An experimental study was carried out on a working unglazed transpired solar collector (UTSC) to determine what effects ambient wind has on its performance. The monitoring system included instruments to measure temperatures, collector outlet flow rates, solar radiation, wind speed, and wind direction; as well as an ultrasonic anemometer placed near the centre of the collector. Efficiency was defined as the fraction of incident solar heat flux that went to preheating the transpired air. Our observations indicate a high degree of turbulence near the wall which feeds the near wall region. This is supported by observations of efficiency which decrease monotonically with increasing turbulence intensities. It was also observed that peak efficiencies did not occur at the lowest wind speeds. Both these findings seem to contradict existing laminar boundary layer models for UTSC performance. (C) 2002 Published by Elsevier Science Ltd.