URBAN CLIMATE AND SUMMER TEMPERATURES IN LISBON DURING THE 20TH CENTURY. TRENDS, NUMBER OF HOT NIGHTS AND DIURNAL TEMPERATURE RANGE. There is ample evidence of the cities’ effects upon the local climate and, in particular, upon temperature, by giving rise to the so-called “urban heat island” (UHI), which has a strong negative impact upon human bioclimatic comfort and health. In this study, temporal evolution of summer (June, July, August and September- JJAS) temperature trends in Lisbon are studied. During the course of the 21st Century, a temperature increase was found to have occurred in this city (most notably since 1980 and particularly as regards the mean minimum temperature), alongside an increase in the number of nights in which the temperature has reached values higher than 20 ºC and a reduction of the diurnal temperature range, all of which can be attributed to urban growth and/or global warming.