It increases during the in-game night (approximately 11 P.M.), and decreases at dawn (approximately 9 A.M.). Places such as the Lost Cave, Maze, Shrine of the Sight, and the End Times Biome, are special because it is much closer and darker than above, but still appears to recede or fall respectively to the time of day.
During the Alpha Testing Stage, it was very little and believed to be only seen 6,000 studs away from a player's perception.
When the Beta Testing Stage began, its range decreased, possibly only 1,000 studs from a player's sight, and was a slightly lighter shade of purple than it was during release. Later in the stage, it decreased again, limiting the player's vision to 300 studs.
Late in release, the fog was switched back to its early beta style (more purple, less range) after multiple users requested it to be changed, primarily on the basis of nostalgia.