What a Dense Fog Advisory actually means

A Dense Fog Advisory is issued by your local National Weather Service office when visibility is expected to fall to a quarter mile (about 400 meters) or less across a wide area, typically for several hours. At a quarter mile you cannot see the next exit sign on the interstate; at the 100-foot visibilities that the worst advisories describe you cannot see the car ahead until you are almost on it. Fog is the least dramatic hazard the NWS covers and one of the deadliest per event, because chain-reaction crashes happen at full highway speed. About 55,000 Americans a month search the phrase, mostly between October and February. Every US city page here shows the live NWS alerts beside the hourly forecast, so you can see the advisory's end time before you leave.

The four kinds of fog behind an advisory

• Radiation fog. The common one. On a clear, calm night after rain or over wet ground, the surface cools, the air next to it reaches its dew point and fog forms in the lowest few hundred feet. It is thickest around sunrise and usually burns off within one to three hours as the sun warms the ground. Valleys and river bottoms get it first and keep it longest.
• Valley fog that will not lift. In winter, when the sun is low and the valley is capped by warmer air aloft, radiation fog can persist for days. California's Central Valley "tule fog", from November to February, is the textbook case and the state's most dangerous driving weather; Fresno sits in the middle of it.
• Advection fog. Warm, moist air moves over a cold surface: sea fog rolling onto the coast, or spring fog over snow-covered or cold ground. This one does not care about sunrise; it lasts until the wind shifts or the air mass changes. San Francisco in summer is advection fog.
• Freezing fog. Fog at temperatures below 32°F that deposits ice on roads, bridges and windshields. The visibility threshold is the same, but the surface is the bigger hazard, and offices call it out in the advisory text.

When it clears

The advisory text gives an end time, usually "until 10 AM". For radiation fog that estimate is reasonably good: fog needs sun or wind to break it, and the office knows roughly when the sun will do the work. Two things push the end time later. A thin cloud deck above the fog blocks the sunlight that would burn it off; and light winds keep the moist layer in place rather than mixing it out. Advection fog has no natural end time, which is why coastal advisories sometimes run for a day or more. Once a breeze of 10 mph or more arrives, fog rarely survives; a windy morning is a clear morning.

Driving in it

• Low beams, never high beams. High beams reflect off the water droplets and light up the fog in front of you, cutting what you can see. This is the rule most drivers get backwards. Fog lights, if you have them, point low and help.
• Slow down enough to stop within what you can see. At 100 feet of visibility that is about 25 mph. Multi-car pileups in fog happen because the first driver sees the stopped traffic too late and the fifth driver never sees it at all.
• Increase following distance and avoid the temptation to follow the taillights ahead closely; they may be leaving the road.
• Use the right edge line as a guide, not the center line, so you are not drawn toward oncoming traffic.
• If you cannot see, get off the road at an exit or parking lot, not the shoulder, and turn off your lights so no one follows you in.

Forecasting fog the night before

Fog is one of the hardest things a weather model predicts, because it depends on a few degrees of cooling in the bottom 200 feet of the atmosphere, and most models are coarser than that. The tell-tale setup is easy to read yourself, though: rain or wet ground during the day, skies clearing in the evening, wind dropping below 5 mph, and a forecast low within two or three degrees of the dew point. When the hourly forecast on a city page shows temperature and dew point converging toward dawn with calm wind, expect fog whether or not the icon says so. Our guide to why weather forecasts disagree explains why the models split on exactly this kind of small-scale event, and our accuracy scoreboard shows which model has done best in your city.

Advisories, watches and warnings share one grammar across every hazard. If the terms still feel interchangeable, start with watch vs warning, and what the difference means; a companion is what a Wind Advisory means, since wind is the thing that ends fog.