Fog forms when the air near the ground becomes so cool and moist that water vapor condenses into tiny suspended droplets, creating a low cloud that reduces visibility.

What fog actually is

Fog is essentially a cloud sitting on the ground, made of countless microscopic water droplets (or sometimes ice crystals) floating in the air.

When enough of these droplets accumulate, they scatter light and make it hard to see, typically cutting visibility to less than about 1 km.

The basic recipe: cool + moist air

Fog needs two main ingredients:

  • Moist air (plenty of water vapor present).
  • Cooling of that air until it reaches its “dew point” – the temperature at which water vapor starts to condense into liquid droplets.

Once air temperature and dew point are very close (often within about 2–3 °C), water vapor condenses on tiny particles in the air like dust, salt, or pollen, forming the fog droplets.

Common ways fog forms

There are several classic “setups” that cool and saturate the air enough for fog.

  1. Radiation fog (calm, clear nights over land)
 * After sunset, the ground loses heat to space and cools rapidly.
 * The air touching the ground cools with it until it reaches the dew point.
 * Tiny droplets form, especially in valleys and low spots where cool air collects.
  1. Advection fog (moist air moving over a colder surface)
 * Warm, humid air flows over colder land or water.
 * The lower layer of that air cools to its dew point, and fog forms, often along coasts or over cold currents like near Newfoundland.
  1. Evaporation / steam fog (cold air over warmer water)
 * Cold air moves over warm, wet surfaces (like lakes or newly opened sea ice).
 * Water evaporates into the cold air, adding moisture until it’s saturated, and condensation appears as wispy fog just above the surface.
  1. Upslope fog (air pushed up terrain)
 * Moist air is forced up a hill or mountain.
 * As it rises, it cools, reaches the dew point, and forms a blanket of fog along the slope.
  1. Frontal fog (near weather fronts)
 * Rain from warmer air falls into cooler air near the surface.
 * The raindrops partly evaporate, adding moisture until that cooler air becomes saturated and foggy.

Why fog appears and disappears

Fog sticks around when:

  • The air stays near its dew point (cool and moist).
  • Winds are light (strong winds tend to mix the air and break up the fog).
  • There’s no strong sunlight to warm and dry the air.

Fog dissipates when:

  • Sunlight warms the ground and the air, lifting the temperature above the dew point.
  • Drier air mixes in and lowers humidity.
  • Stronger winds stir the fog layer and spread the droplets out so they’re no longer dense enough.

Mini example: a foggy autumn morning

On a clear autumn night, the ground cools quickly and chills the air just above it.
If the air was already fairly humid, that cooling brings the temperature down to the dew point, causing water vapor to condense into droplets and forming a shallow layer of radiation fog in the early morning.

After sunrise, the ground warms, the air temperature rises away from the dew point, and the fog gradually “burns off,” leaving clear skies.

TL;DR: Fog forms when moist air near the ground cools to its dew point so that water vapor condenses onto tiny particles in the air, creating a ground- level cloud that can form in different ways (nighttime cooling, air moving over warmer or colder surfaces, or along slopes and fronts). Information gathered from public forums or data available on the internet and portrayed here.