Six radar wind profilers were deployed around Houston for the Texas 2000 Air Quality Study. Mixing depths (boundary layer height or zi) were derived from the profiler reflectivity. Maximum mixing depths varied from day to day by more than 50%. The temporal pattern of mixing depths was generally similar at all sites except that the LaMarque site, southeast of Houston near Galveston Bay and nearest to the Gulf coast, had lower mixing heights on days with persistent southerly flow. The afternoon transition from convective to stable conditions is commonly dependent on the sea breeze, which reaches the different sites at different times in the afternoon. These phenomena are presented and discussed in the context of their implications for the air quality goals of the study.