How Smog Forms Over Cities
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Smog is a mix of pollutants rather than a single substance, combining particles, ground level ozone and other reaction products suspended in the air. Vehicle exhaust and industrial emissions supply the raw ingredients, and sunlight often drives the chemical reactions that turn those emissions into visible haze.
Geography and weather play a large role in how smog behaves. Cities in valleys or basins can trap pollutants beneath a layer of warmer air, a pattern known as an inversion, which prevents the usual upward mixing that would otherwise disperse emissions into the wider atmosphere.
This is why smog tends to build over several still, sunny days and then clear quickly once wind or rain arrives. The pattern explains why the same city can have strikingly different air quality from one week to the next, without any change in how much pollution is actually being emitted. It also explains why forecasts pay close attention to wind and pressure patterns, since these often predict smog severity better than emissions data alone.