You cannot reliably tell smog, wildfire smoke, and fog apart by color alone, but a short checklist can. Ozone smog is invisible and builds through a sunny afternoon, wildfire smoke carries a tan or orange tint and can spike at any hour, and marine fog is simply suspended water droplets that clear from the coast inward by late morning. The fastest real diagnosis combines the time of day, the season, whether the air smells like anything, and a quick check of humidity, visibility, and the dominant pollutant on AirNow, not a glance out the window.
What is the difference between smog, smoke, and fog?
The three phenomena differ at the level of physical cause, not just appearance. Smog over Los Angeles is chemically produced: sunlight cooks vehicle and industrial emissions (nitrogen oxides and volatile organic compounds) into ground-level ozone and secondary particles over the course of several hours. Wildfire smoke is a combustion plume, a direct mixture of fine particulate matter (PM2.5), carbon monoxide, and other gases released by burning vegetation or structures, carried into the basin by wind rather than formed locally by sunlight. Fog is neither pollutant nor combustion byproduct at all, just ordinary water vapor that has condensed into suspended droplets once the air cools to its dew point.
All three can reduce visibility and tint the sky, and in practice they frequently mix, which is exactly why a single glance rarely settles the question. For the chemistry behind the first of these in more depth, see what is smog and how does LA's photochemical haze form, and for the water-droplet side, see fog vs. mist vs. haze.
| Signal | Smog (photochemical) | Wildfire smoke | Fog / marine layer |
|---|---|---|---|
| Typical color | Grayish brown haze; the ozone itself is colorless | Tan, gray, or orange, darker and more saturated close to a fire | Flat white or light gray |
| Smell | Usually none distinct, sometimes a faint acrid or tire-like edge near freeways | Campfire or burnt-wood odor, often the clearest single tell | Damp, salty, or no smell at all |
| Typical time of day | Builds through the morning, peaks in the afternoon | Any hour; depends on the fire and wind, not sunlight | Thickest overnight and at dawn, burns off by late morning |
| Typical season | Late spring through early fall, strongest on hot, stagnant days | Peak wildfire season, roughly summer through fall, but possible year-round | Most persistent in late spring and early summer (May Gray, June Gloom) |
| How it clears | Fades as the sun angle drops and photochemical production slows toward evening | Clears with a wind shift, rain, or the fire being contained; can linger for days | Burns off from the coast inland as morning sun heats and mixes the marine layer |
Treat this table as a set of probabilities, not a verdict. Southern California's basin and mountain geography routinely stacks these three phenomena in separate layers of the same sky, so the honest answer to "which one am I looking at" is often "check the data," covered later in this article.
What does each one do to visibility and air quality?
Ground-level ozone, the core pollutant in LA smog, is an invisible gas. A clear-looking sky can still carry an unhealthy ozone reading, especially inland in the afternoon, so smog's visible haze actually comes from a secondary layer of fine particles that forms alongside the ozone, not from the ozone itself. Wildfire smoke behaves differently: it is dominated by PM2.5, particles small enough to scatter light efficiently and drive both a hazy sky and an elevated Air Quality Index (AQI) reading at the same time, which is why a visibly smoky day and a bad AQI day for wildfire smoke usually arrive together. Fog is the outlier.
Because it is pure water, it can cut visibility to a quarter mile or less at a coastal airport while the AQI reads clean, since the National Weather Service and AQI monitors are measuring two different things: atmospheric moisture versus pollutant concentration. None of the three can be diagnosed from a photo alone, which is the central reason this page exists rather than promising a color-based shortcut.
Why can all three appear as haze?
All three phenomena reduce visibility through the same basic optical process: particles or droplets suspended in the air scatter sunlight in every direction instead of letting it pass straight through, which is what a human eye interprets as haze, glare, or a flattened, washed-out sky. Humidity makes this worse for both smog and smoke, since hygroscopic particles absorb water and swell in size as relative humidity rises, scattering even more light for the same particle count. Backlighting matters too. Looking toward the sun through any hazy layer, smog, smoke, or fog, exaggerates the graying effect compared with looking away from it, which is why the same afternoon can look dramatically hazier driving west toward the coast than driving east.
Los Angeles's basin geometry compounds the confusion. The San Gabriel, San Bernardino, and Santa Ana Mountains box in the South Coast Air Basin, and a persistent temperature inversion often caps it from above, so smog, smoke, and moisture can all get trapped in the same shallow layer near the surface rather than mixing out. A marine layer can sit at the surface in Downtown Los Angeles while photochemical haze builds a few hundred feet above it, or a wildfire smoke plume can ride in aloft, thousands of feet up, while the ground-level air stays comparatively clear.
A cross section of the basin on any given afternoon can show two or three of these layers stacked at once, which is the physical reason a single description like "hazy sky" is rarely precise enough to say what is actually happening.
Which observations help tell them apart?
No single number settles it, but combining a handful of readings gets close. Relative humidity and the dew point spread are the first filter: air close to its dew point points toward fog or mist, while a wide spread combined with reduced visibility points toward a dry particulate cause, smog or smoke, instead. Reported visibility from an airport or NWS station distinguishes dense fog (commonly under 1 kilometer, about 0.62 miles) from a lighter haze. Cloud base matters too.
A low, flat, gray ceiling that a hiker can climb above within a few hundred feet of elevation gain is behaving like fog or marine stratus, while a hazy layer with no defined ceiling that persists at altitude is behaving like smoke or smog. The AQI's dominant pollutant is the most direct answer of all: a PM2.5-driven reading points to wildfire smoke, an ozone-driven reading points to photochemical smog, and a clean AQI reading alongside a genuinely gray sky points to fog with no pollutant involved.
Satellite smoke and fire detection products can confirm an active plume and its transport path, which a ground-level monitor alone cannot show. Low-cost consumer air sensors are useful for a rough trend but are not calibrated to the standard the AQI uses, so treat them as a supplement to, not a replacement for, an official monitor.
Can smog, smoke, and fog overlap?
Yes, and in the Los Angeles basin this is closer to routine than exceptional. A morning marine layer can sit over Downtown Los Angeles and the coastal plain while photochemical ozone precursors accumulate underneath it, waiting for the fog to burn off and full sun to arrive before the smog reaction accelerates through the afternoon. During an active wildfire, smoke can arrive above the marine layer entirely, transported by upper-level wind while a separate, unrelated fog bank sits at the surface a few hundred feet below it, so a hazy, dimmed sky and a foggy coastline can have two completely independent causes on the same day.
The health-relevant story can also flip as the day goes on: a location might start the morning under clean but foggy air, spend midday under rising ozone as the fog clears and the sun does its photochemical work, and finish the evening with smoke drifting in from a distant fire as winds shift overnight. That kind of sequence is exactly why checking a live reading matters more than trusting the sky's look at any single moment.
What should a Los Angeles reader check now?
Start with AirNow for the current AQI and, critically, its dominant pollutant, ozone points toward smog and PM2.5 points toward smoke. The South Coast AQMD publishes regional forecasts, smoke advisories, and monitor-by-monitor detail for communities like Pasadena and Woodland Hills, both of which can post different readings than the coast on the same afternoon. The NWS Los Angeles/Oxnard office and NWS San Diego office report visibility and cloud base, which settle the fog question directly, while the California Air Resources Board maintains statewide air quality standards and historical monitoring data for anyone checking a pattern over time rather than a single day.
If the AQI shows an elevated PM2.5 or ozone reading, follow official health guidance for sensitive groups rather than judging risk by how the sky looks. For the mechanism behind each phenomenon in full depth, see what is smog and how LA's photochemical haze forms, why wildfire smoke turns the LA sky orange, and fog vs. mist vs. haze.
The short version is worth repeating before you head out the door: color and haziness alone cannot separate smog, smoke, and fog with confidence, but smell, timing, season, and thirty seconds with AirNow usually can. Before a hike, a beach day, or just deciding whether to open the windows, check WeatherEscape's current conditions alongside AirNow's dominant pollutant reading for your part of the basin, since a gray sky over Pasadena in June is very likely marine fog, while the same gray sky in September could be smoke from a fire nowhere near the city.