It may look like just another foggy morning. But beware, if the temperature is at or below freezing, you could be stepping out into a treacherous, glazed landscape. Freezing fog is double-trouble: dropping visibility so you can't see the black ice.
"Fog may be a common occurrence in the late fall and winter. But if you didn’t look at the thermometer before you step out, when you actually get on to the sidewalk, you’re slipping and sliding," said Meteorologist Britta Merwin. "So freezing fog is a pretty big deal."
Freezing fog starts out the same way as fog. At night, the ground radiates heat into space, especially on a calm, clear night without insulating clouds or the wind mixing the radiated heat back to the surface.
Cooler air can't hold as much moisture as the same air at a higher temperature. So, the water vapor condenses into tiny water droplets and eventually fog, which is essentially a cloud on the ground.
Fog below freezing: A dangerous setup
"When fog forms in temperatures that are below freezing, the tiny water droplets in the air remain as liquid," states the U.K. Met Office. "They become supercooled water droplets remaining liquid even though they are below freezing temperature. This occurs because liquid needs a surface to freeze upon."
Thus, the supercooled droplets can freeze instantly on exposed surfaces.
The slippery surfaces are responsible for many accidents.
Freezing rain forms a thicker layer of ice over surfaces, tougher to remove.
Rime is very common on the windward side of objects during freezing fog events and may also look like a white plume, nicknamed "frost feathers," according to Britannica.com.
"Rime is composed of small ice particles with air pockets between them; this structure causes its typical white appearance and granular structure," states Britannica.com. "Because of the rapid freezing of each individual supercooled droplet, there is relatively poor cohesion between the neighboring ice particles, and the deposits may easily be shattered or removed from objects they form on."
Airports use deicing equipment to guard against the rime forming on the wings and fuselage of planes taking off from areas with freezing fog. The FAA issued a ground stop for Seattle's Sea-Tac Airport for three hours during a recent early morning freezing fog event.
"They didn't even start letting the planes land until about 11:15 in the morning because you've got to warm things up," said Merwin. "That's how you resolve this."
The fog dissipates when the Sun warms the air, increasing its ability to hold moisture and/or when wind picks up or thermals from the warming sun mix the saturated fog layer with drier air.