Visibility is a distance, not a description
Visibility tells you how far a suitable object can be seen and identified through the air. Fog is cloud at ground level that reduces visibility below 1000 metres in the Met Office definition. Dense fog means visibility below 100 metres. Those thresholds help meteorologists describe conditions, but they do not mean the view changes neatly at a boundary.
Mist, haze, heavy rain, snow, spray, smoke and blowing dust can also reduce visibility. A forecast icon may show fog while the detailed text or observation gives the more useful number. Always check the valid time and units: 500 metres is a very different travel condition from five kilometres.
A city-wide value cannot resolve every street, hill or runway. Visibility can change quickly near rivers, coastlines, open fields and dips in the terrain. The observation may also come from an airport or model grid point some distance from you.
Why fog forms in different ways
Fog forms when tiny water droplets are suspended close to the ground, but the route to saturation varies. Radiation fog often develops on clear, calm nights as the land and the air just above it cool. It commonly thins after sunrise, although winter fog can persist when weak sunlight and a temperature inversion keep cold air trapped.
Valley fog collects where colder, denser air drains into low ground. Advection fog forms when moist air moves across a colder surface; it is common around coasts and can move inland with the wind. Upslope fog develops as air is forced up terrain and cools. Evaporation fog can form when cold air passes over warmer water or wet ground.
These types behave differently. A sunrise rule that works for shallow radiation fog may fail for sea fog being carried ashore. This is one reason a short hourly forecast and local official update are more useful than a generic statement that fog always burns off in the morning.
Read the temperature and dew point together
Fog becomes more plausible near the surface when air temperature approaches the dew point, because the air is nearing saturation. Light wind and overnight cooling can support radiation fog. Yet a small temperature-dew-point gap does not guarantee fog: cloud cover can limit cooling, wind can mix the air, and moisture may vary sharply with height and location.
Likewise, a forecast that does not display fog at a city point is not proof that every road will remain clear. Patches can form below the scale of the model. If the route crosses valleys, bridges, elevated ground or the coast, check the official forecast for those areas as well as the destination city.
Why fog forecasts carry extra uncertainty
The timing depends on small changes in temperature, wind and cloud. A difference of a degree or a little extra breeze can determine whether saturation occurs. Once fog forms, its top depth and the amount of sunlight reaching the surface influence how quickly it clears.
Forecast models represent these processes over grid boxes. They can signal a favourable setup while missing the precise start, density or edge of a fog bank. Treat an hourly visibility value as an estimate for that place and time, not a guaranteed minimum for an entire journey.
Driving decisions need current local information
The US National Weather Service advises drivers who must travel in fog to slow down, allow extra time, use low-beam headlights and leave more distance from the vehicle ahead. High beams can reflect from droplets and increase glare. In extremely dense fog, its guidance recommends moving completely off the road to a safe location rather than stopping in a traffic lane.
Road rules and official advice differ by country, so follow the responsible transport and weather authorities where you are. Airport and marine operations use specialised visibility observations and thresholds; a general city forecast is not a substitute for those products.
Before travelling, check the latest official warning, not just the icon saved earlier in the day. Conditions can change after a forecast was issued, and the safest decision may depend on your route rather than the weather at either endpoint.