] the weather, too?"
"It's definitely not new," the NWS replied. "It's just a bit rarer here rather than across the Midwest."
"When was the last time one of these was issued? Can’t remember PHI issuing this," one Facebook user asked the NWS on its page.
As it turned out, it had been awhile. The NWS Mount Holly had gone nearly two years, or 672 days, since it last issued a freezing fog advisory, and that was only the second such advisory issued since 2005.
Many people have heard of fog or witnessed it themselves, but freezing fog may be a less familiar weather phenomenon. Freezing fog can pose just as many hazards as snow and ice during the winter months, but what exactly is it?
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.
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).
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“In addition to the hazard of reduced visibility that comes with fog, freezing fog brings additional dangers,” AccuWeather Meteorologist Ryan Adamson said.
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.
“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.
“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.
Freezing fog can also create a distinct type of ice referred to as rime ice. According to the National Weather Service in Spokane, Washington, rime forms as supercooled water droplets freeze on contact, commonly forming needlelike structures.
"Ice structures may be wind driven with rime occurring on the downwind side of what the droplets are freezing on to," the NWS 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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