Fog forms when air near the ground cools enough that its water vapor condenses into tiny suspended droplets, creating a cloud at the surface. The trigger is almost always the same: something brings the air temperature down to the point where it can no longer hold all its moisture, and the excess water vapor latches onto microscopic particles floating in the atmosphere. What varies, and what makes fog so interesting, is how that cooling happens. Warm ground radiating heat away on a clear night, moist air sliding over a cold ocean current, wind pushing air up a mountainside: each produces fog through a distinct chain of events, and understanding which one is at work explains why fog behaves so differently from place to place.
Radiation Fog and Clear-Night Cooling
The fog you wake up to on a calm autumn morning is almost always radiation fog. After sunset, the ground loses heat by radiating it into space, and on clear, still nights that heat escapes quickly. The thin layer of air sitting directly on the cooling ground chills along with it. Once that air reaches saturation, water vapor begins condensing into droplets just above the surface. The process involves a delicate balance between the ground’s heat loss, a small trickle of warmth rising from the soil below, and gentle turbulence mixing moisture and coolness through the lowest few meters of air. Too much wind stirs in warmer air from above and prevents the fog from forming; too little mixing keeps the moist layer paper-thin, producing only dew on the grass.
Radiation fog tends to be thickest in valleys and low-lying areas, where cold air pools because it is denser than the warmer air above. River valleys are classic fog traps: the water provides a steady supply of moisture, and the terrain funnels cool air downhill after dark. This type of fog usually burns off within a few hours of sunrise. As sunlight warms the ground, the air temperature climbs back above the dew point and the droplets evaporate from the bottom up. On overcast mornings, though, the fog can linger all day, because clouds block the solar heating that would normally dissipate it.
Advection Fog and Warm Air Over Cold Water
Advection fog forms when warm, moist air moves horizontally over a cooler surface. The classic example is coastal fog along the California coast or the coast of Newfoundland: warm air from over the open ocean drifts inland or across a cold ocean current, and the bottom of the air mass chills rapidly. This creates a stable fog layer that can be remarkably persistent, lasting for days in some locations.
Research on sea fog along the Korean Peninsula illustrates the mechanism well. Water vapor carried by prevailing winds from the warm open sea encounters cooler waters near the coast, and the lower layer of the air mass cools enough for condensation to begin. Crucially, the fog in these cases is not generated locally at the coast but develops as moisture transported over long distances meets the cooling influence of cold upwelling water near shore.1PLOS ONE. Numerical study on advective fog formation and its characteristic associated with cold water upwelling Advection fog is the reason San Francisco’s Golden Gate Bridge disappears under a white blanket nearly every summer: the Central Valley heats up, drawing in marine air that crosses the cold California Current and condenses before it reaches the city.
Because advection fog is driven by large-scale air movement rather than local nighttime cooling, it does not follow the same daily cycle as radiation fog. It can form at any time of day or night, and it often requires a change in wind direction or a shift in the position of warm and cold water masses to clear.
Upslope and Mountain Fog
When wind pushes air up the side of a hill or mountain, the air expands as atmospheric pressure drops with altitude. That expansion cools the air. If the air is moist enough, the cooling brings it to saturation, and fog forms on the slopes. Anyone who has hiked into a cloud bank partway up a mountain has walked into upslope fog.
Observations at subtropical mountain forests confirm this process: upslope winds cool as they rise, relative humidity climbs, and orographic clouds develop that register as fog at ground-level stations on the slopes.2Journal of Hydrology. Up and down: Bidirectional fluxes of fog droplets at two subtropical mountain forest sites Numerical simulations of fog events at Beppu Bay in Japan verified that moist air lifted by upslope winds and cooled as it expanded was sufficient to produce the observed fog.3Meteorological Applications. Numerical simulations of upslope fog observed at Beppu Bay in Oita Prefecture, Japan Upslope fog is common in the Appalachian Mountains, parts of the Pacific Northwest, and tropical cloud forests, where persistent trade winds push moist air against elevated terrain.
Steam Fog and Cold Air Over Warm Water
Steam fog is the mirror image of advection fog. Instead of warm air flowing over cold water, cold air flows over water that is significantly warmer. You have probably seen this on a small scale: wisps of “steam” rising from a heated swimming pool on a cool morning, or from a lake on the first cold night of autumn. What is actually happening is evaporation from the warm water surface adding moisture to the cold air just above it. That cold air cannot hold the extra moisture, so it condenses almost immediately into visible droplets.
At larger scales, steam fog (also called sea smoke) forms when very cold air masses sweep across open water. Studies have documented it occurring when the air temperature is anywhere from about 5 to 40 °C below the water temperature, in winds ranging from calm to gale force. The resulting fog can extend from just above the surface up to around 1,500 meters in extreme cases, and it often has a distinctive columnar or banded appearance, with pillars of mist rising from the water surface.4Quarterly Journal of the Royal Meteorological Society. Sea smoke and steam fog The underlying physics comes down to mixing: two parcels of air that are both unsaturated but at very different temperatures can, when blended together, produce a mixture that is supersaturated, forcing droplets to form.
Steam fog tends to be short-lived compared to advection fog, because the warming effect of the water surface below eventually raises the air temperature enough to evaporate the droplets. In Arctic regions, however, where the temperature contrast between open water and the air mass above can be extreme, steam fog can be both dense and persistent.5Atmospheric Research. Comparison of advection and steam fogs: From direct observation over the sea
Why Tiny Particles Matter
Water vapor does not condense into droplets spontaneously in clean air. It needs something to condense onto: a microscopic particle called a condensation nucleus. Dust, sea salt, soot, pollen, and sulfate particles from pollution all serve this role. In air with no particles at all, you would need to cool it far below the dew point before droplets would form. In the real atmosphere, there are always plenty of particles, so condensation begins right around the dew point.
The number and size of these particles influence what the fog looks like and how it behaves. Experiments during field campaigns have found that the fraction of aerosol particles that activate into fog droplets depends far more on particle size than on chemical composition. Larger particles activate more readily regardless of what they are made of.6Atmospheric Chemistry and Physics. Experimental study of the aerosol impact on fog microphysics
But there is a counterintuitive twist when the air is heavily polluted. Up to a point, adding more particles means more fog droplets form for a given amount of cooling. Beyond a critical particle concentration, however, the available water vapor gets spread so thinly across so many nuclei that each individual droplet stays extremely small, and the total number of activated droplets actually decreases.7Atmospheric Chemistry and Physics Discussions. Influence of aerosols on the formation and development of radiation fog Highly polluted air can also produce a thin, persistent haze that technically is not fog but looks and feels similar, blurring the boundary between the two.
Freezing Fog and Supercooled Droplets
Fog does not automatically freeze the moment air temperatures drop below zero. Water droplets can remain liquid well below 0 °C in a state called supercooling, and freezing fog is simply fog made of these supercooled liquid droplets. When the droplets contact a surface, however, they freeze on impact, coating everything in a thin shell of ice called rime. This makes freezing fog a serious hazard for roads, bridges, power lines, and aircraft.
Just how cold fog can stay liquid is surprising. Observations over the central Greenland Ice Sheet documented supercooled liquid fog forming at air temperatures between about −35 and −38 °C. In that case, the fog likely existed as a mixed layer of liquid droplets near the top and frozen ice crystals settling below, where skin temperatures plunged to around −50 °C, too cold for liquid water to persist.8Atmospheric Chemistry and Physics. Supercooled liquid fogs over the central Greenland Ice Sheet Ice fog, by contrast, is composed entirely of ice crystals rather than liquid droplets. It forms at extremely low temperatures, typically below about −30 °C, and is common in Arctic and subarctic cities where vehicle exhaust and power-plant steam provide moisture that freezes almost instantly in the frigid air.
When Fog Feeds Ecosystems
Fog is not just an inconvenience for drivers. In some ecosystems, it is a critical water source. The coastal redwood forests of California offer the most studied example. During the dry summer months, when rain essentially stops, fog rolling in from the Pacific condenses on the enormous surface area of redwood needles and branches, dripping to the forest floor in what ecologists call fog drip.
Measurements in these forests found that fog drip accounted for roughly a third of the total annual water input, on average, when redwood trees were present. Where trees had been removed, fog contributed only about 17 percent, showing that the trees themselves dramatically amplify how much fog water reaches the ground by intercepting droplets on their vast canopy. During summer, when rain was absent, about two-thirds of the water inside understory plants came from fog. Even the redwoods themselves drew 13 to 45 percent of their annual water use from fog, and that dependence increased in years when rainfall was unusually low.9Oecologia. Fog in the California redwood forest: ecosystem inputs and use by plants Without regular fog, these forests would face severe drought stress every summer.
The same principle applies in fog-dependent deserts. Parts of the Atacama Desert in Chile and the Namib Desert in southern Africa support plant and animal life that depends almost entirely on moisture from fog. Some organisms have evolved remarkable adaptations for capturing it: certain beetles in the Namib tilt their bodies into the wind, letting fog droplets collect on bumpy surfaces on their backs and trickle down to their mouths.
Pollution, Fog, and Haze
The interplay between air pollution and fog has a long and grim history. London’s infamous “pea-soup” fogs in the 19th and early 20th centuries were natural fog made catastrophically worse by sulfur dioxide and soot from coal burning. The same chemistry is now playing out in parts of China, where severe haze events share a mechanism with those historic London fogs.
Research comparing the two situations has shown that sulfur dioxide dissolved in fog droplets or wet aerosol particles reacts with nitrogen dioxide to produce sulfate, a reaction that proceeds efficiently only under high humidity with enough ammonia to neutralize the acidity. Under polluted conditions, this process generates sulfate rapidly and also promotes the formation of nitrate and organic matter on particles, which in turn thickens the haze layer.10PubMed Central. Persistent sulfate formation from London Fog to Chinese haze The result is a feedback loop: pollution seeds particle growth, which scatters more light and reduces visibility, and the wet, particle-rich atmosphere keeps the chemistry running.
In the other direction, cleaning up pollution can reduce fog. An analysis of multidecadal visibility data across Europe found that low-visibility events, including fog, mist, and haze, declined significantly over the past 30 years. The decline tracked closely with reductions in sulfur dioxide emissions from power plants and industry. The researchers estimated that the improvement in visibility could have contributed about 10 to 20 percent of Europe’s recent daytime warming, and roughly half of eastern Europe’s warming, by letting more sunlight reach the ground.11Nature Geoscience. Decline of fog, mist and haze in Europe over the past 30 years Cleaner air means fewer particles for water vapor to condense onto, fewer fog events, and more sunshine, with real consequences for regional climate.
How Climate Change Is Shifting Fog Patterns
Whether climate change will bring more or less fog depends on where you are. Fog is sensitive to temperature differences between surfaces and the air above them, and as the planet warms, those differences are changing in different directions in different regions. An analysis of global fog-day data over the past four decades found that the driving factors of fog, primarily temperature and moisture, have shifted regionally in ways that explain local trends in fog frequency.12Journal of Geophysical Research: Atmospheres. Validity of Global Fog‐Day Trends Indicated by the Global Surface Summary of the Day (GSOD) Data Set
For marine fog specifically, climate modeling suggests that changes in large-scale atmospheric circulation patterns will reorganize where warm air gets pushed over cold water and vice versa, shifting the geographic distribution of sea fog. One consistent finding is that in a warmer climate, the liquid water content inside marine fog increases for a given set of conditions, meaning individual fog events could become thicker and harder to see through even if their overall frequency changes.13Atmospheric Science Letters. Changes in marine fog in a warmer climate Coastal communities and shipping routes that currently deal with fog may see shifts in when and how often it occurs, while areas that rarely experience fog could begin encountering it as ocean temperature patterns evolve.
Fog Harvesting and Biomimetic Engineering
In regions where fog is abundant but rain is scarce, people have learned to harvest water directly from fog. The basic technology is a large mesh screen stretched vertically in the path of fog-bearing winds. Fog droplets impact the mesh fibers, coalesce, and drip into a collection trough. The approach has been deployed in Chile, Morocco, and parts of sub-Saharan Africa to supply drinking water to remote communities.
Engineering the mesh correctly turns out to matter a lot. Testing of stainless steel rectangular meshes with various openness levels showed that the best collection performance came from meshes with a shade coefficient, the fraction of the mesh area blocked by fibers, in the range of 50 to 60 percent.14Water Resources Management. Evaluating Mesh Geometry and Shade Coefficient for Fog Harvesting Collectors Too open and the droplets pass through; too dense and the wind deflects around the screen instead of pushing fog through it.
Researchers are now looking to biology for ways to improve on simple mesh screens. The Namib Desert beetle’s back has a pattern of bumps and valleys with different wetting properties: the bumps attract water and the valleys repel it, channeling droplets efficiently toward the beetle’s mouth. Engineers have mimicked this approach by 3D-printing surfaces that combine water-attracting and water-repelling zones. One study fabricated hybrid surfaces using two different plastics and plasma treatment, achieving fog collection rates of about 366 grams per square meter per hour by exploiting the pressure gradient that forms between hydrophobic and hydrophilic regions.15PubMed Central. Desert Beetle-Inspired Hybrid Wettability Surfaces for Fog Collection Fabricated by 3D Printing and Atmospheric Pressure Plasma Treatment Other designs draw inspiration from spider silk, whose spindle-knot structures drive tiny droplets toward collection points, and from the vein patterns of leaves, which channel water efficiently across a flat surface.16PubMed. Biomimetic Fog Collector with Hybrid and Gradient Wettabilities
Fog Dispersal at Airports
Fog grounds flights, delays highways, and causes chain-reaction pileups. Unsurprisingly, there has been a long history of trying to make it go away on demand. During World War II, the British military burned enormous quantities of fuel along runway edges to heat the air and evaporate fog, a system called FIDO (Fog Investigation and Dispersal Operation). Modern approaches have tried to be less extravagant. One line of research has explored using electrically charged particles injected into fog: the charged droplets are attracted to the ground or to collection surfaces, effectively pulling moisture out of the air. Thermal systems, which heat the air to push it above the dew point, remain the most proven technology for airport fog dispersal, but they consume enormous amounts of energy and are rarely cost-effective for routine commercial operations.
The fundamental challenge is scale. A fog bank may contain only a small amount of liquid water per cubic meter, but it extends over a huge volume of air. Clearing a runway-sized corridor means treating millions of cubic meters of air continuously, because undisturbed fog immediately flows back in from the sides. Most airports have concluded that improved instrument-landing systems and automated guidance are more practical investments than fog-removal technology, though research into targeted dispersal continues.
Why Some Places Are Foggier Than Others
Geography determines fog frequency more than anything else. The foggiest places in the world tend to sit where the ingredients for one of the major fog types are reliably present. Grand Banks off Newfoundland, where the warm Gulf Stream meets the cold Labrador Current, produces advection fog on more than 200 days per year. The coast of northern Chile and southern Peru is fog-shrouded for long stretches because the cold Humboldt Current chills the marine air from below. Inland, the Central Valley of California traps cool air in winter under a warm-air lid, creating so-called tule fog that can reduce visibility to near zero for days.
Urban areas tend to experience less radiation fog than rural areas, for a reason that surprises many people: cities are warmer. The urban heat island effect keeps nighttime temperatures a few degrees higher than surrounding countryside, often enough to prevent air from reaching the dew point. A century ago, when cities burned more coal and had dirtier air loaded with condensation nuclei, urban fog was more common. As the European visibility data show, cleaner air has been one of the strongest anti-fog forces in developed countries over the past several decades, fundamentally changing the character of city weather in places that once defined themselves by their fog.