Currently Explains: Fog

We’re officially in the thick of fog season. Literally. 

These clouds of warm, moist air often form near waterways and are most likely to occur when the dew point level is no more than 5 degrees F (-15 degrees C).

You’re most likely to find fog along the misty mountain ranges of North and South America throughout the fall and winter. It typically forms overnight, when the air near the ground cools, and the water vapor condenses into tiny droplets of water.

There are several different types of fog:

  1. Advection fog — when moist air moves over a colder surface (most common).
  1. Radiation fog — this often occurs on clear nights with calm winds. The air temperature cools down to the dew point temperature, causing 100 percent relative humidity near the surface, and resulting in foggy conditions that will begin to burn off after sunrise. 
  1. Super fog — a combination of warm polluted air or smoke combines with cooler air
  1. Freezing fog — fog composed of ice crystals instead of water droplets formed in the extremely cold arctic air. Freezing fog can coat all exposed surfaces with a layer of ice, which can make travel hazardous and slippery. Black ice, which is hard to see when driving, biking, or walking, often occurs in freezing fog situations. 

Navigating through fog can quickly become dangerous. By definition, fog has a visibility of less than 1km (a little over half a mile), but can increase all the way to a “Category X” fog, where visibility is less than 20m (about 66 feet).

There are more than 38,000 vehicle crashes, and 600 deaths, every year due to fog. The thick clouds can obscure even the sharpest of vision and make it hard to navigate any terrain.

The best way to stay safe is to follow a trusted weather source and keep your eyes peeled for fog forecasts. This way, you can plan your travel accordingly, around this extreme weather.

The National Weather Service, for example, will issue Dense Fog Advisories when the fog is thick enough for hazardous travel.

Should you be forced to go out into fog, here are some things to keep in mind, according to the National Weather Service:

  • Use your low-beam (fog) headlights to make your car more visible to others on the road.
  • Leave plenty of distance between you and other vehicles on the road.
  • Remain in your lane by paying extra attention to the yellow lines on the pavement.
  • In extreme situations, turn on your hazard lights and carefully pull off to the side of the road, or into a driveway, until the coast is clear.

Fog illustration by Houssem Zouaghi.


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