Climatic impact of big cities

Cities occupy vast areas which sometimes extend over areas of several thousand km² and which have up to several million inhabitants. These vast spaces can give rise to specific meteorological phenomena: urban thunderstorms, artificial snowfall, smog (polluted fog, Figure 1), etc... It is not surprising if they are also able to significantly influence air temperatures. On an annual scale, town are hotter than their countryside from several degrees (annual mean difference). During some special meteorological conditions however, the temperature differences can be as large as more than 10°C. This is specially the case for big cities (Figure 2).