General Fog Information

Fog forms when moisture in air cools below its dew point and starts to condense.

Learn more about how fog forms in the video to the right!

Fog occurs frequently in the Bay Area, especially along the coast, like in San Francisco.

Fog Types

Fog is categorized by how it is formed.

  • Radiation fog or Ground fog is a relatively shallow fog that forms at night. It forms when the ground cools through radiation It may be dense enough to hide the entire sky or may conceal only part of the sky.[1]
  • Advection fog (see animation on left) forms when warm moist air moves over a colder surface and condenses. This is what happens in the Bay Area when warm air from the inland moves over the cooler Pacific Ocean and into San Francisco Bay.[1]
  • Upslope fog happens when moist air goes up a topographic barrier such as a mountain and cools to its dew point.[1]
  • Evaporation fog is when warm moist air mixes with cooler, dry air and saturates. One form is known as steam fog and you can also see it from your breath when the air is cold.[1]

Fog can be catastrophic. In 2001 118 people died at Milan's Linate airport when an airliner operated by Scandinavian airline SAS was accelerating for takeoff. It clipped the wing of a private plane that had strayed onto the wrong runway in heavy fog.[2]

SFO Facts

  • Fog can be a major problem at airports; SFO faces numerous delays per year.
  • Each year $4.2 billion is lost due to weather causing air traffic delays.[3]
  • Weather related crashes cost on average $42 billion annually in the US.[3]
  • SFO is ranked #7 out of 10 for the worst weather of US airports.[4]
  • SFO was the most delayed airport in the country in August 2010.[4]
  • Fog causes delays in SFO due to the spacing of the two parallel runways. On foggy days, at least 2500 ft of spacing is required for safety but the SFO runways only have 750 ft of spacing.[4]
  • In the Bay Area, fog rolls through the San Bruno gap and over the SFO airport as shown in the animation to the left.

Real life image of fog rolling over the San Bruno gap and onto SFO (

Benefits of Fog

  • Fog provides water for many types of vegetation such as redwood trees.
  • Parque Nacional Fray Jorge, Chile is located in desert conditions but plants and trees are able to thrive only in the "fog forest".[5]
  • Fog harvesting uses a mesh to condense and collect water.
  • In developing countries fog harvesting is a source of drinking water.[6]
  • In Mpumalanga, South Africa, a fog harvesting system was able to collect almost 800 l/d of water.[7]
  • In Chungungo, Chile 75 fog collectors were able to provide, on average, 33 L of clean water per person per day.[7]

Interesting Videos & Websites

  • An interesting video of fog rolling into San Francisco: Click here
  • A video of an airplane landing in SFO in the fog: Click here
  • Different types of fog in San Francisco: Click here
  • Fog rolling through the Golden Gate Bridge: Click here
  • This website has great time-lapse pictures of San Francisco as well as a live view: Click here
  • Bay Area fog forecasts: Click here
  • Another time lapse: Click here



This website is part of a design project from CEE 105, Applied Fluid Mechanics by Jennifer Frederick, Tracy Hoang, and Vasileios (Bill) Valatsos. For more information visit this page.