The fog that drifted back across Melbourne on Tuesday afternoon was a side-effect of Melbourne's own special weather phenomenon, known as the 'Spillane Eddy'. The Spillane Eddy occurs under light synoptic wind conditions, often when a large high pressure system is over Melbourne in winter. On Tuesday morning, light northeast winds flowing down off the ranges to the north and east of Melbourne carried fog areas down over Port Phillip Bay. With the sun being on a low angle at this time of year and offering little in the way of solar heating, the fog lingered through the day. A very weak southerly sea breeze then carried the fog back across the city during the afternoon. In some circumstances, this pattern can be repeated day-after-day, resulting in a recirculation of air pollution from cars and industry. Often the EPA will issue 'smog alerts' when this occurs. Air quality was only listed as 'fair' across many suburbs on Tuesday. While fog was still present across Melbourne on Wednesday morning, it should quickly clear as northerly winds increase ahead of a cold front moving into the Bight. There will be no return of the fog this afternoon due to the increasing winds.