Fog increases during the in-game night (approximately 11 PM) and decreases at dawn (approximately 9 AM). Fog in places such as the Lost Cave, Maze, and the End Times Biome are special because the fog is much closer and darker than above the surface, but still appears to recede or fall in respectively to the time of day.
During the Beta Testing Stage, the fog decreased, possibly only 1,000 studs from a player's sight, and was a slightly lighter shade of purple than it is now. Later in the stage, it decreased, only 300 studs away from a player's vision. This is now the current and present setting of Fog in the game.