For each city, the exact causes of pollution may be different. Depending on the geographical location, temperature, wind and weather factors, pollution is dispersed differently. However, sometimes this does not happen and the pollution can build up to dangerous levels. A temperature inversion occurs when air close to the earth is cooler than the air above it. Under these conditions the pollution cannot rise and be dispersed.
Cities surrounded by mountains also experience trapping of pollution.
Inversion can happen in any season. Winter inversions are likely to cause particulate and cabon monoxide pollution. Summer inversions are more likely to create smog. Another consequence of outdoor air pollution is acid rain . The effects of acid rain on the environment can be very serious.