What Is Fog?
Fog is a cloud at ground level. It forms when tiny water droplets remain suspended in the air near the surface, reducing how far you can see.
In weather observations, fog usually means visibility has fallen below 1 km. The droplets are extremely small, often only a few micrometres across, so they float in the air rather than falling like rain. The result can range from a light veil that softens distant objects to dense fog that makes roads, runways, and coastlines disappear.
Fog is not a separate substance from a cloud. The difference is mainly where it forms: a cloud is above the ground, while fog reaches the surface.
How Fog Forms
Fog needs three ingredients:
- Moisture: The air must contain enough water vapour to become saturated.
- Cooling: Air near the surface must cool to its dew point, the temperature at which water vapour begins to condense.
- Limited mixing: Light winds allow droplets to build up. Strong winds usually disperse fog, although some types of fog need a steady wind to develop.
As air cools, it can hold less water vapour. Once the temperature reaches the dew point, some of that vapour condenses onto tiny particles in the air, such as dust, salt, smoke, or pollution. These droplets form the suspended cloud we see as fog.
A small gap between the air temperature and the dew point is an important clue. When the two values are close, only a little more cooling is needed for condensation to begin. See Humidity and Dew Point Explained for more on how this process works.
The Main Types of Fog
Different combinations of cooling, moisture, wind, and terrain produce different types of fog.
Radiation Fog
Radiation fog forms when the ground loses heat on a clear night and cools the air directly above it. It is most common when winds are light and the air is already moist.
The coldest air settles in valleys and low-lying areas, which is why radiation fog often develops there first. It usually becomes thickest near dawn and lifts after sunrise as the ground warms. If the sun is weak, the fog can last much longer.
Advection Fog
Advection fog forms when warm, moist air moves horizontally over a colder surface. The air is cooled from below until it reaches its dew point.
This is common along coastlines, where moist air passes over cold ocean water, but it can also happen when warm air moves across snow-covered ground or a cold land surface. Unlike radiation fog, advection fog does not need clear skies and can persist through the day.
Valley Fog
Valley fog is a terrain-related form of fog that collects in valleys and basins. Cold, dense air drains downhill during the night and pools at lower elevations. If enough moisture is present, the air becomes saturated and fog develops.
Valley fog can remain trapped for hours or even days when an inversion forms. In an inversion, warmer air sits above colder air near the ground, preventing the cold, foggy air from mixing upward.
Upslope Fog
Upslope fog forms when moist air is pushed uphill by the wind. As the air rises, it expands and cools. If it reaches its dew point, cloud droplets form at the surface.
This type of fog is common on the windward sides of hills and mountains. It can cover large areas and continue as long as the moist air keeps flowing uphill.
Steam Fog
Steam fog, sometimes called evaporation fog, forms when cold air moves over warmer water. Water evaporates into the cold air, which quickly becomes saturated. The moisture then condenses into fog.
It can appear over lakes, rivers, and other bodies of water, especially in autumn and winter. The fog may look like steam rising from the surface, although it is made of suspended water droplets rather than hot vapour.
Freezing Fog
Freezing fog is fog that forms when the suspended droplets remain liquid even though the air temperature is below 0°C (32°F). When those supercooled droplets touch roads, trees, buildings, or aircraft, they can freeze and create a coating of ice.
Freezing fog is different from freezing rain. Freezing rain falls from the sky and freezes when it reaches a surface, while freezing fog forms and remains near the ground.
Fog, Mist, Haze and Low Cloud
These terms describe similar-looking conditions, but they are not interchangeable.
- Fog is a cloud at ground level that reduces visibility to below 1 km.
- Mist is made of water droplets too, but visibility remains above 1 km. It is usually thinner and clears more easily.
- Haze is made mostly of dry particles such as dust, smoke, or pollution, not water droplets.
- Low cloud forms close to the ground and can obscure hills or buildings without necessarily reaching the surface.
The boundaries can vary slightly between weather services, but the visibility measurement is the key distinction. See Visibility Explained for more on how these conditions are measured and reported.
Fog can also occur alongside haze or smoke. In that situation, water droplets and dry particles both scatter light, making the air look thicker and visibility fall further.
How Fog Is Forecast
Fog is one of the harder weather conditions to predict because it depends on small differences in temperature, moisture, wind, terrain, and surface conditions. A forecast model may correctly predict a humid, calm night but miss the exact valley or coastline where fog becomes dense.
Forecasters look for several signals:
- Temperature and dew point are close: The air is near saturation.
- Light winds: There is enough stability for droplets to accumulate.
- Clear skies overnight: The ground can cool rapidly and produce radiation fog.
- Moist ground or nearby water: Extra moisture makes saturation more likely.
- Low-lying or sheltered terrain: Valleys and basins collect cold air.
- A temperature inversion: Stable air can trap fog near the surface.
Fog often forms overnight and is thickest around sunrise, but this is not universal. Advection fog, coastal fog, and fog beneath a persistent inversion can last throughout the day. Rain can also leave the air saturated and produce fog when winds remain light.
The How Weather Forecasts Work resource explains why local effects like fog are more difficult to predict than large-scale temperature trends.
Why Fog Matters
Driving
Dense fog reduces the time available to see hazards, road markings, and other vehicles. Slow down, increase your following distance, and follow local road safety guidance. High beams often reflect from the droplets and create glare, so low beams are usually more effective.
Aviation
Visibility and cloud base are critical for take-off and landing. Airports use dedicated instruments to measure runway visibility, while pilots use instrument procedures when the view of the runway is not sufficient for a visual approach.
Shipping
Fog can hide coastlines, buoys, and other vessels. Ships use radar, navigation lights, sound signals, and reduced speeds when visibility is restricted.
Air Quality
Fog itself is made of water droplets, but the stable air that allows fog to form can also trap pollutants close to the ground. If fog occurs in a city or industrial area, check the Air Quality Index alongside the visibility forecast.
When Does Fog Clear?
Radiation fog usually clears when sunlight warms the ground and the air near the surface. The droplets evaporate back into water vapour, and mixing increases as the temperature rises.
Fog can also clear when:
- Wind becomes strong enough to mix the air.
- Drier air moves into the area.
- Cloud cover prevents further surface cooling but does not bring additional moisture.
- A weather front changes the temperature, wind, or humidity.
Advection and valley fog can be much more persistent. If the air remains saturated and the surface stays cold, fog may continue well into the afternoon or return again the following night.
How Airpult Shows Fog Conditions
On Airpult, you can combine the visibility, humidity, dew point, cloud cover, wind, and hourly forecast to understand when fog is most likely at a location.
A forecast showing calm winds, high humidity, and a small temperature-to-dew-point gap overnight is a useful signal for possible fog. The visibility forecast then tells you how seriously it may affect travel and outdoor plans. Use the explore page to search for any location and check its forecast.
