Fog in Adelaide this morning reduced visibility to just 100 metres at the airport, and despite it is starting to lift early flights may still be delayed. Fog was thickest at about 3am in cold and very still air. By 4:30am a light breeze caused it to thin out enough and this improved visibility from 100 metres to 900 metres. But by 5am the wind dropped off enough to allow fog and low cloud to thicken and return visibility to 300 metres. The temperature at the time was only 5.6 degrees. Fog has been widespread across the Adelaide Plains and Hills overnight after recent rain. It will take longer to clear from sheltered areas further from the coast, making driving hazardous for a few more hours. Visibility at Parafield has improved from 150 metres at 4am to 400 metres at 5am. Visibility should improve enough by about 8am throughout.