During winter and early spring in New Orleans, dense, early morning fog often occurs, making driving difficult for commuters. This happens most often when warm, moist air from the south moves in over land areas that have cooled down during the night.
When warm air rides over colder land, the air cools down and eventually drops to what is known as the dew point temperature, where the air is saturated with water and can’t hold any more. That’s when the water vapor must condense into water droplets.
Those droplets form a cloud right at the surface – and there’s your fog.
– Alexandra Cranford, WWL-TV