It has certainly felt more autumnal in the last few days. High pressure has brought settled, calm conditions and barely a breeze. This combined with mild, moist air from the south-west (or Azores) and the increasingly lengthy nights has meant prime conditions for mist and fog to form.
The low cloud, mist and fog was extensive last night - thickening up through the early hours to give terrible visibility through the morning rush hour.
It will be a similar scenario into tomorrow morning, with much of the fog only slowly dispersing from rural spots. It will be a slow commute or crawl to work in first thing in the morning.
Mist and fog form in a similar process to clouds - and are essentially made up of air condensing nearer the ground. Fog is much thicker than mist and can take nearly all morning to clear at this time of year. Fog reduces visibility to less than 1km, while mist reduces visibility to no less than 1km.
As the breeze picks up later in the week - mixing up the air - mist and fog will be less of a problem.