Data standing in fog on the Enterprise bridge.
Laas as fog on the Promenade of Deep Space 9.
Fog is a concentrated suspension of water vapor, and is formed when water condenses into tiny liquid water droplets in the air.
Fog is highly unlikely on a planet with no cloud formations, bodies of water, or changes in surface temperature. Despite lacking these features, Korob created fog on the planet Pyris VII by telepathically drawing the image of fog from the USS Enterprise landing party in 2267. (TOS: "Catspaw")
Several years later in 2270, the Enterprise computer created fog on the bridge of the ship after a strange space cloud caused it to gain artificial intelligence and start playing practical jokes on the crew. (TAS: "The Practical Joker")
In 2370, fog suddenly rolled in on the Bridge of the USS Enterprise-D when the ship's environmental controls malfunctioned due to feedback from one of Caldos II's weather substations. Data estimated the fog's temperature to be one degree Celsius. (TNG: "Sub Rosa")
Michael Eddington described the planet Athos IV, located in the Badlands, as "a grim little fogbound piece of rock in the middle of nowhere." The heavy fog on the planet made it difficult for Eddington and Captain Sisko to identify Jem'Hadar soldiers guarding the Maquis compound. (DS9: "Blaze of Glory")
Laas, a Changeling, turned himself into fog on the Promenade of Deep Space 9 in 2375, much to the displeasure of many around him. (DS9: "Chimera")
In 2376, Tom Paris created a holodeck program of a 19th century Irish town called Fair Haven, complete with fog. Harry Kim thought the program needed more fog to make the program authentic, but Tom disagreed as he found fog depressing. Tuvok had no opinion on the matter. (VOY: "Fair Haven")