Fog is not steam rising from the ground, nor simply a cloud that has “fallen”. It is a suspension of tiny water droplets—or, in severe cold, ice crystals—in air next to the surface. The World Meteorological Organization classifies it as a hydrometeor that reduces visibility. Physically it resembles a cloud, but forms in or reaches the layer where people live and travel.
This is an educational guide, not a current warning. When fog may affect a journey, check the visibility forecast, official warnings and current traffic or transport information.
The essential condition: air close to saturation
Suspended droplets appear when air approaches saturation. It may cool towards its dew point, gain water vapour, or undergo both processes. Microscopic condensation particles and suitable mixing are also needed. Strong wind can disperse a shallow layer, although complete calm does not always help it deepen or spread.
The temperature–dew-point spread is a useful clue, not an automatic forecast. Our guide to relative humidity and dew point explains why both values must be read with wind, cloud and terrain.
Radiation fog: night-time cooling
Under clear skies, the ground loses heat by radiation after sunset. Air touching it may cool to saturation and form a shallow fog. Light wind is often favourable because it mixes moisture through a thin layer without breaking it up. Damp fields, river plains and hollows are common settings.
Sunshine can warm the ground and evaporate the droplets, but fog does not invariably “burn off” in the morning. A persistent inversion, upper cloud, abundant moisture or weak heating can keep it in place for hours.
Valley fog: terrain collects cold air
At night, cooled, denser air can drain downslope and pool on a valley floor. Low temperature, moisture from rivers or soil, and an inversion that limits vertical mixing can combine there. Hills may be sunny above a sea of fog.
Nearby places at different elevations can therefore have opposite visibility. A clear route along a slope may enter dense fog abruptly as it descends into the layer.
Advection and coastal fog
Advection is horizontal transport. When moist air moves over a colder sea, coast or chilled land, its lowest layer cools and may become saturated. Unlike radiation fog, advection fog requires airflow and may persist in daylight or move well inland.
A small shift in wind direction can carry a bank from offshore onto a beach, harbour or airport. It may feel sudden even though the fog was already present nearby.
Upslope fog
When wind forces moist air up a hillside, it expands and cools. Droplets form as saturation is reached around the slope. From the valley it may look like low cloud; to somebody crossing the hill it is fog.
It can cover passes, paths and exposed roads while another side of the mountain remains clear. The orientation of the terrain to the wind is crucial.
Evaporation and precipitation fog
Evaporation fog, sometimes called steam fog or sea smoke, develops when cold air passes over relatively warm water. Added vapour quickly condenses into low wisps; it is not smoke. In precipitation fog, some rain evaporates into colder air beneath a cloud, raises its humidity and helps saturate the near-surface layer.
Mechanisms frequently overlap. The names identify the dominant process rather than rigid, mutually exclusive boxes.
Fog, mist and haze are different
Fog and mist mainly consist of water droplets, but observers distinguish them by visibility. The Met Office uses visibility below 1,000 metres for fog and 1,000 metres or more for mist. Operational thresholds can differ by service and industry, so use the definition supplied with the observation.
Haze is mainly caused by dry particles such as dust, salt or pollution. It may reduce visibility under different humidity and colour conditions and should not automatically be called fog.
Why visibility can change so quickly
- Banks and edges: moisture, wind and temperature vary over short distances.
- Terrain: a bend or descent can take you into the layer.
- Wind: it moves and mixes fog, changing its thickness.
- Light: droplets scatter light; full beam can create a bright veil.
- Sampling: visibility at one sensor may not represent an entire road, runway or valley.
How to read a fog forecast
Consider temperature, dew point, wind, cloud, recent rain, soil moisture and terrain together. A clear, light-wind night with moist air favours radiation fog; moist flow over a cold sea points towards advection. No simple rule guarantees formation or density.
Local forecasts and warnings are updated with observations. Drivers should also use our dedicated guide to driving safely in fog: speed and following distance must match what is actually visible, not the road’s usual pace.
Key points
- Fog is a surface-level suspension of droplets or crystals that reduces visibility.
- Cooling, added moisture or air rising can create it.
- Radiation, valley, advection, upslope, evaporation and precipitation fog describe different mechanisms that may overlap.
- Visibility can vary abruptly; one weather value cannot predict fog by itself.