Perth and Melbourne shared a common problem as the sun rose above the horizon this morning. The sky wasn't completely visible. A road weather alert was issued in Perth this morning as thick fog caused dangerous driving conditions. Visibility was reduced to just 50 metres in Pearce at 7am WST and below 3000 metres at Perth Airport. The main ingredients needed for fog to form are moisture, light winds and sufficient cooling near the surface. A unique set of weather features conspired to bring all three of these in Perth this morning. Firstly, west to southwesterly winds developed on Wednesday afternoon, allowing moisture-laden air to flow across the city. Winds then became light during the evening, which allowed the surface temperature to drop. As the air cooled, airborne water vapour was condensed into liquid droplets, which were suspended in the air as fog. Perth Airport typically experiences around 10-11 fog days between April and October each year, with the highest frequency occurring between May and July. Thick fog this early in the year is not common. While Perth was shrouded in fog his morning, the sky over Melbourne was also a bit murky. Smoke from planned burns in central Victoria was carried over the city by light northerly to northeasterly winds overnight. While the smoke helped produce a golden sunrise this morning, it also reducing air quality in the city. According to the Environmental Protection Authority of Victoria, air quality was very poor in parts of eastern and western Melbourne. This is the highest category on the state's air quality index. Smoke will linger in Melbourne today but should clear from tomorrow as northerly winds start to strengthen ahead of an approaching front.