Air quality in cities is extremely variable. From smog to blue skies the biggest factor influencing air pollution is the weather.
Once pollutants are emitted into the atmosphere, the weather determines what happens next. The main factors are wind, humidity and temperature.
The most polluted cities are typically low lying or bordered by mountains – or in some cases both. For instance, Beijing and Delhi.
Terrain is a key factor in air quality, influencing the weather and sometimes reducing airflow.