Observations at the San Juan, Puerto Rico international airport show that the annual mean temperature has increased by about 2.1°C (3.8°F) from 1956 to 1983. The chief contributors to the increase are an increase in daily minimum temperature from 1956 to about 1970 and an increase in daily maximum temperature from about 1970 to 1983. In addition, there is evidence for veering of the wind direction so that overland trajectories are more frequent. No comparable temperature change has been measured at a cooperative station 8 km to the southwest in the city of San Juan nor at other stations in Puerto Rico, thus ruling out a synoptic-scale phenomenon. It is hypothesized that as the airport expanded in size and the vegetation diminished, a mini heat island evolved. The heat island combined with the development of adjacent residential and commercial areas and a changing wind direction account for the observed annual mean temperature increase.

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