Beautiful and breezy cities on small tropical islands, it turns out, may not be exempt from the same local climate change effects and urban heat island effects seen in large continental cities such as Los Angeles or Mexico City. A surprising, recent discovery indicates that this is the case for San Juan, Puerto Rico, a relatively affluent coastal tropical city of about two million inhabitants that is spreading rapidly into the once-rural areas around it. A recent climatological analysis of the surface temperature of the city has revealed that the local temperature has been increasing over the neighboring vegetated areas at a rate of 0.06°C per year for the past 30 years.This is a trend that may be comparable to climate changes induced by global warming. These results encouraged the planning and execution of an intense field campaign in February 2004, referred to as the San Juan Atlas Mission, to verify the spatial and temporal extent of this urban heat index. Results of this field campaign recently have been analyzed and are the main topic of this article. These results reveal the warming of a tropical coastal city that is significantly higher than typical temperatures in vegetated areas.This may be the first set of high-resolution thermal images taken in a tropical coastal city. Figure 1 shows that the daytime surface temperatures of a portion of San Juan at fi ve-meter resolution are as high as 60°C,and that differences between urbanized and limited vegetation areas are in excess of 30°C.