Understanding the hazards of frozen fog and mist

Gothic horror flicks from olden days with misty scenes of fog-draped castles and thick forests conjure thoughts of impending doom. Oh, the power of that imagery, derived simply from a shroud of water droplets suspended in air!

While not carrying the overt brawn of, say, a snowstorm, fog and mist can bring even greater impact, and not merely by setting the mood.

A famous routine by comedian George Carlin opens the door to a sports metaphor. He deftly noted the differences between baseball and football by contrasting the terminology of each. It was soft vs. tough, underscoring the disparate expectations of experience.

As an analogy for weather, it’s pitch-perfect. With a near-silent presence, fog or mist or drizzle lands softly in our minds like a routine flyball vs. predictions of blizzards or thunderstorms that rattle menacingly in our psyche days before the end-zone crush. We are lulled into calm by the former, while jarred to attention by the latter.

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Yet those gentle weather words can signal significant impact — the most obvious, low visibility; the more sinister, veneers of ice.

As for translucency, mist carries an advantage over fog, offering slightly better sightlines. By definition, the tiny droplets in mist are less densely packed together, so we can see through them more easily. The term “mist” is used if visibility is better than five-eighths of a mile. Anything less than that and it’s fog. But that fine line is usually breached, with the description going from mist to fog, or back again, often depending simply on the stillness of the air. In either case, the reductions to the depth of our view can occur startlingly fast.

Mountaintops and valley swales are especially prone to the sudden encampments and blinding hazard, although theirs is far from an exclusive club. Fog can occur anywhere there is sufficient low-level saturation in the air.

Neither fog nor mist is precipitation. That term defines water, in liquid or solid form, that falls to the ground. Rather, the feather-light droplets are suspended, as in a cloud, not quite heavy enough to allow gravity to overcome the natural buoyancy of the encircling air that keeps them afloat. Their cousin drizzle has slightly larger, heavier droplets that soft-land on the ground, and because they fall out of the air, drizzle is not typically accompanied by the same degree of diminished visibility.

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The hazardous impacts of fog are most striking when traveling, with countless stories of multi-car collisions occurring as a result of suddenly restricted visibility. Even a routine experience of driving through the mountains can switch between miles-ahead clarity and abrupt veiled views. The danger is real, and the most we can do is be aware and react quickly.

The particular peril of frozen fog and mist

Even greater impacts from fog and mist occur when temperatures fall below freezing, often resulting in an imperceptible icy glaze on ground surfaces. Nearly as many wintertime accidents occur from fractional coatings of ice as from major winter storms, but we tend to ignore the icy peril until an actual encounter. This happens for two reasons: It forms quickly, often with little warning, and it’s nearly invisible, leaving us unprepared to appropriately respond.

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Fortunately, with increased messaging via social media and weather blogs, public awareness is improving, yet the seriousness of a risk that develops so quickly is not easily discerned.

Freezing fog is becoming particularly dense across portions of Nelson, Albemarle, and Augusta counties. A Freezing Fog Advisory is in effect for these locations until 9 AM Tue. Exercise caution and reduce speeds given the potential for icy roadways tonight. #VAwx pic.twitter.com/XXMlPhDUMm

A big part of the problem is the innocuous appearance of fog. Freezing fog, mist and drizzle are liquid, even when the surrounding air is below freezing. The droplets are supercooled — the temperature within each is just barely below freezing. This happens because the droplets need a trigger to force their transition from liquid to ice to overcome the surface tension that keeps them hanging at that so-close tipping point. That changeover is spurred on contact with subfreezing surfaces and objects such as roads, walkways, trees and power lines. The subtle process is practically unseen, yet the resultant razor-thin ice is as treacherous as snow.

How an icy breath of ‘freezing fog’ created a mystical scene in the South

According to data published by the Federal Highway Administration, on average there are over 150,000 crashes and 41,000 people injured each year because of ice on the road. That’s 13 percent of all weather-related accidents, and these figures do not include snow, which is something we most often attribute to wintertime highway calamity.

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And it’s not just traffic accidents.

“Nearly 85 percent of the cases we see each year involve personal injury in which a plaintiff slipped and fell on ice or snow,” Alicia Wasula, certified consulting meteorologist and president of STM Weather, a forensics weather consulting firm in Albany, N.Y., wrote in an email. “Many of these slip and fall incidents occurred during ‘nuisance’ type weather situations, rather than big ‘blockbuster’ storms. In these cases, a light glaze of ice was sufficient to result in a slippery surface, particularly on untreated areas.

“In these situations, knowledge of how to obtain National Weather Service watches, warnings and statements and how to use the information therein to prepare ahead of time (such as by pretreating sidewalks with salt) can prevent the hazardous situation from forming, and ultimately prevent injuries and litigation that often follows.”

The risk of freezing fog, more patchy in nature, also extends into the Northeast.

The concern is that light moisture that came down Monday morning saturated the air near ground level, but temperatures have cooled below freezing.

Fog may freeze on contact with surface objects. pic.twitter.com/KJwHCe6teH

From individual slip-and-falls to major highway accidents, the ways we react to the dangerous impacts of freezing fog and the like have seen only small improvements over the years. That these weather phenomena remain such a consequential, too-often unrecognized risk tells us that we in the meteorological community must continue to strive to give more timely warnings that are effectively messaged; otherwise, poor public response to these hidden hazards will continue.

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So, the next time you encounter misty landscapes, whether in the movies or in your travels, take a second to appreciate the deceptive power behind the scene. Mother Nature finds ways to foil our perceptions when least expected.

Jim Duncan recently retired from his 40-year career as chief meteorologist with NBC12 WWBT-TV in Richmond. He runs his own meteorological consulting firm, Jim Duncan, LLC, serving clients in media, education and other industries.

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