In a dense fog advisory, slow down, use low beams, leave extra distance and delay the trip if you can.
What the advisory means
The National Weather Service issues a Dense Fog Advisory when fog is expected to reduce visibility to a quarter mile or less over a widespread area. It sits at the advisory level: a hazard that calls for caution, below a warning. See our guide to watches, warnings and advisories for how the levels differ.
Marine dense fog advisories
For marine products the NWS uses the same name for widespread or localized fog that limits visibility to regionally or locally defined levels, not exceeding 1 nautical mile.
Driving in dense fog
The NWS advises: slow down and use low-beam headlights; keep your distance from other vehicles; avoid changing lanes or passing in dense fog unless absolutely necessary; drive only as fast as you can stop within the distance you can see; avoid cruise control; and consider delaying travel until conditions improve. Fog can thicken or clear quickly, so watch for sudden changes, especially in valleys, on bridges and near rivers.
If your car breaks down
Pull well onto the shoulder, use only hazard lights and, if possible, leave the vehicle and move away from the road.
Boating in fog
Visibility can drop to a few feet and disorient boaters. Turn on all running lights, even in the day, slow down, listen for other boats and use radar if you have it.
Types of fog
Fog forms when water vapor condenses into tiny droplets near the ground. The NWS lists radiation, advection and freezing fog among the common types. Valley fog forms when cooler air drains downhill and is trapped in a stable layer near the ground. "Super fog" forms when condensed water vapor mixes with smoke and moisture from smoldering organic material, and can cut visibility to a few feet.
Check your own location
See current official alerts and the impact for your ZIP code.
Common questions
What does a dense fog advisory mean?
It means fog is expected to reduce visibility to a quarter mile or less over a widespread area, according to the National Weather Service glossary.
Is dense fog bad?
It can be. Fog, especially dense fog, can be hazardous to drivers, mariners and aviators, and the NWS says it contributes to numerous travel accidents every year.
What should I do if I break down in dense fog?
The NWS advises pulling well onto the shoulder, using only hazard lights and, if possible, leaving the vehicle and moving away from the road.
Sources
Reviewed by the Storm Incoming editorial team on October 27, 2026. Follow local emergency officials if their instructions differ.