1 Definition and characteristics
1.1 Basic concept
Fog is a suspension of tiny water droplets, or sometimes ice crystals, in air near the ground. It is essentially a cloud at the Earth’s surface and is usually described by its effect on visibility rather than by its exact composition. When enough suspended moisture is present, distant objects lose clarity and can disappear from view.
1.2 Visibility thresholds
In meteorology, fog is commonly defined when horizontal visibility drops below about 1 kilometer. More intense varieties may reduce sight to only a few meters, while lighter forms may leave nearby shapes visible but indistinct. The practical threshold is important because it affects travel, aviation, and marine operations.
1.3 Fog as a surface cloud
Fog forms under the same basic principles as clouds. The difference is mainly location: a cloud becomes fog when its base touches the ground. This makes fog a local phenomenon strongly shaped by surface conditions such as terrain, vegetation, water, and built environments.
1.4 Distinction from haze and mist
Fog is denser than mist and usually causes a much greater reduction in visibility. Mist often appears as a thinner suspension of droplets and allows farther viewing distances. Haze, by contrast, is commonly associated with dry particles such as dust, smoke, or pollution, though it may be mixed with moisture in some situations.
2 Formation
2.1 Saturation and condensation
Fog develops when air reaches saturation, meaning it can no longer hold all of its water vapor in gaseous form. At that point, vapor condenses onto tiny particles suspended in the air. This process produces microscopic droplets that remain aloft because they are so small and because air movement keeps them dispersed.
2.2 Cooling mechanisms
Fog often forms when air cools to its dew point, the temperature at which saturation occurs. Cooling can happen in several ways, depending on weather conditions and local geography. The most common mechanisms involve radiative loss of heat, movement of air over cooler surfaces, or air being lifted over higher ground.
2.2.1 Radiation cooling
Radiation fog forms when the ground loses heat during the night, especially under clear skies and light winds. The air in contact with the cooling surface also cools, and if it reaches saturation, droplets appear. This type is common in sheltered areas and often dissipates after sunrise as the surface warms.
2.2.2 Advection cooling
Advection fog develops when moist air moves horizontally over a colder surface. The air cools from below until it becomes saturated, often over land, water, or snow-covered terrain. It is frequently widespread and persistent because the air mass can continue moving while remaining near the saturation point.
2.2.3 Upslope cooling
Upslope fog occurs when moist air is forced upward along a slope or mountain side. As the air rises, it expands and cools, which encourages condensation. This type may linger on windward slopes and ridgelines, where topography helps maintain the lifting process.
2.3 Moisture sources
Fog requires sufficient water vapor, which may come from evaporation from oceans, lakes, rivers, wet soil, or vegetation. Areas near large bodies of water are especially prone to fog because the air often contains abundant moisture. In cold weather, melting snow and ice can also contribute moisture to the lower atmosphere.
2.4 Role of nuclei
Condensation typically begins on tiny airborne particles called condensation nuclei. These can include sea salt, dust, smoke, and other aerosols. Without such particles, water vapor would have more difficulty forming droplets, so their presence helps fog develop and influences droplet size and concentration.
3 Types of fog
3.1 Radiation fog
Radiation fog is a common night and early morning fog formed by surface cooling. It is most likely under clear skies, calm conditions, and high ground moisture. Valleys, fields, and inland lowlands often experience it, and it may vanish quickly once solar heating resumes.
3.2 Advection fog
Advection fog forms when moist air passes over a cooler surface and is cooled from below. It may occur along coasts, over cold ocean currents, or across snow-covered land. Unlike radiation fog, it can appear during both day and night and may cover large regions.
3.3 Upslope fog
Upslope fog appears as air is lifted along terrain and cooled to saturation. It is closely tied to topography and wind direction. This fog often clings to hillsides and mountains, creating a broad veil over elevated land.
3.4 Evaporation fog
Evaporation fog forms when relatively cold air passes over warmer water or moist ground and picks up water vapor. The added moisture causes the local air to become saturated, even if it was initially unsaturated. It is sometimes observed over lakes, rivers, and damp surfaces after rain.
3.5 Steam fog
Steam fog is a specific kind of evaporation fog seen when cold air moves over warm water. The water warms and humidifies the air just above the surface, producing a visible plume-like layer. It is often common in late autumn or winter over lakes, rivers, and unfrozen seas.
3.6 Freezing fog
Freezing fog consists of supercooled droplets that can freeze on contact with surfaces below 0 °C. It can coat roads, trees, wires, and aircraft with glaze or rime. Because it combines low visibility with icing, it is especially hazardous in cold conditions.
3.7 Valley fog
Valley fog collects in low-lying terrain where cool air drains downhill and becomes trapped. The cold air near the ground may remain sheltered by surrounding slopes, allowing moisture to accumulate. These pockets can persist longer than fog on exposed land, particularly during calm weather.
4 Physical properties
4.1 Droplet size and concentration
Fog droplets are extremely small, often only a few micrometers across. A dense fog contains a large number of droplets per unit volume, which greatly weakens visibility. The exact droplet concentration depends on temperature, humidity, turbulence, and the availability of nuclei.
4.2 Temperature and humidity conditions
Fog is favored by high relative humidity, cool temperatures, and stable air. When air is near saturation, only a slight drop in temperature is needed to produce condensation. Wind can either help form fog by moving moist air over a cooler surface or disperse it by mixing in drier air.
4.3 Optical effects
Fog modifies the way light travels through the atmosphere by scattering and diffusing it. This creates a bright, gray appearance and softens contrasts. It can also produce distinctive visual phenomena when sunlight, moonlight, or headlights interact with the droplets.
4.3.1 Light scattering
The small droplets in fog scatter light in many directions, which reduces clarity and makes distant objects fade into a uniform background. Headlights and streetlights may appear blurred or surrounded by halos. The scattering also explains why fog often seems more opaque in brighter illumination.
4.3.2 Fogbows
A fogbow is a pale optical arc produced when sunlight is scattered by tiny fog droplets. It resembles a rainbow but appears whiter and more diffuse because the droplets are very small. Fogbows are usually seen opposite the Sun when conditions align properly.
4.3.3 Halos and glories
Halos and glories are less common optical effects associated with interactions between light and atmospheric particles. In fog, some luminous rings or colored patterns may appear under special viewing conditions. These phenomena depend on droplet size, viewer position, and the angle of incoming light.
5 Geography and climate
5.1 Coastal fog
Coastal regions often experience fog because moist marine air meets cooler land or ocean currents. Cold upwelling waters can intensify this effect by lowering the temperature of the air above them. Such fog may be frequent, dense, and persistent along certain shorelines.
5.2 Mountain and valley environments
Mountainous terrain promotes fog through uplift, cooling, and trapped air in valleys. Windward slopes may be wrapped in low cloud or fog for long periods, while valleys can collect nighttime fog that lingers until daytime heating begins. Local relief strongly shapes how fog forms and moves.
5.3 Urban fog
Cities can influence fog through added heat, aerosol particles, and modified airflow. Buildings may alter wind patterns and trap moist air in sheltered areas, while pollution particles can provide additional condensation nuclei. In some places, the urban heat island effect may reduce fog, but in others it can interact with moisture to sustain low visibility.
5.4 Seasonal patterns
Fog frequency varies with season, climate, and geography. In many regions, it is most common during cooler months when nights are long and air temperatures are lower. In maritime climates, fog may occur year-round, especially where ocean temperatures, currents, and coastal winds favor saturation.
6 Impacts and hazards
6.1 Road transportation
Fog is a major hazard for road travel because it obscures lane markings, vehicles, signs, and obstacles. Drivers must reduce speed, increase following distance, and use low-beam headlights or fog lamps when appropriate. Sudden changes in visibility can make multi-vehicle collisions more likely.
6.2 Aviation
Aircraft operations are highly sensitive to fog because takeoff, landing, and ground movement depend on clear visual references. Dense fog can delay flights, reduce airport capacity, and require instrument-based procedures. Even brief fog events can disrupt schedules at major hubs.
6.3 Marine navigation
At sea, fog complicates navigation by hiding shorelines, buoys, other vessels, and hazards such as reefs or piers. Mariners rely more heavily on radar, instruments, charts, and sound signals when visibility is poor. Coastal harbors may be especially affected when fog settles over shipping lanes.
6.4 Agriculture and ecosystems
Fog can benefit some ecosystems by providing moisture to plants and soils, particularly in arid or coastal regions. Certain forests rely on frequent fog for water input. At the same time, prolonged fog can delay drying, increase disease pressure on crops, and reduce the effectiveness of some field operations.
6.5 Public safety
Low visibility from fog can interfere with emergency response, outdoor events, and pedestrian movement. It may also increase the risk of accidents near roads, railways, airports, and waterways. In freezing conditions, fog can create slippery surfaces through ice deposition.
7 Observation and forecasting
7.1 Weather instruments and visibility measurement
Fog is observed using standard meteorological instruments, visibility sensors, and human reports. Airport sites often measure how far a target or light can be seen through the air. Thermometers, hygrometers, and wind instruments help identify the conditions that support fog formation.
7.2 Satellite and radar limitations
Satellites can sometimes detect fog from cloud-top patterns or temperature contrasts, but they may struggle to distinguish fog near the surface from low cloud. Radar is useful for precipitation but does not directly measure fog droplets because they are too small. As a result, surface observations remain important.
7.3 Forecasting methods
Fog forecasting combines knowledge of humidity, temperature, wind, terrain, and surface conditions. Forecasters look for signs such as nighttime cooling, calm air, moist ground, and nearby weather patterns that favor saturation. Numerical models can help, but local features often make prediction difficult.
7.4 Fog warnings and advisories
When visibility is expected to fall to hazardous levels, weather services may issue fog advisories or warnings. These alerts help drivers, pilots, mariners, and planners prepare for reduced visibility. Timing is important because fog can appear or lift quickly, especially near sunrise or changing winds.
8 Related phenomena
8.1 Mist
Mist is a lighter form of suspended moisture with greater visibility than fog. It may appear similar at a glance, but objects remain clearer and farther away. In practical terms, the difference is mainly one of density and visibility range.
8.2 Smog
Smog is an atmospheric mixture of smoke, pollution, and sometimes moisture that reduces visibility and can affect air quality. Unlike ordinary fog, it includes substantial amounts of airborne contaminants. In some settings, fog and pollution may combine to create a murky urban atmosphere.
8.3 Low stratus clouds
Low stratus clouds are flat cloud layers near the ground that can resemble fog. When a stratus layer reaches the surface, it is effectively fog. The distinction depends on whether the cloud base is above or at ground level.
8.4 Dew and frost
Dew and frost are surface deposits formed by cooling near the ground, but they do not remain suspended in the air. Dew is liquid water condensed on surfaces, while frost is ice deposited or frozen from moisture. Both can accompany conditions favorable to fog, though they are separate phenomena.