Fog, Mist, and Smog

Created by Sarah Choi (prompt writer using ChatGPT)

Fog, Mist, and Smog: Formation, Physics, Health, and Forecasting

Fog, mist, and smog can look similar at street level—a veil that dims landmarks and softens sound—but they arise from different mixtures of droplets, gases, and particles. Understanding how they form explains why some mornings begin with a ghostly shroud and why certain afternoons sting the eyes and lungs. This article clarifies definitions, unpacks the physics and chemistry, ties each phenomenon to the landscape and season, and offers practical guidance for safety, health, and forecasting.

Definitions and Key Differences

Meteorologists distinguish these terms by particle type and how far you can see:

Fog is a cloud in contact with the ground, composed mainly of liquid water droplets (or ice crystals in very cold conditions). By convention, surface visibility falls below 1 kilometer. Fog forms when air near the ground cools to its dew point or when water vapor is added until saturation is reached.

Mist is like “light fog”: it also consists of suspended water droplets, but visibility is 1–10 km. It often occurs as air approaches saturation without fully reaching it or as thicker fog begins thinning.

Smog (smoke + fog) describes polluted, visibility‑reducing air. Historically it referred to sulfurous smog—a mixture of fog and smoke from coal burning. Today, the more common type is photochemical smog, driven by sunlight acting on nitrogen oxides (NOx) and volatile organic compounds (VOCs) to produce ozone (O₃) and secondary particles. Smog may or may not include water droplets; its hallmark is pollution and health impact, not simply saturation.

Related terms help sharpen the picture: haze (dry aerosols lowering visibility, usually 1–10 km), freezing fog (supercooled droplets that freeze on contact), and vog (volcanic smog rich in sulfur gases and sulfate aerosols).

How Fog and Mist Form: Cooling, Moisture, and Nuclei

Air becomes fog when the relative humidity reaches ~100% in the surface layer. Two pathways dominate:

  1. Cooling the air to its dew point. Clear nights with light winds favor radiative heat loss from the ground. The air in contact with the surface cools, humidity rises, and once saturated, droplets form on cloud condensation nuclei (CCN)—tiny particles of salt, dust, soot, or organics. This produces radiation fog, common in valleys and over grasslands just before sunrise.
  2. Adding moisture to the air. When very cold air passes over relatively warm water, evaporation injects vapor that quickly condenses into steam fog (also called Arctic sea smoke). Advection fog forms when mild, humid air moves over a colder surface (ocean currents, snowpack, or cold ground), cooling from below until saturation. Upslope fog emerges as wind pushes air up a slope, expanding and cooling it to the dew point. In coastal zones, cool ocean currents and upwelling make advection fog frequent in warm seasons.

Fog microphysics depend on droplet number and size. More CCN yield many small droplets, producing a milkier look and longer persistence (smaller droplets fall slowly). Fewer CCN yield fewer, larger droplets and a tendency to drizzle. In temperatures well below freezing, fog can contain ice crystals (ice fog) or supercooled droplets. In supercooled fog, droplets freeze upon contact with surfaces, building rime—a rough, white accretion on trees, fences, and turbines.

Stability, Inversions, and the Daily Rhythm

Fog thrives under stable conditions. A temperature inversion—warmer air over cooler surface air—traps moisture and pollutants near the ground, suppressing vertical mixing. Radiation fog often peaks around sunrise and dissipates as solar heating breaks the inversion and mixes drier air downward. Advection fog can persist all day if the source of coolness (cold sea surface, snow cover) and onshore flow continue. Urban heat islands can inhibit radiation fog locally by keeping nights slightly warmer, while nearby parks and rivers still fog.

Terrain, Water, and Local Flavor

  • Valleys and basins collect cold, dense air at night, focusing radiation fog that may remain while hillsides above bask in sun—classic valley fog.
  • Coasts and bays see advection sea fog when warm, humid air pools over cold water; sea breezes carry it inland in the morning and retreat offshore later.
  • Lakes and rivers generate steam fog in early winter or cool spring mornings when water is warmer than air.
  • Mountains host upslope stratus; on ridgelines, fog can be continuous during moist flow, feeding cloud forests with drip‑throughfall.

Visibility, Optics, and Sound

Fog limits meteorological optical range (MOR) by scattering light off droplets. Dense fog absorbs and scatters higher‑energy light more strongly, muting colors and flattening contrast. Halos around lights, glories, and fogbows appear via diffraction and backscatter from uniformly sized droplets. Sound travels differently too: fog itself does not carry sound farther, but the same stable stratification that makes fog often refracts sound, making it seem louder or quieter depending on temperature gradients and wind.

Freezing Fog, Black Ice, and Infrastructure

Freezing fog is dangerous. Supercooled droplets glaze roads, runways, bridges, rail switches, and cables with clear ice. Bridges and overpasses freeze first because they lose heat from above and below. Even light accretion can disrupt transportation and power systems. In aviation, freezing fog and low ceilings reduce visibility and create icing risks for aircraft operating near the surface.

How Smog Forms: Two Archetypes

Sulfurous (industrial) smog arises when burning sulfur‑rich fuels emits sulfur dioxide (SO₂) and soot. In moist, cool, stagnant air—especially under inversions—SO₂ converts to sulfate (H₂SO₄ and related salts) within fog droplets, thickening the haze. The infamous mid‑20th‑century winter episodes involved coal smoke mixing with fog to create “pea‑soupers,” with severe health impacts.

Photochemical smog dominates many modern cities, especially in warm, sunny seasons. The chemistry pivots on sunlight:

  • Vehicles, power plants, and other sources emit NOx (NO + NO₂) and VOCs.
  • Sunlight photolyzes NO₂ into NO + O. The free oxygen atom combines with O₂ to form ozone (O₃).
  • NO can “titrate” ozone back to NO₂, but the presence of VOCs (including biogenic compounds from trees) shifts reactions so that NO is re‑oxidized without consuming ozone, allowing O₃ to accumulate.
  • Secondary products—peroxyacetyl nitrate (PAN) and secondary organic aerosols (SOA)—form downwind, creating a regional smog plume. Heat accelerates these reactions; stagnant high‑pressure systems trap pollutants near the surface. Topography (basins, coastal mountains) and sea‑breeze recirculation can concentrate smog.

Wildfire smoke adds another modern twist: abundant fine particles (PM₂.₅) can combine with urban emissions to worsen visibility and health, even when ozone is moderate.

Health Impacts and Measurement

For fog and mist, the main risks are visibility‑related accidents and icing; for smog, direct cardiopulmonary health effects dominate. Fine particles (PM₂.₅) penetrate deep into lungs and enter the bloodstream, raising risks of asthma exacerbation, heart attacks, and strokes. Ozone irritates airways, reduces lung function during exercise, and increases hospital visits on hot, stagnant days.

Air quality is summarized by an Air Quality Index (AQI) that blends measurements of PM₂.₅, PM₁₀, O₃, NO₂, SO₂, and CO. Fog visibility is measured by transmissometers or forward‑scatter sensors reporting MOR; airports also report runway visual ranges alongside cloud ceilings from ceilometers.

Forecasting and Nowcasting

  • Set‑up recognition: Forecasters look for clear skies, light winds, and moist low‑level air for radiation fog; onshore flow over cold surfaces for advection fog; persistent upslope flow for hill and mountain fog; and inversions or stagnation under high pressure for smog.
  • Profiles and guidance: Soundings (radiosondes) and model profiles show inversions, surface dew‑point depressions, and wind speeds through the lowest kilometer. High‑resolution models predict fog likelihood using microphysics and surface energy balances; urban schemes account for heat islands.
  • Remote sensing: Satellites detect low clouds and fog at night using fog/stratus RGB composites that exploit thermal contrasts; daytime visible imagery and surface observations refine coverage. Lidar and sodar profile mixing and aerosols. For smog, ground networks and satellites monitor NO₂ columns, aerosol optical depth, and fire smoke transport.

Safety and Practical Guidance

Driving in fog: Slow down; use low‑beam headlights (high beams backscatter and worsen glare); follow pavement edge lines, not taillights; increase following distance; avoid sudden stops. In dense fog, consider delaying travel; use hazard lights only if stopped off the roadway.

Operating near water or mountains: Expect sudden fog banks with wind shifts; carry navigation aids and charts; be prepared for temperature drops and condensation on equipment.

Freezing fog precautions: Treat surfaces early with anti‑icing agents; walk with traction aids; clear steps and handrails; in aviation, monitor holdover times and runway friction.

Smog and health: On high‑AQI days, reduce outdoor exertion, especially for sensitive groups (children, elderly, those with heart or lung disease). N95/FFP2 respirators reduce inhaled PM₂.₅; they do not remove ozone—activated‑carbon filters help with gases. Indoors, use HEPA purifiers (and carbon media if ozone/odors are issues), keep windows closed during peaks, and time ventilation for cleaner periods.

Climate and Long‑Term Trends

Climate variability and change influence fog and smog. Warmer nights may reduce radiation fog in some places by weakening nocturnal cooling, yet coastal marine fog trends depend on ocean upwelling, currents, and aerosols. Heat waves and stagnant high‑pressure systems can increase smog episodes, while stronger wildfire seasons export smoke widely. Urban growth modifies local fog by altering roughness, heat storage, and CCN.

A Simple Mental Model

Think of fog and mist as the atmosphere’s way of saying “the lowest air has run out of room for vapor.” Cooling from below or adding moisture tips it over the edge, and tiny droplets appear—how thick and persistent they become depends on inversions, wind, and droplet numbers. Think of smog as “chemistry trapped in a lid.” Emissions accumulate under a cap; sunlight and time transform them into eye‑stinging ozone and fine particles; a front, wind shift, or deep mixing scrubs the bowl and resets the air.

Closing Thoughts

The veil that hides a lighthouse at dawn and the haze that blurs a skyline at noon both tell stories about physics and chemistry close to the ground. Reading those stories—temperature, moisture, inversions, emissions, light—helps us travel safely, protect our health, and anticipate when the sky will clear. With better observations, cleaner energy, and thoughtful urban design, we can keep the mystery of morning fog while lifting the burden of unhealthy smog.