Fog and mist are both water droplets suspended in the air, and they differ only in density. Fog cuts horizontal visibility below 1 kilometer (about 0.62 miles), and mist is the same process at a lighter concentration that leaves visibility at 1 kilometer or more.
Haze is different: it is reduced visibility from dry particles such as dust, smoke or pollution. In Los Angeles, a gray summer morning over Downtown Los Angeles is almost always marine fog or mist off the Pacific. A brownish tint over Woodland Hills on a still, hot afternoon is more likely haze from ozone and particles.
What Is the Difference Between Fog, Mist, and Haze?
Fog and mist are the same thing, water droplets suspended near the ground, and visibility decides which label applies. The National Weather Service Los Angeles/Oxnard office and the NWS glossary define fog as a ground-level cloud of water droplets, or ice crystals in freezing weather, that cuts horizontal visibility below 1 kilometer. Mist is the same droplets at a lighter concentration, with visibility of 1 kilometer or more. Mist is fog that has not thickened enough to earn the stricter label.
Haze is reduced visibility from dry, fine particles in the air, including dust, smoke, sulfates and the particles produced by photochemical smog. Haze does not need saturated air or high humidity. A hazy day in the San Fernando Valley around Woodland Hills can happen under dry, sunny skies with humidity far below what fog needs. Water versus dry particles is the fastest way to sort out what is graying a Los Angeles skyline.
| Term | What it's made of | Visibility threshold | Typical LA setting |
|---|---|---|---|
| Fog | Suspended water droplets (surface-based cloud) | Below 1 km (about 0.62 miles) | Coastal mornings at Downtown Los Angeles and along the shoreline, especially late spring through early summer |
| Mist | Same water droplets as fog, lighter concentration | At or above 1 km, but still visibly gray or damp | Thinning edges of a marine layer, or fog that is forming or clearing |
| Haze | Dry particles: smog, dust, smoke, sulfates | Variable; tied to PM2.5 and ozone concentration, not humidity | Warm, stagnant afternoons in inland basins like Pasadena and the San Fernando Valley |
How Do Humidity and Visibility Help Distinguish Fog, Mist, and Haze?
Relative humidity near 100% is the strongest sign that gray sky is fog or mist, not haze, because water droplets form and last only when air is at or near saturation. Haze forms across a much wider humidity range, because it depends on how many dry particles are in the air.
The dew point spread, the gap between air temperature and dew point, tells them apart. A spread near zero points to fog or mist. A wide spread with poor visibility points to haze, smoke or dust. Haze along the horizon also blocks the green flash at sunset, which needs a clean view of the sea.
Droplet density and visibility do the rest of the sorting. Fog droplets are dense enough to scatter light widely and cut visibility sharply, often to a few hundred feet in the thickest coastal fog.
Mist has the same droplets at lower density, so visibility usually stays past half a mile while the air still feels damp and looks gray. Observers, airport stations and even NWS products do not always use "mist" consistently, so check the visibility number, not just the label. At LAX, visibility under half a mile is most common in October, as the guide to LAX weather delays shows.
Why Does Marine Fog Form Along the Southern California Coast?
Marine fog forms along the Southern California coast because cold ocean water chills the moist air above it until the air saturates and condenses into droplets. Wind-driven upwelling keeps the water offshore cold all year. A temperature inversion above traps the cool, moist layer near the surface instead of letting it mix upward. Nighttime cooling chills the marine layer further, so fog is most likely before and around dawn.
The same marine layer produces LA's gray coastal mornings and its stronger season in May and June, covered in the marine layer's gray mornings and May Gray and June Gloom. The Catalina eddy, a coastal wind pattern, can deepen that marine layer against the coast, as the Catalina eddy's role in LA's gray mornings explains. Low stratus cloud a few hundred to a couple thousand feet up is not fog, because fog by definition touches the ground.
A driver heading from the coast into the hills can pass under that cloud deck as fog and then rise above it into clear sky within minutes, because the cloud never touched the ground at the higher elevation.
When Is Gray Air More Likely to Be Smog or Smoke?
Gray or brown air is more likely smog when it builds through a warm, sunny afternoon in an inland basin. Ozone haze forms from sunlight acting on pollution for hours, so it peaks well after the morning traffic that supplied its ingredients.
Wildfire smoke arrives at any hour, often tan, orange or gray depending on particle size and sun angle, and satellite smoke maps show it regardless of humidity. Coastal fog clears from the coast inland as the marine layer burns off, usually by late morning. Smog and smoke do not follow that timing.
Smog, smoke and fog form in completely different ways. Photochemical smog needs nitrogen oxides and volatile organic compounds reacting in sunlight for several hours. Smoke is particles from burning that can travel long distances high in the air.
Marine fog is water condensing over a cold ocean. WeatherLA's explainer on how LA's photochemical smog forms covers ozone chemistry and inland timing, and smog vs. smoke vs. fog compares their health and safety effects. Color and smell do not measure health risk. The current Air Quality Index and its dominant pollutant show what is in the air.
Can Fog, Haze, and Smoke Occur Together?
Fog, haze and smoke often occur together in Los Angeles, because the coast and basin hold several air layers at once. Wildfire smoke can sit well above the surface while marine fog or low stratus fills the lowest few hundred feet. A dim, hazy sky and a foggy coast can then happen at the same time for unrelated reasons. Photochemical haze can also build under an inversion that traps both pollution and leftover moisture near the ground, especially in the San Gabriel and San Fernando valleys, where mountains limit how far polluted air spreads.
One look at a gray sky cannot separate these layers. A driver heading inland on a smoky morning may pass through marine fog near the coast and then into smoky but drier air inland, all in a half-hour drive. Treat a visual guess as a starting point and confirm it with the measurements below.
Which Observations Reveal What Is in the Air?
Four measurements reveal what is in the air: dew point spread, visibility, cloud-base height and the dominant AQI pollutant. A dew point spread near zero points to fog or mist, and a wide spread with poor visibility points to haze, smoke or dust. Reported visibility separates fog from mist when the air is saturated, using the National Weather Service observing definitions. Cloud-base height separates ground-level fog from low stratus above the surface.
For particles, the dominant pollutant on the Air Quality Index points to the source: PM2.5 usually means smoke, PM10 usually means dust and ozone means photochemical smog. Satellite and official smoke-tracking maps confirm wildfire smoke aloft even when it is not obvious at the surface. Time of day also helps. Coastal fog and mist are most common in early morning and clear by late morning, while ozone haze builds through the afternoon in inland areas like Pasadena.
Check current conditions for your destination on WeatherLA and the current AQI before judging from a photo or a hazy commute. For related sky questions, see cumulus vs. cumulonimbus clouds for storm clouds over the basin and sleet vs. hail vs. graupel for what falls in a cold storm. For how Pacific ocean cycles shift the odds of a wet or dry winter, see El Niño vs. La Niña for a SoCal winter.
Fog and mist are water, sorted by how far you can see. Haze is dry particles, sorted by what is in the air, not how humid it is. Before deciding whether a gray morning is marine fog that will burn off or ozone haze that will build, check visibility, dew point, cloud base and the dominant AQI pollutant for that location.