WIND STRUCTURE IN RELATION TO ODOR PLUMES IN TSETSE-FLY HABITATS

Key characteristics of airflow were measured in the African bush in a study of host odour plume structure. Wind speed, speed variance, direc tion, and directional variance were measured by conventional cup anemo meters plus wind-vanes and by a solid state ultrasonic anemometer, on time scales from seconds to minutes. The two technologies gave opposit e relationships between wind speed and turbulence measured as rare of angular direction change in the wind ((o) s(-1)). A positive correlati on between turbulence and wind speed was observed with mechanical anem ometers and wind-vanes, evidently caused by their inherent hysteresis (stalling in weak wind, overswinging after gusts). The same correlatio n was negative with the solid-state anemometer which, being hysteresis free, should have measured the true directional turbulence more accur ately. Such fine-scale turbulence at a fixed point in space (on a scal e of about similar to 15 cm diam.) decreased with wind speed up to sim ilar to 1.5 m s(-1), as does large-scale (similar to 1 m diam.) turbul ence of air moving through space (Brady ed al., 1989). This decrease o ccurred both within vegetation and out in the open, but the slope and intercepts of the relationship depended on vegetation and topography. Variables for describing wind speed and turbulence are considered in t he context of odour plume structure.