Sydney-siders awoke to a murky morning with fog, mist and haze spread across the Sydney basin Friday morning. The prime spots for winter fog saw the lowest visibilities of less than 50 metres at Richmond and 150 metres at Campbelltown before the Friday rush-hour. Further east, the day started with mist, causing visibility to drop as low as 1300 metres at Bankstown and 2500 metres at Holsworthy. Even as the fog and mist lifted, the skies stayed hazy upon sunrise. But what caused the haze and what's the difference between haze and fog? This can be explained by thinking about what happens overnight. As was proved last night, temperatures cool best when the skies are clear and the winds are light. Where there is enough moisture, or where temperatures drop the most, mist and fog forms. Essentially a cloud forms on the ground. At the same time, with all this cold air sinking to the surface, temperatures actually increase with height in the atmosphere. This is what meteorologists call an 'inversion'. Sometimes called a capping inversion, the lowest layer of the atmosphere essentially becomes a tin with a lid. The lid acts to trap pollutants, dust and smoke from hazard reduction burning. High pressure, or anticyclones, like the one sitting over the east coast at the moment can enhance this lid, trapping more pollution for longer periods. Strong anticyclones during winter often cause the worst pollution days for cities around the world. Fortunately for Sydney, the high pressure isn't too strong leading to improved conditions this afternoon. Showers and thunderstorms are expected for parts of the city on Saturday evening which will help to bring better air quality.