In the wake of widespread rain, nights and mornings are being obscured by fog across much of southeastern Australia, including the big cities. Decent, recent rain has given fog season a boost and it is now up to clearing skies and light winds to encourage night-time cooling. Rain ended up being the heaviest in decades for this time of year in some areas, leaving the ground more moist than usual. Typical for this time of year, cool nights help moisture in the air near the ground to condense and form fog. During the next few nights and mornings we can expect to have fog develop in many areas from north Queensland to Tasmania. While fog will generally be thickest in the most prone places, those elevated yet sheltered from the wind, including Orange and Canberra, fog is also likely in parts of Brisbane, Newcastle, Sydney, Canberra, Melbourne and Launceston. Visibility has potential to drop to just a few hundred metres which is low enough to delay flights and even slow down those on the roads. This morning visibility was reduced to just 50 metres in Griffith and 100 metres in western Sydney, Orange, Mildura, Shepparton, Toowoomba and Roma. During the week the fog risk will lessen when fronts pass through and stir up the air (mid-week in the south), but will increase again late in the week when a high pressure system resettles.