Fog forms when the air near the ground becomes so full of water vapor that it can’t hold any more, so the vapor condenses into tiny suspended droplets—essentially making a cloud at ground level.

What fog actually is

  • Fog is a cloud that forms right at or very near the Earth’s surface, made of tiny liquid water droplets (or ice crystals in very cold conditions).
  • It reduces visibility to less than about 1 kilometer; if visibility is better than that, it is usually called mist or haze rather than fog.

Core cause: air reaching saturation

  • Fog appears when air temperature cools to the dew point, or when extra moisture is added, so that the relative humidity reaches 100% and condensation starts.
  • This saturation can happen through three main processes: cooling the air, adding moisture, or mixing two air masses with different temperatures and humidities.

Common ways fog forms

  • Radiation fog : On clear, calm nights the ground loses heat, cooling the air just above it until it reaches the dew point and droplets form, especially in valleys and low-lying areas.
  • Advection fog : Warm, moist air moves (advects) over a colder surface like cold ocean water or snow, the air cools from below, and fog forms along coasts and over cold currents.
  • Evaporation/steam fog : Cold air passes over warmer water or wet ground, extra moisture evaporates into the colder air, and that moist, cooling layer becomes saturated.
  • Upslope/valley fog : Air is forced slowly up a slope or trapped in valleys; as it rises or pools it cools and reaches saturation, sometimes persisting for days.

Conditions that favor fog

  • Light winds help keep moist air near the surface; too much wind mixes drier air down and prevents fog, while no wind at all favors dew instead of suspended droplets.
  • High humidity, wet soils, nearby water, and recent rain add moisture, making it easier for the air to reach saturation as temperatures fall at night or when air flows over colder surfaces.

How fog goes away

  • Fog usually lifts or thins when the air warms above the dew point (for example after sunrise) or when drier air mixes in, lowering the relative humidity below 100%.
  • Stronger winds can also disperse the droplets by mixing them into a deeper layer of drier air, turning dense fog into low cloud or just hazy conditions.

Information gathered from public forums or data available on the internet and portrayed here.