Fog is a cloud that is in contact with the ground. The Atlantis expedition came across a planet covered in fog, which was later revealed to be energy beings. (SGA: "Home")
When investigating a planet believed to be one of Michael Kenmore's facilities, a mysterious fog rolled in, which was later revealed to be secreted by Michael's experiments. (SGA: "Whispers")
Lt. Matthew Scott, Chloe Armstrong and Eli Wallace came across a planet that had a large amount of fog on it, on their way back to Destiny. (SGU: "Human")