Fog Versus Haze: A Practical Guide to Telling Them Apart

Fog and haze look almost identical to a casual observer: both reduce visibility, both soften the horizon, both give distant lights a diffuse halo. But meteorologically they are two entirely different phenomena, produced by different physics, associated with different health impacts, and requiring different responses from anyone driving, flying, or photographing outdoors. Consumer weather apps and air-quality apps routinely confuse them or lump them together as 'low visibility'. Amateur observers can, and should, do better.

Fog is a cloud that touches the ground. It consists of suspended water droplets — the same droplets that make up stratus and other low clouds — held aloft by tiny updrafts and turbulence in a very moist boundary layer. The World Meteorological Organization defines fog as a suspension of water droplets that reduces horizontal visibility below one kilometre. Above one kilometre and up to five, the same suspension is officially called mist. Fog requires relative humidity at or very close to one hundred percent and, in almost all cases, a dew-point depression (the gap between air temperature and dew point) of less than about half a degree Celsius.

Haze, in the strict meteorological sense, is a suspension of dry aerosol particles — dust, smoke, pollen, sulphate, industrial particulates — that scatters light and reduces visibility without any significant water content. Haze can persist at relative humidities well below fifty percent and requires no condensation to exist. In warm dry weather in polluted or fire-affected regions, haze can hold visibility down to a few kilometres for days at a time under a stable inversion.

The three practical thresholds that distinguish fog from haze are humidity, visibility, and the dew-point spread. If relative humidity is above ninety-five percent, the dew-point spread is below about one degree, and visibility is under one kilometre, you are looking at fog. If humidity is under eighty percent, the dew-point spread is greater than three degrees, and visibility is anywhere from a few hundred metres to ten kilometres, you are looking at haze. Anything in between is either mist (moist, thin) or a mixed fog-haze layer (common in urban valleys in winter).

The public-health implications of getting this right are significant. Meteorological fog is chemically inert — it is essentially clean water — and its only direct health risk is reduced visibility for drivers and pilots. Haze, by contrast, often carries fine particulate matter with real cardiovascular and respiratory effects. Treating a haze event as 'just fog' can delay air-quality warnings and mask a genuine pollution problem. Conversely, treating fog as haze can produce spurious alarm about air quality when the actual air is clean.

For photographers, the distinction is aesthetic as well as diagnostic. Fog scatters light almost neutrally across the visible spectrum, producing the classic white-out with cool grey shadows. Haze, especially from wildfire smoke, tends to preferentially scatter shorter wavelengths, producing warm orange or brown tones and unusually red sunsets. If your sunset photograph has an unusually deep red disk and the horizon looks brown rather than grey, you are almost certainly photographing through haze, not fog, regardless of what your weather app calls it.

For amateur observers building a log, always record humidity, temperature, dew point, and visibility together when reporting a low-visibility event. These four numbers unambiguously classify the phenomenon, and the record will remain useful for decades even if the observer's classification at the time was wrong. Community science databases increasingly ask for these raw numbers precisely because so many historical observations were mislabelled fog when they should have been haze, and vice versa.

A few edge cases are worth naming. Freezing fog is meteorological fog whose droplets freeze on contact with surfaces below zero degrees; it is a hazard for aircraft and power lines. Ice fog is fog consisting of suspended ice crystals rather than droplets, common in extreme cold at high latitudes. Smog is a specific mixture of smoke and fog first named for London's coal-smoke episodes; in modern usage it usually refers to photochemical pollution rather than true smoke-fog. Volcanic haze — vog — is a specific type of aerosol haze produced by sulphur dioxide from active volcanoes. Each of these has its own signature in the four core measurements.

The final and most useful skill is real-time self-diagnosis. Step outside into a low-visibility event. Feel your face: fog leaves it damp within seconds, haze leaves it dry. Look at a nearby light source: fog produces a soft, uniform halo, haze produces a sharper, sometimes coloured tint. Smell the air: haze from combustion often carries a distinct scent, fog is odourless. These sensory cues, combined with the four measurements, will produce a correct classification every time.