What is freezing fog? What danger does it pose?
How does freezing fog form, and how does it different from "regular" fog?
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Rising air temperatures and melting snowpack caused days of fog for tens of millions of Americans in Jan. 2024.
You’ve probably driven through fog at some point, and experienced how challenging it can be.
But have you come across freezing fog?
It can pose as much risk as snow and ice, and can disrupt traffic on roadways and for air travel.
Fog in general is air that is saturated to the point that you have very small water droplets that are visible close to the ground said Guy Pearson, AccuWeather's Director of Weather Warning Services at AccuWeather Enterprise Solutions.
“Typically this forms when you have cooler ground temperatures and warm moist air just above the ground and the contrast in temperatures between the ground and air is enough that water droplets condense and can be visible,” Pearson said “The amount of moisture in the air and the amount of difference in temperature between the ground and the air immediately above it then dictates how dense the fog is. Very moist air with a larger difference in temperature between the ground and the air immediately above it will lead to denser fog and lower visibility.”
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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.
What about freezing fog?
Freezing fog is not really a meteorological term, Pearson said. In order for it to form you need items or an object near the surface to be below freezing temperature, he said. This could include, trees, power lines and power poles, fences, tall weeds or cars. The air above the ground has supercooled water droplets in it, that might be just below freezing but are still in liquid form.
Once those droplets come in contact with something at the ground or just above the ground freeze on contact with the surface of that object, Pearson said. Most often, freezing fog does accumulate on trees and power lines and if the wind is calm or maybe even light, can make a very picturesque scene as the supercooled water droplets turn into ice crystals (also called rime ice). If there is some wind, then the water droplets will just form more as a hard layer of ice vs the more crystalized picturesque rime ice.
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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.
What danger does freezing fog pose?
Freezing Fog can be especially dangerous on bridges and overpasses that are not treated. Most of the roadway that is in direct contact with the ground would likely just be wet. However, as cold air can flow under bridges and overpasses reducing the temperature of the surface to below freezing, freezing fog could accumulate quickly and make any elevated surface slick and dangerous very quickly, Pastelok said.
High beams and defrosters
When visibility reduces, turning on the high beams may seem like a good idea. But in reality it will make it even harder to see. High beams emit an intense light that reflects off of the moisture in the air, while regular headlights don’t do that. Some vehicles are equipped with fog lights, which is the best option if available.
Fog is formed when water vapor drops get suspended in the air and there is high humidity on a chilly day. It can cause the fog to build up on your windows and further reduce visibility. Using your windshield wipers can also help.
Speed and following distance
Reducing your speed and the space between you and vehicles in front of you reduces your risk, since fog makes it harder to see what is in front of you and helps give you extra time to react. Slowing down will give you extra time to react to avoid an accident.
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A jogger trots at Pier A Park Park as a wall of fog blankets the Hudson River behind it, Friday, Jan. 12, 2018, in Hoboken, N.J.
Using lane markings to guide you
Road reflectors in the center and outside lane lines can help guide you. If the road lacks lane reflectors you can use the solid white line, which is easier to identify and help keep you from straying over the center line into oncoming traffic.
Pulling over
Stopping on the side of the road might be tempting, but it’s actually a terrible idea. Other drivers may not be able to see you, increasing the odds of a car accident or injuries to yourself and others. Stopping in a parking lot or until the fog clears is safer.
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