Haze happens when tiny particles suspended in the air scatter and absorb sunlight, reducing how far you can see. These particles can come from pollution, dust, wildfire smoke, or even natural sources like trees. What makes any given hazy day look the way it does depends on the type of particles in the air, how much moisture is present, and whether weather patterns are trapping everything near the ground.
How Particles Block Your View
Visibility on a clear day can stretch 90 miles or more. When the air fills with fine particles, that distance shrinks dramatically. The particles responsible are incredibly small, mostly under 2.5 micrometers in diameter (about 30 times thinner than a human hair). At that size, they interact with visible light in two ways: they scatter it in different directions, and they absorb some of it outright. Both effects reduce contrast between objects and the sky, making everything look washed out.
The combined effect of scattering and absorption is what atmospheric scientists call “light extinction,” and it’s the direct measure of how much visibility you lose. The more particles in the air, the more light gets redirected before it reaches your eyes, and the hazier things look.
Humidity Makes Haze Worse
You’ve probably noticed that hazy days often feel muggy, and that’s not a coincidence. When relative humidity climbs, water vapor attaches to particles already floating in the air. The soluble components in those particles absorb water and swell, sometimes growing large enough to scatter significantly more light. At 80% relative humidity, the scattering effect of airborne particles is roughly 1.5 to 1.6 times stronger than it would be in dry conditions. That’s why a moderately polluted day can look much worse when it’s also humid.
This is also what separates haze from fog and mist. Fog is a ground-level cloud made of water droplets that cuts visibility below 1,000 meters (about 0.6 miles). Mist is the same thing, just less dense, with visibility at or above 1,000 meters. Haze, by contrast, is a suspension of extremely small dry particles, not water droplets. In practice, humidity often makes haze thicker, but the base material is particulate matter, not moisture.
Temperature Inversions Trap the Haze
Normally, air near the ground is warmer and rises, carrying pollutants upward where they disperse. A temperature inversion flips that pattern: a layer of warm air sits above cooler air near the surface, acting like a lid. This suppresses the normal mixing and convection that would clear particles out of the lower atmosphere, so pollutants and particles accumulate near the ground where people live and breathe.
Inversions are common on calm, clear nights when the ground cools quickly, and they often persist into the morning. Cities in valleys or basins are especially prone to multiday inversions during winter, which is why places like Los Angeles, Salt Lake City, and Beijing are known for persistent haze episodes. Until wind or a weather front breaks the inversion, the air just keeps getting murkier.
Pollution and Chemical Reactions
A major source of haze in urban and suburban areas is secondary organic aerosols. These aren’t emitted directly from tailpipes or smokestacks. Instead, they form through chemical reactions in the atmosphere: sunlight reacts with volatile organic compounds (from vehicle exhaust, industrial emissions, and even trees and plants), nitrogen oxides, and sulfur dioxide to produce new particles that weren’t there before. This is why haze often builds through the afternoon on hot, sunny days as photochemical reactions have more time to work.
The color of haze can tell you something about what’s in it. Nitrogen dioxide, a byproduct of combustion, is reddish-brown at normal temperatures. When nitrogen dioxide and sulfur dioxide react with tiny water droplets to form nitric and sulfuric acid particles, the resulting haze tends to look yellow or orange-gray, because those acid particles primarily scatter longer wavelengths of visible light. A bluish tint, though rare, can come from ozone, which is colorless to slightly blue.
Wildfire Smoke
Wildfire haze has a character all its own. Wood and vegetation burning at high temperatures release enormous amounts of organic carbon and black carbon into the atmosphere. The organic carbon component includes a substance called brown carbon, which absorbs light strongly in the ultraviolet and visible spectrum. In wildfire plumes, dark brown carbon is roughly four times more abundant than black carbon, and it accounts for 50 to 75% of the light absorption in the smoke. That’s what gives wildfire haze its distinctive brownish, sometimes orange tint, as opposed to the grayish-white look of typical urban pollution.
Wildfire smoke can travel thousands of miles. During major fire seasons in the western United States or Canada, smoke plumes regularly drift across the entire continent, creating hazy skies in cities that are nowhere near the fires themselves. The particles are small enough to stay suspended for days or even weeks during transport.
Dust From Thousands of Miles Away
Desert dust is another natural cause of haze that can seem to appear out of nowhere. Dust storms in the Sahara loft enormous quantities of mineral particles into the atmosphere. The smallest of these, with diameters under 10 micrometers, can stay airborne for a week or more and travel across the Atlantic Ocean to affect air quality in the southeastern United States and Caribbean. Larger particles settle out within a few hundred to a thousand kilometers of their source, but the finest dust makes it remarkably far. When a Saharan dust plume arrives, the sky often takes on a milky, whitish-yellow appearance, and sunsets turn unusually vivid shades of red and orange.
How to Check What You’re Breathing
The Air Quality Index (AQI) is the simplest way to know whether the haze outside is just an eyesore or a health concern. AQI is color-coded and based largely on concentrations of fine particles (PM2.5). A “Good” rating (AQI 0 to 50) corresponds to PM2.5 levels of 0 to 9.0 micrograms per cubic meter over 24 hours. “Moderate” (AQI 51 to 100) covers PM2.5 from 9.1 to 35.4 micrograms per cubic meter. Above that, the air starts posing risks for sensitive groups and eventually for everyone.
In February 2024, the EPA tightened its annual health-based standard for PM2.5 from 12.0 to 9.0 micrograms per cubic meter, reflecting growing evidence that even low levels of fine particle exposure carry health risks. You can check your local AQI in real time through the EPA’s AirNow website or app, which pulls data from monitoring stations across the country. If the haze outside has you wondering whether to keep your windows open or go for a run, that’s the fastest way to get an answer with a number behind it.