Fog happens when the air near the ground cools enough, and has enough moisture, that invisible water vapor turns into millions of tiny suspended droplets, basically making a cloud at ground level.

What fog actually is

Fog is just cloud at the surface rather than high up in the sky.

It’s made of tiny liquid water droplets (or sometimes ice crystals) floating in the air, which scatter light and make everything look hazy or hidden.

Why it gets foggy

Fog usually forms when:

  • The air cools down to its “dew point” so it can’t hold any more water vapor. The excess vapor condenses into droplets and creates fog.
  • There is plenty of moisture in the air already (after rain, near lakes/oceans, over wet ground, or above melting snow).
  • There are tiny particles like dust or pollution in the air for water to cling to and form droplets.

Common situations you notice fog

  • Early morning / late night : Ground cools overnight, cooling the air right above it until it reaches the dew point, so fog appears and then “burns off” as the sun warms things back up.
  • Near coasts (like San Francisco) : Warm, moist ocean air moves over cooler water or land; as it cools, fog forms and is blown inland (called advection fog).
  • Over snow in winter : Warmer, moist air over cold snow gets cooled, the air saturates, and fog develops.

Why fog goes away

Fog disappears when:

  • The sun warms the ground and the air, lifting the temperature above the dew point so droplets evaporate. This is why people say fog “burns off.”
  • Drier air moves in or winds pick up, mixing the moist layer with drier air and thinning the fog out.

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