Urban growth implies significant modifications in the urban climate. To understand the influence of the city of Lisbon on the urban boundary layer, a mesoscale meteorological network was installed in 2004. The main goals of the present study are to update the results of the research published in 2007 and to bring more precise information about the relationship between the Urban Heat Island (UHI) and the regional and local wind systems. The highest frequencies of the UHI were found in the city centre (Restauradores). In the green park of Monsanto, the highest frequency occurred between ？2 and 0°C. During the summer, the effect of the breezes was observed in Belém, lowering the temperature. The “strong” UHI (intensity >4°C) occurred more often during the summer, with median values of 2°C by night and 1.8°C by day. The highest frequencies of UHI occurred for winds between 2 and 6？m/s and were not associated with atmospheric calm, as pointed out in the literature. Winds above 8？m/s inhibit the occurrence of strong UHI in Lisbon. Summer nighttime strong UHI should be further investigated, due to the heat stress consequences on the population and probable increase of energy consumption. 1. Introduction Urban growth and land use changes imply considerable modifications on the atmospheric dynamics and induce local climatic changes. The consequent changes in aerodynamic roughness also induce alterations in the wind flows, which act as one of the major factors in urban climate and microclimates [1]. The authors of this study [1] found a reduction of about 30% in the mean wind speed in the Lisbon urban boundary layer, due to both topography and built-up areas, when compared with the wind speed simulations where only regional winds and the local topography were taken into account. These values may reach 40%, if the roughness length increases to 1.5？m windward of the city. This will contribute to increase the heat stress and urban atmospheric pollution, with major negative consequences on human health and quality of life for the urban population. In recent years the problems caused by urban climate modifications led to an increasing number of studies about thermal bioclimates [2–4], also bringing about new ideas and solutions to solve them [5–8]. Amongst the climatic effects of urbanization, the urban heat island (UHI) pattern is the best documented example of involuntary human climatic modification [2, 9–11]. The urban heat island is defined by the difference of temperature between the city centre and the surrounding areas [12]. This clear and simple definition has
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