One of the conditions for fog forming is a disturbance of the air. In the parks, in wind free conditions we might expect the parks to remain clear whereas traffic moving in the city, coupled with thermal uplift, would be enough to generate fog.
Many years ago the airfield was covered in a very heavy hoar frost. It was calm and no cloud and a brilliant day. In the eastern corner of the airfield 3 aircraft ran up their engines for checks. When we deplanned a few minutes later it was in a complete pea-souper that persisted for hours.
It had simply been initiated by our engine runs.