Fog is a cloud that forms at ground level when tiny water droplets, or sometimes ice crystals, become suspended in the air near Earth’s surface. It develops when air cools enough for water vapor to condense, or when moisture is added to the air until it becomes saturated. By scattering light, these tiny droplets reduce visibility and can make familiar landscapes, roads, and buildings difficult to see.
Fog is a common weather phenomenon that can occur in nearly every part of the United States, from coastal regions and mountain valleys to agricultural plains and urban areas. Its formation depends on the interaction of temperature, humidity, wind, and the characteristics of the surrounding land or water. Different combinations of these factors produce several distinct types of fog, each with its own formation process and typical weather conditions.
What causes fog to form?
Air contains water vapor, the invisible gaseous form of water. The amount of water vapor air can hold before becoming saturated depends strongly on its temperature. Warm air can generally contain more water vapor than cold air before reaching saturation.
When air cools, its capacity to hold water vapor decreases. If it cools enough, it reaches a temperature called the dew point. At this point, the air becomes saturated, and water vapor can begin condensing into liquid droplets, provided suitable surfaces or particles are available for condensation.
The temperature at which condensation begins depends on the air’s moisture content. Air that is already humid needs only a small amount of cooling to reach its dew point, while drier air must cool more substantially.
Water vapor does not usually turn directly into a visible cloud simply because the air reaches saturation. It generally condenses onto microscopic particles suspended in the atmosphere, known as condensation nuclei. These particles may include sea salt, dust, smoke, and other aerosols. The resulting droplets are small enough to remain suspended in the air for a time.
When condensation produces a large concentration of droplets close to the ground, fog forms. The droplets scatter light traveling through the air, reducing the distance over which objects can be seen clearly.
Fog can also form when evaporation adds enough moisture to relatively cool air to bring it to saturation. In that case, the air does not necessarily need to cool first; adding water vapor can produce the same result.
How fog differs from clouds and mist
Fog and clouds consist of the same basic ingredients: tiny liquid water droplets or, under suitable conditions, ice crystals suspended in the atmosphere. The main distinction is their location. Fog is a cloud that reaches the ground or forms close enough to the surface to obscure it.
Clouds typically develop above the ground, where rising air cools and condenses. Fog develops at or near the surface through processes such as nighttime cooling, the movement of moist air over a colder surface, or the mixing of air masses with different temperatures.
Mist is similar to fog but generally allows greater visibility. In common meteorological usage, fog reduces horizontal visibility to less than 1 kilometer, or about five-eighths of a mile. Mist usually leaves visibility at or above that threshold, although everyday descriptions of the two terms are not always precise.
Neither fog nor mist requires rain. Their droplets are typically much smaller than raindrops, and they often remain suspended rather than falling to the ground. However, fog droplets can settle onto surfaces, collect on vegetation, or contribute moisture to the ground when conditions favor deposition.
Radiation fog: Fog that forms as the ground cools
Radiation fog is one of the most familiar types of fog, especially in inland areas during fall and winter. It forms when the ground loses heat, usually during the night, and cools the air directly above it.
After sunset, Earth’s surface emits infrared radiation and gradually loses energy. Under clear skies, relatively little of this energy is returned toward the surface by clouds, allowing the ground to cool efficiently. The air touching the cooling ground also loses heat. If that air reaches its dew point, water vapor condenses and fog develops.
Light winds often favor radiation fog by mixing the cooled air with slightly warmer air above it without disrupting the near-surface cooling process too strongly. If winds are completely calm, however, fog may remain confined to an extremely shallow layer. Moderate winds can help spread the cooling through a deeper layer of air, but stronger winds often mix in warmer, drier air and prevent fog from forming or maintaining itself.
Radiation fog commonly develops in low-lying areas, fields, and valleys where cool, dense air accumulates. A valley may become foggy while nearby hills remain clear because the valley floor experiences stronger cooling or collects air that has cooled and drained downslope.
This type of fog often forms late at night or near sunrise, when surface temperatures are lowest. It may dissipate after sunrise as sunlight warms the ground and the air above it. The warming allows the droplets to evaporate back into water vapor.
Radiation fog does not always disappear quickly after sunrise. Thick fog, weak sunlight, persistent moisture, or a temperature inversion can delay its breakup.
Advection fog: When moist air moves over a cold surface
Advection fog forms when relatively warm, moist air moves horizontally over a colder surface. The air cools from below until it reaches its dew point, allowing water vapor to condense.
Unlike radiation fog, which depends primarily on the nighttime cooling of the ground, advection fog depends on the movement of air from one region to another. It can develop during the day or night and may persist even when the sky is cloudy.
Coastal areas are particularly favorable locations for advection fog. When moist air moves from a relatively warm ocean region over colder coastal waters, it can cool enough to produce widespread fog. Along parts of the Pacific coast, this process contributes to persistent fog during certain seasons.
Advection fog can also form over land. For example, moist air moving inland from a body of water may encounter colder ground and cool to its dew point. Snow-covered surfaces can promote fog formation when relatively mild, humid air moves over them.
Wind is an important factor in this process. Unlike radiation fog, which often develops under light winds, advection fog generally requires enough horizontal movement to transport moist air over a colder surface. Moderate winds may help the fog spread over a broad area, although strong winds can mix the air enough to alter or dissipate the fog.
Because the cooling surface may remain cold for hours or days, advection fog can persist longer than typical radiation fog. Its duration depends on the continued supply of moist air, the temperature of the underlying surface, and changes in wind direction and speed.
Upslope fog: When air rises along elevated terrain
Upslope fog develops when moist air moves up a hillside, mountain slope, or elevated plateau. As the air rises, atmospheric pressure decreases, allowing the air to expand. Expansion causes the air to cool, even without direct heat loss to the surrounding ground.
This cooling is called adiabatic cooling. If rising air cools to its dew point, water vapor condenses into droplets and fog forms along the terrain.
Upslope fog is common where moist winds encounter elevated land. In the United States, it can develop along the eastern slopes of the Rocky Mountains and in other areas where moist air is forced upward by terrain.
The process is closely related to cloud formation. When rising air becomes saturated, the resulting cloud may cling to a mountain slope or cover a ridge. Whether it is described as fog or a low cloud depends largely on its relationship to the ground and its effect on visibility.
Upslope fog can cover extensive areas and last as long as moist air continues flowing toward and up the terrain. Unlike radiation fog, it does not depend on clear nighttime skies or strong surface cooling. It may form during the day and persist under overcast conditions.
Evaporation fog: When water adds moisture to cold air
Evaporation fog forms when water vapor enters relatively cold air and raises its moisture content to saturation. It is often associated with water that is warmer than the air above it.
When water evaporates, molecules escape from the liquid surface and enter the atmosphere as vapor. If the air above the water is cold, the added moisture can bring it to saturation. Some of the vapor then condenses into tiny droplets, producing a visible cloud.
A common example is steam fog, which appears as wisps of vapor rising from a lake, river, or pond on a cold morning. The water may be considerably warmer than the air, allowing evaporation to supply moisture rapidly to the cold atmosphere.
The visible plume is not primarily water vapor itself. Water vapor is invisible; the white appearance comes from tiny droplets formed when the moist air mixes with the colder surrounding air.
Steam fog often develops over lakes and rivers in fall and early winter, when water retains heat after the air has cooled. Similar effects can occur above heated pools or other warm water surfaces exposed to cold air.
Evaporation fog can also form when precipitation falls through dry air. Some raindrops evaporate, adding moisture to the air and cooling it. If the air becomes saturated, a layer of fog may develop near the ground. This process is sometimes called precipitation fog, particularly when it occurs as rain falls into cooler air near the surface.
Evaporation fog differs from radiation and advection fog because moisture addition plays a central role in its formation. Cooling may still occur, but the added water vapor is an essential part of the process.
Frontal fog: When weather fronts bring moisture and cooling
Frontal fog is associated with weather fronts, which are boundaries between air masses with different temperatures and other properties. It often develops when precipitation from relatively warm air falls into colder air near the surface.
As rain falls into the cooler air, some of it evaporates. This evaporation adds water vapor to the air and absorbs heat, cooling the surrounding atmosphere. If the air becomes saturated, fog can form.
Frontal fog may occur ahead of a warm front, where warm air and precipitation move over a colder surface air mass. It can also develop in other frontal settings when precipitation, moisture, and temperature conditions combine to bring the near-surface air to saturation.
The fog may cover a broad region because fronts can extend across large distances. It can also be persistent when precipitation continues and the supply of moisture remains strong.
Not every front produces fog. Its formation depends on factors such as the temperature and humidity of the air masses, the amount of precipitation, and how much evaporation occurs before the rain reaches the ground.
Freezing fog: Fog made of supercooled water droplets
Freezing fog occurs when fog droplets are liquid even though their temperatures are below 32°F (0°C). Such droplets are called supercooled water droplets because they remain liquid below the normal freezing point.
Liquid water does not always freeze as soon as its temperature falls below 32°F. Freezing often requires an appropriate ice-forming nucleus or another disturbance that initiates the transition. Small atmospheric droplets can therefore remain liquid at subfreezing temperatures.
When these droplets come into contact with roads, trees, bridges, power lines, or other surfaces that are below freezing, they may freeze on contact. The resulting deposit is known as rime ice.
Freezing fog can create hazardous travel conditions because it may coat roads and sidewalks with a thin layer of ice that is difficult to see. It can also accumulate on exposed structures and vegetation, increasing the risk of damage when icing becomes substantial.
Freezing fog is different from ice fog, which consists of tiny ice crystals suspended in the air. Ice fog generally occurs in very cold conditions, particularly in polar and subarctic environments. It can develop when water vapor from sources such as combustion, warm water, or other moisture emissions enters extremely cold air and forms ice crystals.
Valley fog: When cold air collects in low areas
Valley fog is a common feature of mountain valleys and other low-lying landscapes. It develops when cool, moist air accumulates near the ground and reaches saturation.
At night, slopes and elevated ground lose heat. Air cooled near the slopes becomes denser and flows downhill under gravity, collecting in valleys and basins. This process is called cold-air drainage.
If the collected air contains enough moisture, its temperature may fall to the dew point, producing fog. Clear skies and light winds often support this process, making valley fog closely related to radiation fog.
Valleys can trap cold air beneath warmer air above, creating a temperature inversion. Under a temperature inversion, air temperature increases with height through a layer of the atmosphere rather than decreasing as it usually does. The inversion can limit vertical mixing and help fog persist near the valley floor.
Valley fog may fill a low area while higher terrain remains above the fog layer in sunshine. From a mountain or elevated viewpoint, the fog can resemble a lake of clouds, with ridges and peaks rising above its surface.
Although valley fog often develops overnight, it can remain well into the day when sunlight is weak or the fog layer is deep. It may dissipate as the ground warms, the inversion weakens, or winds mix the air more thoroughly.
Valley fog describes a setting and formation pattern rather than an entirely separate physical mechanism. Many examples combine radiation cooling, cold-air drainage, and local moisture conditions.
How fog droplets form and remain suspended
Fog becomes visible when water vapor condenses into a large number of microscopic droplets. These droplets are generally much smaller than ordinary raindrops, allowing them to remain suspended in the air for extended periods.
Condensation usually begins on tiny airborne particles. Water molecules collect on these particles, gradually forming droplets. The availability and properties of these particles can influence how many droplets form and how large they become.
The droplets scatter light in different directions. When enough light is scattered away from its original path, objects appear faint, hazy, or invisible at a distance. The reduction in visibility depends on factors including droplet concentration, droplet size, and the distance light travels through the fog.
Fog droplets do not simply fall to the ground as soon as they form. Their small size means air resistance strongly affects their movement. Although gravity causes them to settle slowly, air currents and turbulence can keep them suspended.
Over time, droplets may collide and merge, a process known as coalescence. Under suitable conditions, this can increase droplet size and contribute to the formation of drizzle. However, fog does not always produce precipitation, and most fog droplets remain small enough to stay airborne for considerable periods.
Fog can also deposit moisture on surfaces. As droplets encounter leaves, grass, buildings, or other objects, they may collect there. In some environments, this deposition provides an additional source of water for vegetation and soils, especially when rainfall is limited.
What determines how thick fog becomes?
Fog thickness depends on the amount of moisture in the air, the strength and depth of cooling, and how air moves near the surface. A thin layer of fog may form when only a shallow layer of air reaches saturation. More extensive cooling or moisture supply can produce a deeper layer.
The difference between air temperature and dew point is an important clue. When the two are close, the air is near saturation and may form fog with only slight additional cooling or moisture. However, a small temperature–dew point difference does not guarantee fog because suitable atmospheric conditions and condensation processes must also be present.
Wind can either help or hinder fog formation. Gentle mixing may spread cooling and moisture through a deeper layer, allowing fog to develop more extensively. Stronger mixing may bring in warmer or drier air and cause droplets to evaporate. In other situations, persistent winds transport moist air over a cold surface and sustain advection fog.
Cloud cover influences the surface energy balance. At night, clouds absorb and emit infrared radiation, reducing the rate at which the ground loses heat compared with clear-sky conditions. This often makes radiation fog less likely to form or less dense, although fog can develop beneath clouds when other conditions are favorable.
Local geography matters as well. Valleys collect cool air, coastal areas experience temperature contrasts between land and water, and mountain slopes force air to rise. These features create conditions that favor particular fog types.
Air pollution and airborne particles can influence fog droplets by providing condensation nuclei. However, fog formation cannot be explained by pollution alone. The air must also have suitable temperature and moisture conditions.
Why fog often forms overnight and disappears after sunrise
Nighttime is favorable for many types of fog because the ground cools after sunset. As the surface loses heat, the air above it may cool toward its dew point. If sufficient moisture is present, fog can form during the late evening or early morning.
The lowest temperatures often occur near sunrise, which is why radiation fog commonly reaches its greatest extent around that time. Once the Sun rises, solar energy begins warming the ground and the air near it.
As the air warms, its saturation vapor pressure increases. If the amount of water vapor remains roughly unchanged, the relative humidity decreases, and the droplets can evaporate. Fog then thins and may disappear.
Sunlight does not always clear fog immediately. A dense layer can block some incoming solar radiation, slowing the warming process. Moist ground, persistent temperature inversions, and continued inflow of humid air can also delay dissipation.
Some fog forms through processes that do not depend on nighttime cooling. Advection fog may persist throughout the day if moist air continues flowing over a cold surface. Upslope fog can remain as long as air is forced upward over terrain, and frontal fog may last while precipitation and suitable moisture conditions continue.
Fog therefore dissipates when the conditions sustaining its droplets change, not simply because a particular time of day arrives.
How fog affects transportation and daily life
Fog is a significant weather hazard because it reduces visibility without necessarily producing rain, strong winds, or other obvious signs of severe weather. Drivers may encounter a road that appears clear until they enter a dense patch of fog, leaving little time to react to stopped vehicles, curves, or obstacles.
The danger is particularly pronounced when fog develops unevenly. Visibility can change rapidly across short distances, and drivers may misjudge their speed or following distance when visual reference points disappear.
When visibility is poor, drivers should slow down, increase following distance, and use low-beam headlights. High beams can make visibility worse because light reflects and scatters off the droplets toward the driver. Drivers should avoid sudden maneuvers and use road markings as a guide when they remain visible.
Fog also affects aviation and maritime transportation. Airports may need to delay or restrict operations when visibility or cloud ceilings fall below the limits required for particular procedures. Ships and recreational boaters may have difficulty identifying shorelines, navigation markers, and other vessels.
In agriculture and natural ecosystems, fog can provide moisture through droplet deposition. Coastal redwood forests, for example, can benefit from fog that deposits water on foliage and soil. The importance of fog as a water source varies by ecosystem and depends on the frequency, duration, and characteristics of fog events.
Fog may also affect local temperatures. A dense fog layer reduces the amount of sunlight reaching the ground during the day, limiting surface warming. At night, fog can absorb and emit infrared radiation, altering how quickly the ground cools. Its net effect depends on the surrounding weather conditions and the characteristics of the fog layer.
How fog forecasts are made
Forecasting fog requires meteorologists to assess whether the air near the surface is likely to become saturated and whether the resulting droplets will persist.
Temperature and dew point provide important clues. When the two approach one another, the air is nearing saturation. Forecasters also examine wind speed and direction, cloud cover, recent rainfall, soil moisture, nearby bodies of water, and the movement of weather systems.
Local terrain is another important consideration. A forecast may indicate a high likelihood of fog in valleys while nearby hills remain clear. Coastal fog forecasts must account for sea surface temperatures, wind direction, and the movement of marine air over land.
Weather models estimate atmospheric temperature, moisture, and wind, but predicting fog remains challenging because fog often forms in shallow layers close to the ground. Small errors in surface temperature, humidity, or wind can determine whether fog develops, how dense it becomes, and how long it lasts.
Observations from weather stations, visibility sensors, satellites, and other instruments help forecasters identify existing fog and evaluate atmospheric conditions. Satellites can reveal broad patterns of fog and low clouds, although distinguishing fog from low cloud layers can be difficult, particularly at night or over complex terrain.
Because fog can vary substantially over short distances, a regional forecast cannot always describe conditions at an individual location precisely. Local observations and transportation advisories remain important when visibility is changing rapidly.
Why understanding fog matters
Fog forms when atmospheric moisture, temperature, and air movement combine to produce suspended water droplets or ice crystals near the ground. The underlying physics is straightforward, but the specific processes that bring air to saturation vary from one environment to another.
Radiation fog develops as the ground cools, advection fog forms when moist air moves over a colder surface, upslope fog appears as air rises over terrain, and evaporation fog develops when added moisture brings cold air to saturation. Freezing fog presents an additional hazard because its supercooled droplets can freeze on contact with exposed surfaces.
Recognizing these differences helps explain why fog forms in some places and not others, why it may disappear quickly or linger for hours, and why its effects range from reduced visibility to meaningful moisture inputs for ecosystems.
Although fog can make the landscape appear unfamiliar, it is a natural expression of the same atmospheric processes that produce clouds. The essential ingredients are moisture, cooling or moisture addition sufficient to reach saturation, and conditions that allow tiny droplets to form and remain suspended near Earth’s surface.