A climatological analysis of the differences of air temperature between rural and urban areas (dT(U–R)) corroborates the existence of an urban heat island (UHI) in the tropical coastal city of San Juan, Puerto Rico that has been increasing at a rate of 0.06 1Cyear
1 for the last 40 years with predicted differences as high as 8 1C for the year 2050. The Regional Atmospheric Model System (RAMS) was used to validate the presence of this UHI and to simulate and compare three different land use scenarios consisting of potential natural vegetation, present, and projected future to quantify the impact of the urban development in the regional climate of Puerto Rico. RAMS simulated the UHI conditions at the lower and upper atmosphere revealing significant changes in sensible heat fluxes and sinks, and an increasing low turbulent-kineticenergy zone (LTKEZ) over the urbanized area of San Juan.