Haze is a suspension of tiny dry particles or liquid droplets in the atmosphere that reduces visibility and gives the sky a milky, washed-out appearance. Unlike fog, which is made of water droplets close to the ground, haze consists mostly of fine particles smaller than a few micrometers across, things like dust, soot, salt crystals, and organic compounds that scatter and absorb sunlight before it reaches your eyes. The causes range from entirely natural (wildfires, dust storms, volcanic emissions, even chemicals released by trees) to overwhelmingly human (vehicle exhaust, coal burning, industrial smokestacks, agricultural fires). What makes haze particularly interesting is how much it depends on weather: the same city can go from clear skies to a thick, soupy haze overnight without any change in emissions, simply because the atmosphere stopped mixing.
How Haze Reduces Visibility
The core of haze is light scattering. When sunlight or any other light hits a particle floating in the air, the light bounces off in various directions rather than continuing straight to your eye. The result is that distant objects look faded, colors wash out, and the horizon takes on a whitish or brownish tint depending on what the particles are made of. Early optical research on haze found that the scattering is strongly lopsided: more than three-quarters of light hitting a haze particle gets pushed forward, while less than a quarter scatters backward. This forward-scattering pattern is characteristic of particles larger than the wavelengths of visible light, meaning most haze particles are at least a few tenths of a micrometer across.
This matters for what you actually see. Because haze scatters so much light forward, it creates a bright, glowing veil between you and whatever you’re looking at. A mountain range 30 kilometers away doesn’t just get dimmer; it gets buried behind a wall of scattered light that the atmosphere itself seems to generate. That’s why haze makes everything look flat and featureless rather than simply dark.
What Is Actually Floating in the Air
Haze particles fall into two broad camps. Primary particles are emitted directly: soot from a diesel engine, mineral dust kicked up by wind, sea salt from ocean spray, or ash from a fire. Secondary particles, on the other hand, form in the atmosphere itself when gases react with each other or with sunlight. These secondary aerosols are a major player in severe haze events. Research in heavily polluted regions has shown that chemical reactions in the atmosphere, driven by both sunlight and water-based chemistry, can rapidly produce new particles and cause existing ones to grow, sometimes explosively enough to turn a mildly hazy day into a dense pollution event within hours.
The chemistry gets layered. Photochemical reactions involving sunlight tend to dominate during the early stages of a haze episode, producing less-oxidized organic particles. As the episode drags on, further chemical processing in the atmosphere creates more-oxidized compounds that keep the haze going even after the initial burst of pollution eases.
On the inorganic side, compounds like ammonium nitrate and ammonium sulfate form when ammonia from agriculture meets sulfur dioxide and nitrogen oxides from industry and traffic. These inorganic particles are often hygroscopic, meaning they soak up water from the air and swell in size, which makes them even better at scattering light and choking visibility. Nitrate production, for example, has been linked to both daytime photochemical reactions and nighttime reactions on particle surfaces.
Natural Sources
Not all haze is human-made. Wildfires and prescribed burns are a major natural contributor. Smoke from burning forests and grasslands produces enormous quantities of fine particles that can drift hundreds or even thousands of kilometers from the fire itself. In parts of the western United States, wildfire smoke is responsible for a substantial share of regional haze, particularly during summer and early autumn.
Dust storms are another natural source. Desert regions like the Sahara generate massive plumes of mineral dust that get lofted into the upper atmosphere and carried across oceans. A particularly dramatic example occurred in June 2020, when an enormous Saharan dust storm sent particles as high as six kilometers into the atmosphere, where the African Easterly Jet carried them rapidly westward across the Atlantic toward the Americas. The aerosol concentrations during that event were among the highest ever recorded by satellite instruments.
Volcanic eruptions inject sulfur dioxide and ash into the atmosphere, sometimes at altitudes where they can circle the globe for months. And on a quieter scale, forests themselves release volatile organic compounds that react in sunlight to form secondary organic aerosols. On hot summer days, these biogenic emissions can create a faint blue haze over forested mountains, which is how the Blue Ridge Mountains and the Blue Mountains of Australia got their names.
Human-Made Sources
In urban and industrial areas, human activity usually dominates haze formation. A detailed source-tracking study in Beijing identified the major contributors to fine particle pollution during haze episodes:
- Secondary inorganic aerosol: Roughly a quarter of fine particles, formed from chemical reactions of pollutant gases in the atmosphere.
- Vehicle emissions: About a fifth of fine particles, from exhaust and brake and tire wear.
- Coal combustion: Close to a fifth of fine particles, from power plants and heating.
- Industrial pollution: About one-eighth of fine particles, from factories and manufacturing.
- Biomass burning: About one-ninth, from agricultural waste burning and wood fires.
All of these sources contributed more on haze days than on clear days, while natural sources like mineral dust actually made up a smaller fraction of the total during haze events.
Agricultural burning deserves particular attention because of how concentrated its effects can be. Burning crop stubble before or after harvest is a common practice in many countries, and when it happens over a wide area at once, it can trigger severe haze episodes that blanket entire regions. In parts of China, India, and Southeast Asia, agricultural burning season is synonymous with haze season.
Ammonia from farming also plays a quieter but important role. When ammonia emissions are high, they react with sulfur dioxide and nitrogen oxides from industry and vehicles to form ammonium nitrate and ammonium sulfate particles. Modeling work has estimated that cutting ammonia emissions in half, something achievable through better fertilizer management, could reduce fine particle pollution by roughly 11 to 17 percent when paired with modest reductions in sulfur dioxide and nitrogen oxides.
Why Weather Makes or Breaks a Haze Episode
Emissions alone don’t determine whether you get haze. The atmosphere’s ability to mix and disperse those emissions matters just as much, and sometimes more. Three weather factors are especially important.
Temperature inversions are the biggest culprit. Normally, air near the ground is warmer than the air above it, so it rises and carries pollutants upward where they can disperse. During an inversion, a layer of warm air sits on top of cooler air near the surface, acting like a lid. Pollutants get trapped underneath and accumulate. Under persistent winter high-pressure systems, these inversions can last for days, allowing progressive buildup of fine particles and compounding air quality problems day after day.
Humidity is the second key factor, and its role is counterintuitive. You might expect rain to wash haze away, and eventually it does. But before that happens, high relative humidity makes haze worse. Many haze particles are hygroscopic: they absorb moisture from the air and swell. Field measurements have shown that at 85 percent relative humidity, the light-scattering ability of haze particles roughly doubles compared to dry conditions. One study found scattering increases ranging from about 1.3-fold to nearly 1.9-fold at that humidity level, with an average increase of about 1.6-fold. Even when rain begins and washes some particles out of the air, the remaining particles can swell so much in the wet conditions that visibility actually gets worse before it gets better.
Wind, or rather the lack of it, rounds out the picture. Stagnant conditions under high-pressure systems mean there’s no horizontal wind to blow pollutants away and no vertical mixing to dilute them. Many of the worst haze events worldwide happen when weak winds, temperature inversions, and moderate humidity all coincide.
What Haze Does to the Climate Around It
Haze doesn’t just sit passively in the atmosphere; it interacts with it in ways that can reinforce itself. Dense haze layers absorb and scatter sunlight before it reaches the ground, cooling the surface while warming the air higher up. Multiyear observations in northern China documented an average surface temperature drop of about 2.2 degrees Celsius under polluted conditions, alongside a warming of roughly 0.7 degrees Celsius in the upper part of the atmospheric boundary layer.
This matters because it strengthens the very temperature inversion that trapped the pollution in the first place. Cooler air at the surface and warmer air above means less mixing, which means more pollution stays trapped, which means more light absorption, which means even more surface cooling. Researchers call this a feedback loop, and it helps explain why severe haze episodes tend to worsen rapidly once they get started and then persist for days.
Dense haze also cuts the amount of solar energy reaching the ground. Measurements at an urban site in the Indo-Gangetic Plain found that haze and foggy conditions reduced the direct radiative effect at the surface by a meaningful margin compared to clear-sky days. For agriculture, this means less light for photosynthesis. For solar energy installations, it means reduced output during the same winter months when energy demand peaks.
Health Effects
The health concerns around haze center on the size of the particles involved. During Singapore’s 1997 haze episode caused by Indonesian fires, scanning electron microscopy found that 94 percent of particles in the haze were smaller than 2.5 micrometers in diameter, small enough to bypass the body’s normal filtering mechanisms and penetrate deep into the lungs. When particulate concentrations tripled from moderate to high levels during that event, researchers documented increases of roughly 12 percent in upper respiratory illness, 19 percent in asthma visits, and 26 percent in rhinitis.
Longer-term studies in Beijing examined the cumulative effects of different haze intensities on mortality over five-day windows. Cardiovascular mortality risk increased by about 2.4 percent during light haze, rising to roughly 10 percent during heavy haze. Respiratory mortality showed a similar gradient. These effects accumulated over multiple days of exposure, meaning prolonged haze events were disproportionately more dangerous than brief ones.
Exposure to haze-related pollution has been linked broadly to increased hospital admissions and mortality for both lung and cardiovascular disease. The mechanism involves both the direct irritation caused by particles lodging in lung tissue and the systemic inflammation that follows when ultrafine particles enter the bloodstream. People with pre-existing respiratory conditions, the elderly, and young children tend to be most vulnerable.
Transboundary Haze
Haze doesn’t respect national borders, and some of the most politically fraught haze problems are transboundary. Southeast Asia has experienced almost annual haze pollution since the early 1980s, caused primarily by grass, forest, and peat fires in Indonesia. Peat fires are especially problematic because peat is essentially compressed organic matter that can smolder underground for weeks, producing dense, particle-laden smoke that drifts across Malaysia, Singapore, and beyond.
During Singapore’s worst haze episodes, outdoor particle levels spike high enough that the government issues health advisories, schools close, and people are urged to stay indoors. Even indoors, the protection is limited. Measurements during Singapore’s 2019 smoke haze episode found that elevated fine particle levels outdoors led to deteriorated indoor air quality as well, since outdoor particles infiltrate buildings through ventilation systems, open windows, and gaps in the building envelope.
Saharan dust provides a different kind of transboundary haze. Massive quantities of mineral dust leave North Africa every year and cross the Atlantic, reaching the Caribbean and the southeastern United States. Airborne measurements during research campaigns have confirmed this long-range transport process, which is driven by large-scale wind patterns in the mid-troposphere. When Saharan dust arrives over the Americas, it can push air quality readings into unhealthy ranges and create the sort of milky, brownish-orange sky that looks nothing like local pollution.
How Haze Is Tracked
Monitoring haze involves a layered approach. On the ground, networks of air quality stations measure concentrations of fine particles in real time. These ground-level readings are complemented by instruments that measure how much sunlight the atmosphere blocks at various wavelengths, a quantity called aerosol optical depth, and by upward-pointing lasers that profile where particles sit in the vertical column of air above a station. Researchers have combined these ground-based tools with satellite measurements to build three-dimensional pictures of haze events, tracking not just how thick the haze is but where in the atmosphere it lives and how it changes with altitude.
Satellite data is especially valuable for covering large, remote, or ocean areas where ground stations don’t exist. During events like the 2020 Saharan dust storm, satellites were the primary tool for tracking the plume’s movement across the Atlantic. They also help distinguish between different types of haze: dust, smoke, and urban pollution have different optical signatures that satellite algorithms can tease apart.
Regulation and Visibility Goals
In the United States, haze in wilderness areas and national parks has been a regulatory target since the late 1990s. The EPA’s Regional Haze Rule, first issued in 1999, requires states to reduce human-caused emissions enough to achieve steady improvement in visibility at protected wilderness areas, with the goal of returning to natural visibility conditions by 2064. Progress is tracked on the statistically 20 percent worst-visibility days using a logarithmic scale of light extinction.
The rule was later revised to sharpen its focus. The original version tracked the haziest days overall, but those days were sometimes dominated by wildfire smoke or natural dust, factors states can’t control through emissions reductions. The updated rule shifted to tracking the 20 percent most anthropogenically impaired days, filtering out natural events to better measure whether human-caused haze is actually declining.
This distinction between natural and human-caused haze is important for policy. A wildfire year can temporarily undo years of progress on the old metric even if industrial and vehicle emissions have been falling steadily. The revised approach acknowledges that the goal isn’t to eliminate all haze, which would be impossible given natural sources, but to get the human contribution down to something approaching background levels.
Internationally, regulations are less uniform. The ASEAN Agreement on Transboundary Haze Pollution, signed in 2002, was supposed to coordinate responses to fire-driven haze in Southeast Asia. But enforcement has been weak, and Indonesia, the primary source country, didn’t ratify the agreement until 2014. The fires have continued, and so has the haze.
Haze on Other Worlds
Haze is not unique to Earth. Observations from the James Webb Space Telescope have confirmed that Pluto possesses extensive atmospheric haze despite its extremely thin, nitrogen-dominated atmosphere. The haze there is thought to result from photochemistry involving nitrogen and methane, somewhat like the chemistry that produces haze on Saturn’s moon Titan, though the composition turns out to be quite different. Pluto’s haze shows spectral features consistent with ices of complex hydrocarbons rather than the tholin-like particles found on Titan.
Mars experiences its own haze events driven by global dust storms that can envelop the entire planet for weeks. And Titan’s thick orange haze, produced by ultraviolet-driven chemistry high in its atmosphere, is dense enough that the moon’s surface is completely invisible at visible wavelengths from space. Studying haze on other worlds helps atmospheric scientists understand the general physics of particle-light interactions and aerosol formation under conditions that can’t be replicated in an Earth laboratory, while also providing a surprisingly useful mirror for understanding how haze behaves in our own atmosphere under extreme conditions.