When a city is slightly below the elevation of its surroundings (a valley), the region around the city will cool at a higher altitude. This blankets the atmosphere inside of the city, leading to an inversion. Usually, hot air rises above cold air, but the valley creates a space where hot air is trapped under a 'dome' of colder air. This traps pollution, particulates, and smoke, leading to exacerbated conditions inside the city. Coupled with the urban heat island that creates higher temperatures within city areas, the temperature inversion causes a large threat. 

Temperature inversions are prominent in the winter, where the natural surroundings cool at a faster rate and lead to larger differences between surroundings and city. This can lead to smog/hazy conditions that create regions of poor air quality within the city. Having experienced this firsthand, I was shocked by the scale at which natural forces could dominate our daily lives. It was truly humbling and showed the necessity for greater climate action in cities.