Urban green demonstrably lowers the air temperature in a city. A direct link can also be made between the formation of smog and air temperature. Warm air carries pollution and particulate matter to higher layers of air, causing a cloud of smog. Cooler air from outside the city is heated at the city limit and does not penetrate into the city centre. At night this warm air pocket causes an inversion, resulting in an inability of the city to cool down, and clean air from outside of the city is not able to mix in. Krusche et al., 1982
