Fictional Land - How can I keep dense fog in an area for half a day? This question is now here on Worldbuilding Stack Exchange 


I am a writer, and this question is for a fictional work. However, given its nature, I believe it is better suited here than Writers Stack Exchange. 

In my fictional world, I have an area that is supposedly covered with fairly thick fog during most of the night and nearly all of the morning (it can leave a bit before noon if necessary). My question is this: What factors would I need to achieve this effect? 

Factors: 
It is a fantasy world, so the factors need to be natural. The area is within a city, and is very densely inhabited. The area is enclosed by large trees, and large roots run all over the ground. The fog does not need to reach any higher than 7 feet, but it does need to cover nearly the whole area (a generous approximation would be 288 ft by 228 ft - about 65,000 square feet). The walls of the area (giant roots/tree trunks) vary in distance from each other a lot. 

Let me know if you need more details on the surroundings.
The heat of the Sun can "burn off" fog. So a lack of direct sunshine, cool temperatures &amp; humidity (evaporation of water from a nearby body of water or saturated or wet ground) are key to maintaining fog. The air must be moist!