As temperatures begin to drop below freezing, fog can become a much more dangerous threat, turning a normal commute into a travel nightmare.

Fog often forms when air near the ground cools enough that water vapor in the air condenses into tiny suspended water droplets. This typically occurs at night and in the early mornings when Earth’s surface loses heat.

But when the air temperature is below freezing (32 degrees Fahrenheit), these water droplets are “supercooled,” meaning they’re liquid even though it’s below freezing. When this happens, it’s known as freezing fog.

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Freezing fog can be extremely dangerous because when these water droplets land on surfaces like roads, trees and sidewalks, they instantly freeze on contact, creating a thin, nearly invisible layer of ice.

Freezing fog can cause black ice to form on sidewalks and roadways. Black ice is difficult to see because it’s thin and clear, but it’s also particularly dangerous to drive on.

Airplanes are also very vulnerable to freezing fog. When freezing fog coats a plane's wings, a layer of ice can form that prevents the wings from generating the lift that makes flight possible.

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That’s why airports around the country have de-icing equipment to treat planes flying in hazardous winter conditions.

Freezing fog is most common in valleys and low-lying areas, where the coldest air settles overnight, but it can show up anywhere temperatures dip to freezing under calm, clear skies.

The National Weather Service may issue a freezing fog advisory when fog is expected to develop, surface temperatures are at or below freezing and visibility is at or below 1 mile.

Safety tips

If you are in a freezing fog advisory, or if conditions look foggy and it’s near or below freezing, here are some safety tips for if you have to head outside:

• Watch for slick steps, sidewalks and railings.
• Slow down while driving and leave extra space behind the car ahead of you.
• Use low-beam headlights or fog lights. High beams reflect off the fog and make visibility worse.
• Treat bridges, overpasses and shaded stretches as icy, even if the rest of the road looks wet.
• Brake gently and early. Avoid sudden braking or sharp steering.
• Delay your trip if you can, especially early morning when fog is thickest and temperatures are lowest.

Miriam Guthrie is an associate meteorologist with weather.com who graduated from Georgia Tech with her undergrad and graduate degrees in Earth and Atmospheric Sciences.