Dense Fog Advisory: when to slow down, delay travel, or get off the road

Read the exact product

Outlook, forecast, advisory, watch, and warning do not mean the same thing

Fog risk often appears first in a local forecast, then may be covered by an advisory when it becomes widespread or especially hazardous. The National Weather Service generally communicates fog with forecasts and advisories—not a dedicated fog watch or fog warning.

Outlook

A broader planning product may mention the potential for fog before its timing or extent is certain. It is not a travel restriction or a guarantee that an advisory will follow.

Forecast

Describes expected fog, visibility, timing, and temperature for a location. Check again before travel because patchy fog can change quickly and may be denser in valleys or near water.

Dense Fog Advisory

Issued by a local NWS office when widespread dense fog develops. Visibility frequently falls to one-quarter mile or less, making travel difficult. Delay travel when possible.

Freezing Fog Advisory

Fog is present with surface temperatures at or below freezing. Tiny droplets can freeze on roads, vehicles, walkways, aircraft, and other exposed surfaces.

Watch or warning

If a watch or warning is also active, read its exact hazard name and instructions. It may address freezing rain, winter weather, flooding, wind, or another threat; it does not replace the fog forecast or advisory.

Visibility can change suddenly

Patchy fog can be dangerous even outside an advisory area

An advisory is not the line between safe and unsafe travel. Fog can be locally dense before it becomes widespread enough for an advisory, and visibility can change over a short distance. Bridges, valleys, low terrain, and areas near water deserve extra attention.

• Do not assume a clear starting point means the entire route will be clear.
• Check the forecast for the route and destination, not only the current observation at home.
• Allow more time or delay departure when dense fog is forecast near the travel window.
• Expect radiation fog most often overnight and in the early morning, especially in sheltered valleys and near calm water.
• Remember that sunrise may improve some fog, but it is not an immediate all-clear.

NWS says radiation fog is common across the United States and is most prevalent during fall and winter. It forms as air near the ground cools and reaches saturation under stable, light-wind conditions.

If you must drive

Slow down, use low beams, and create more stopping distance

Dense fog shortens how far you can see without shortening how far the vehicle needs to stop. Drive for the visibility you actually have, not the posted speed or the pace of traffic around you.

1. Reduce speed smoothly and allow extra time to reach the destination.
2. Turn on low-beam headlights so both headlights and taillights are visible. Use fog lights if equipped.
3. Do not use high beams; reflected light can create glare and make the road harder to see.
4. Leave substantially more room behind the vehicle ahead for unexpected stops.
5. Use lane markings as a guide while continuing to scan for stopped traffic, pedestrians, and road edges.
6. Avoid sudden braking, lane changes, or passing, and never follow another vehicle's taillights too closely.

When visibility approaches zero

Get completely out of the traffic lane before stopping

NWS guidance says the best choice in extremely dense fog is to turn on hazard lights and pull into a safe location such as a parking lot. Never stop in the traveled portion of the road.

1. Signal early and move into a parking lot, driveway, rest area, or another location fully separated from traffic when possible.
2. If no safer location is available, pull as far off the roadway as possible.
3. After stopping, leave hazard flashers on but turn off other lights.
4. Set the parking brake and remove your foot from the brake pedal so illuminated brake lights do not draw traffic toward the vehicle.
5. Remain belted and avoid standing near the roadway while visibility is poor.

Do not stop beneath a bridge, on a curve, over a hill crest, or in another place where approaching drivers cannot see the vehicle in time. If a crash or disabled vehicle blocks a lane, call 911 from a safe location.

Freezing fog

Low visibility may come with nearly invisible ice

In freezing fog, supercooled droplets can freeze instantly on exposed surfaces. NWS warns that this can create black ice on roads and ice on bridges, vehicles, stairs, rails, sidewalks, and aircraft.

• Delay travel when possible, even if the road appears only wet.
• Reduce speed before bridges, ramps, shaded areas, and untreated surfaces.
• Leave extra stopping room and make steering, braking, and acceleration gradual.
• Use handrails carefully and assume untreated outdoor steps and walkways may be slick.
• Do not scrape or de-ice a vehicle while standing where traffic could slide toward you.

Boats and super fog

Road and water guidance are different—and smoke can make fog worse

NWS considers fog dense for mariners when visibility is less than one mile. Boaters need more distance to stop or turn and may become disoriented. Slow down, use running lights, listen for other vessels and navigation signals, stay out of shipping lanes, and use marine forecasts and navigation equipment.

Super fog is a separate, especially dangerous mixture of smoke and moisture from damp, smoldering vegetation with cooler, nearly saturated air. NWS says it can reduce visibility below 10 feet and move through low terrain. Treat any sudden wall of near-zero visibility as a reason to leave the travel lane safely, not to push through it.