Fog forms when air cools to the point where its moisture condenses into tiny suspended water droplets, typically around microscopic airborne particles like dust, soot, or sea salt. The World Meteorological Organization defines it as a suspension of very small water droplets in the air that reduces visibility at Earth’s surface, and those droplets range from a few tenths of a micron to a few tens of microns across. But the cooling can happen in several different ways, which is why fog shows up in such different settings: over cold ocean water, in river valleys after sunset, on mountain slopes, and even in arid deserts on the right night. The details of how the air loses heat, and what particles are floating in it, determine whether fog forms at all and how long it lasts.
The Two Ingredients Every Fog Needs
Strip away the complexity and fog requires just two things: air that has cooled enough to become saturated with moisture, and particles for that moisture to condense onto. The temperature at which air becomes fully saturated is called the dew point, and when the air temperature drops to meet it, water vapor begins condensing into liquid droplets. If that condensation happens at ground level rather than up in the atmosphere, the result is fog rather than a cloud. In fact, fog and low clouds are physically the same phenomenon; the distinction is purely about altitude.
The particles that fog droplets form around are called condensation nuclei, and they matter more than most people realize. Research in the Arctic has shown that highly hygroscopic (water-attracting) particles can trigger cloud and fog formation even when particle concentrations are extremely low, below ten particles per cubic centimeter, a threshold previously thought to be the minimum for any condensation to occur. In heavily polluted areas, the picture flips: a study of fog events in Paris found that local traffic and wood-burning emissions produced particles with low hygroscopicity, meaning the droplets that did form had to grow much larger before becoming fog droplets, with the critical wet diameter varying widely between fog events. The chemistry of whatever is floating in the air shapes what the fog looks like and how it behaves.
Radiation Fog and the Clear-Night Setup
The most common type of fog in many inland areas forms on clear, calm nights. After sunset, the ground radiates heat into the sky, and without clouds to reflect that heat back down, the surface cools rapidly. The thin layer of air just above the ground chills along with it. If that air is moist enough, it eventually reaches its dew point and fog begins to form. This is radiation fog, named for the radiative cooling that drives it.
The recipe is specific: you need clear skies so there is nothing to trap outgoing heat, light winds so the cooling stays concentrated near the surface rather than getting mixed through a deep layer, and enough moisture in the air to reach saturation without extreme cooling. High-pressure weather systems are the classic setup because they bring stable, settled conditions with little cloud cover. Modeling work on radiation fog events in China’s Yangtze River Delta confirmed that strong surface stability dominates before fog forms and that heat exchange between the ground and the air above is a key driver of the cooling process. River valleys, low-lying fields, and areas near bodies of water tend to be fog magnets on these nights because moisture collects in the low spots and cold air drains downhill.
Advection Fog and Cold Surfaces
Radiation fog needs the air to sit still and cool in place. Advection fog takes the opposite approach: warm, moist air moves horizontally over a surface cold enough to chill it below its dew point. The most dramatic examples happen over oceans. When warm, humid air masses flow over cold sea surfaces, the bottom of the air column cools rapidly and fog develops, sometimes blanketing hundreds of square kilometers.
This process creates some of the foggiest coastlines on Earth. The Yellow Sea between China and Korea is a textbook example. Seasonal fog there forms when southerly winds push warm, humid air northward over cold water, with the pattern triggered abruptly when the right wind pattern sets up on the west side of a high-pressure system. Similar dynamics play out along the coast of California, the Grand Banks off Newfoundland, and anywhere warm air streams meet cold ocean currents.
Coastal upwelling, where cold water from the deep ocean rises to the surface near shore, can intensify the effect. During a summer upwelling event along the Baltic Sea coast, water temperatures plunged to around 12°C, and when a warm air front moved across the region, thick advective fog formed in the coastal areas as the temperature contrast between the warm air and cold water drove rapid condensation. The more dramatic the temperature difference between the air and the surface beneath it, the more likely advection fog becomes.
Upslope Fog and Mountain Terrain
When moist air is pushed up a slope by wind, it expands and cools as it rises. If it cools enough to reach saturation, fog or low clouds form on the hillside. This is upslope fog, and it is responsible for the persistent cloud banks that cling to mountain ranges and coastal hills. Simulations of upslope fog events observed at Beppu Bay in Japan confirmed that the fog was caused by moist adiabatic cooling of air lifted by upslope winds. Unlike radiation fog, which needs calm air, upslope fog specifically requires wind to push air uphill.
Valley fog is a related but distinct phenomenon. Cold air pools in valleys overnight, and if the valley floor is moist, radiation fog fills the basin. Mountain passes and narrow river valleys are especially prone because the surrounding terrain funnels cold air downward and traps it. Anyone who has driven through a river valley on an autumn morning and suddenly entered a dense fog bank has experienced this firsthand.
How Air Pollution Reshapes Fog
The relationship between pollution and fog is more tangled than it might seem. Aerosol particles from industrial emissions, vehicle exhaust, and agricultural burning provide additional condensation nuclei, and more nuclei generally means more fog droplets. But those extra droplets tend to be smaller, which changes how the fog behaves in ways that can either help or hinder its persistence.
Research into the effects of urbanization and air pollution on fog in China found that increasing aerosol concentrations produce more condensation nuclei and more fog droplets, but the droplets are smaller, which inhibits the sedimentation that normally helps fog thin out. The result is fog with a higher liquid water content that can linger longer. The same work noted that pollution levels in China had not yet reached a critical threshold that would suppress fog by making droplets too small and too numerous to sustain themselves, a tipping point seen in some laboratory and theoretical studies.
In India’s Indo-Gangetic Plain, one of the foggiest and most polluted regions on the planet, researchers found that meteorological factors alone explained only about 30 to 59 percent of fog occurrence. When air pollution levels were factored in, the association jumped to 60 to 91 percent, with relative humidity, fine particulate matter (PM2.5), and coarser particles (PM10) exerting the strongest influence. This region’s notorious winter fog season, which disrupts transportation for weeks at a time, is not just a weather event; it is partly an air-quality event.
How Fog Burns Off
Fog dissipation is essentially the reverse of formation: the air needs to warm above its dew point, or the fog layer needs to get mixed with drier air from above. In the case of radiation fog, sunrise does most of the work. As the sun heats the ground, the surface warms the air above it, the temperature rises past the dew point, and the fog begins to evaporate from the bottom up. Field measurements during a European fog experiment showed that daytime dissipation is linked to the combination of warming from solar heating, which drives a surface sensible heat flux above about 10 watts per square meter, and wind shear that generates mechanical turbulence to mix the fog layer with drier air overhead.
Advection fog, on the other hand, does not burn off with sunshine in the same way because the cold surface that caused it is still there. Sea fog can persist for days if the wind keeps pushing warm air over cold water. It usually dissipates only when the wind shifts, the air mass changes, or the fog drifts inland where the surface is warm enough to erode it from below. This is why coastal fog can blanket a shoreline all morning and then retreat a few miles inland as the land heats up, only to return when the sea breeze picks up again in the afternoon.
Fog as an Ecosystem Lifeline
In some ecosystems, fog is not just a weather nuisance but a critical water source. California’s coastal redwood forests are the most studied example. Research found that during the study period, about 34 percent of the annual water input to the forest came from fog dripping off the redwood trees themselves. Without the trees to intercept the fog, that figure dropped to around 17 percent, which demonstrates how the towering canopy acts as a giant fog-catching surface. During summer, when rainfall in coastal California is essentially zero, roughly 19 percent of the water within the redwood trees and about 66 percent of the water in understory plants came from fog that had dripped from the canopy into the soil. For the redwoods, fog water accounted for 13 to 45 percent of their annual transpiration needs.
This relationship extends beyond California. Cloud forests in tropical mountains, fog deserts along the coasts of Chile and Namibia, and high-altitude grasslands all depend on fog interception for part of their moisture. In these environments, any long-term decline in fog frequency threatens the entire ecosystem, not just the convenience of human travelers.
Harvesting Fog for Drinking Water
The same property that makes fog ecologically important has inspired engineers to capture it as a water source for communities in arid regions. Fog collectors are typically large mesh panels erected on ridgelines or hillsides perpendicular to the prevailing wind. Fog droplets hit the mesh fibers, coalesce into larger drops, and drip into a collection trough below.
The design of the mesh matters considerably. Research evaluating different mesh geometries and their shade coefficients, which describe how much of the mesh area is covered by material versus open space, found that the optimum shade coefficient for fog collection falls between 50 and 60 percent. Too little coverage and the droplets pass through without being caught; too much and the wind deflects around the mesh instead of flowing through it. Rectangular, square, triangular, hexagonal, and Raschel mesh types were compared, and stainless steel rectangular meshes at the optimal shade coefficient performed best. Fog-collection projects now operate in Chile, Morocco, Peru, and several other countries, though yields depend heavily on local fog frequency and wind patterns.
Fog and Aviation
Few weather phenomena cause as much disruption to air travel as fog. Runways require minimum visibility thresholds for takeoffs and landings, and when fog drops visibility below those limits, flights are delayed, diverted, or canceled. Despite decades of research, fog forecasting remains remarkably difficult. A long-term study at King Fahd International Airport in Saudi Arabia found that terminal aerodrome forecasts, the standard operational forecasts that pilots and airlines rely on, successfully predicted only 49 percent of observed fog days over a nearly 20-year period from 2006 to 2025.
That hit rate is sobering. Fog forecasting is hard because the phenomenon depends on small-scale processes, surface temperatures, soil moisture, local aerosol concentrations, wind variations over short distances, that weather models struggle to resolve. A radiation fog event might form over one field but not the one next to it. Sea fog might develop along a narrow coastal strip while the air just a few miles inland stays perfectly clear. Improving fog prediction remains one of the tougher challenges in operational meteorology.
What Breathing Fog Actually Does to You
People sometimes worry that breathing fog is harmful, and the answer depends almost entirely on what is dissolved in the droplets. Clean fog in a rural area is essentially a fine mist of nearly pure water. The droplets are large enough that most deposit in the nose and upper airways, and breathing them in is physiologically unremarkable.
Acid fog is a different question. In polluted urban areas, fog droplets can absorb sulfur dioxide, nitrogen oxides, and other pollutants, producing acidic solutions with pH values as low as 2 or 3, roughly as acidic as lemon juice. Controlled exposure studies have tested what this does to the lungs. In one experiment, asthmatic volunteers were exposed to sulfuric acid fogs at concentrations mimicking real Brussels fog (pH 2.5 and pH 3.5) for one hour at rest, and researchers found no significant changes in lung function or bronchial responsiveness. In a separate study that pushed harder, exposing both healthy and asthmatic subjects to sulfuric acid aerosols at concentrations up to 2,000 micrograms per cubic meter during heavy exercise, large fog-sized droplets (around 10 microns) still produced no marked lung function changes, though both groups reported increased respiratory symptoms in a dose-related pattern. The same study found that smaller droplets, around 0.9 microns, did cause measurable decreases in lung function among asthmatics at the highest concentrations.
The practical takeaway is that typical outdoor fog, even moderately polluted fog, does not appear to cause acute lung function problems in most people during short exposures. The risks increase with pollution levels, exercise intensity, and especially with very fine aerosol sizes that penetrate deeper into the lungs. People with severe asthma exercising heavily in heavily polluted fog face the greatest concern, though even in controlled experiments the effects were modest.
Why Some Cities Are Losing Their Fog
One of the more counterintuitive trends in climate science is that many historically foggy cities have seen fog become less frequent over the past several decades. San Francisco, London, and parts of the U.S. East Coast have all recorded declining fog days. The reasons are debated, but warming temperatures are the leading suspect for some regions: if nighttime low temperatures do not drop as far, the air is less likely to reach its dew point, and radiation fog becomes less frequent.
Air-quality improvements may also play a role. The thick “pea soup” fogs of industrial-era London were heavily amplified by coal smoke particles that served as condensation nuclei. As cities cleaned up their air, they removed some of the particle supply that had been seeding fog formation. The relationship is not straightforward, as the research from China and India shows that moderate pollution can enhance fog, but extremely high particle concentrations or a shift in particle chemistry could tip the balance the other way. Meanwhile, some regions are seeing more fog, particularly coastal areas where shifting ocean currents or changing wind patterns alter the temperature contrasts that drive advection fog. Fog trends are local and specific, and blanket statements about whether fog is increasing or decreasing globally do not hold up.
Fog Versus Mist Versus Haze
Meteorologists draw formal lines between fog, mist, and haze, though in everyday experience they blur together. Fog officially requires visibility below one kilometer. If visibility is reduced but still between one and two kilometers, that is mist. Haze refers to visibility reduction caused by dry particles, not water droplets, though in practice humid haze can transition to mist and then fog as humidity climbs.
These distinctions matter operationally. Airport visibility categories are built around these thresholds, and the instruments used to measure them, called visibility sensors or transmissometers, quantify what is known as meteorological optical range. Researchers have explored using Doppler wind lidar, a laser-based wind measurement tool, as a proxy for visibility by analyzing how much of the laser beam is scattered back. The method works for rough estimates within a few kilometers of accuracy but is not precise enough for safety-critical applications like aviation. For now, direct visibility sensors remain the standard at airports and weather stations where exact readings matter.