Abstract : Urbanization is responsible of the urban heat island which is characterised by a higher temperature in the urban areas than in the surrounding rural areas. This phenomenon has been studied for two field experiments. The first one took place in Marseille during summer 2001. An optimized network was spread out to assess the urban thermodynamic island of this coastal city. During the night, urbanization was the main factor controlling the spatial variability of air temperature but during the day, this parameter was controlled by the sea/land transition. During daytime, measurements of turbulent fluxes are affected by this cool and moist flow. The second campaign was carried out in Toulouse on a one-year period between February 2004 and March 2005. The impact of anthropogenic heat wastes on the surface energy balance during winter has been studied. They can reach 100 W m-2 and compensate more than half of the solar energy deficit.