A man walking during fog

Here’s a thought: have you ever encountered what seemed like an awkward or random layer of ice during the winter season? There’s no rain or snow and yet, the surface outside is already covered in a thin glaze of ice.

That could be freezing fog, aka ice due to fog. 

Although it’s a fascinating work of nature, freezing fog can be dangerous, especially for people who drive during the winter season. Plus, frozen fog is challenging winter weather to forecast. 

With the winter season fast approaching, it’s important to learn more about freezing fog — is it the same as ice fog? Why is it bad? Why does it happen?

What is Freezing Fog?

A simple fog is composed of water droplets that are suspended in the air near the ground, which reduces visibility. Fog forms due to different factors: temperatures, weather patterns, climate and topography.

Fog forms when saturation happens due to the added moisture in the atmosphere’s lowest layer. Fog can also form when the wind is light or calm or when the night sky is clear, which causes the surface to lose heat.

Freezing fog forms in the same way as the normal fog does, but there are other factors added to the equation. So, what conditions cause freezing fog?

Frozen fog happens when the temperature drops below or is at the surface of the freezing mark. For example, a foggy morning with a 30-degree temperature has fog formed from supercooled cloud droplets. These tiny droplets, which are suspended in the air, remain in liquid form, even if the temperatures are below freezing.

Once they touch an object or a surface that is freezing, they instantly freeze, thus forming the freezing fog. 

This happens because liquid needs a solid object to freeze upon. When freezing fog droplets touch surfaces or objects, they form a white deposit of feathery ice crystals. These crystals are called “rime,” which are often seen on vertical surfaces.

Under the right conditions, freezing fog can also form a rare natural spectacle called light pillars.

Days of freezing fog are common across the valleys located in the western part of the US. In this area, the moist and cold air often gets trapped in the lower elevations, especially during the winter season.

What is the Difference Between Ice Fog and Freezing Fog?

When the fog occurs at temperatures between 14 to 32 degrees, it is considered a frozen form of fog. As mentioned, when the liquid droplets touch solid surfaces or objects, they freeze almost immediately. Freezing fog is also often accompanied by freezing drizzle, especially if there are enough water droplets that fall to the surface. Freezing drizzle often creates a thin glaze of ice on roads, trees and sidewalks.

On the other hand, ice fog is fog formed in temperatures below 14 degrees. Water droplets in fog cannot remain in liquid form at this temperature. As a result, the ice crystals don’t freeze in contact with objects or surfaces since they are already frozen. Unlike freezing fog, ice fog is dense and can create a light dusting of snow.

Why is Freezing Fog Bad?

Person Walking on Road Between Trees with fog
Photo by Rene Asmussen from Pexels

Although sheets of ice and crystals during the winter are pretty to look at, freezing fog can be dangerous. The main hazard from fog is reduced visibility, especially during morning rush hours. When fog can’t be cleared, you can’t see where you’re going. Freezing fog can also negatively impact airports since lack of visibility causes delays at the runways. 

In addition to the danger of reduced visibility, frozen fog can make roads slippery. The formation of freezing fog deposits moisture onto roadways, which causes the surface to freeze. These create icy spots on main roads, overpasses and bridges.

Untreated pathways and sidewalks can also develop a slippery sheet of ice during frozen fog events, which puts unsuspecting pedestrians at risk for trips and slips. If the frozen fog situation persists for several hours (or days) or is too thick, ice can accumulate on bushes, trees, power lines, signs and other exposed surfaces. The extra weight on power lines and trees can cause them to come down, which leads to power outages.

How to Stay Safe In Freezing Fog

Freezing fog should not be easily brushed off. Instead, practice safety and mindfulness, whether you’re staying inside of your home or driving out. 

Here are some ways to stay safe.

Check for Freezing Fog Advisories

The National Weather Service often issues a “Freezing Fog Advisory” when there is a high chance of freezing fog developing in your area. Always stay tuned to these advisories so you’ll know if you should stay at home and reschedule the trip until weather conditions improve. 

If the National Weather Service issues an advisory, wait for conditions to improve before you leave the house. In most cases, the freezing fog takes one to two hours to dissipate.

Drive Safely

Road with fog
Photo by Markus Spiske from Pexels

Driving in icy conditions can be dangerous. Although freezing fog doesn’t always result in icy roads, it can still create sheets of slippery ice. If you must travel despite the freezing fog, consider the following practices:

  • Reduce your speed. You won’t know where the frozen fog or black ice is, so reduce your speed. Exposed areas like open roads and bridges can freeze quickly. Also, pay attention to sheltered areas (such as roads under the trees) since these can take longer to thaw. By reducing your speed, you’re reducing your risk for skidding and vehicle destabilization due to icy encounters.
  • Check your lights. Many cars automatically switch to dipped lights if the visibility is poor. However, not all vehicles have this feature so, during a foggy drive, the car’s lights don’t always come on. If you are unsure, manually switch on your lights when you are in the fog.
  • Increase your distance. The freezing fog can make it difficult to judge how fast you should go. Always exercise caution to prevent crashing or bumping into vehicles pulling out in front of you. Also, take a longer look when crossing traffic in the middle of the fog.

Freezing fog is a fascinating work of Mother Nature, but it can be dangerous. So always practice precaution when encountering this frozen fog.

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