Abstract. In this note we reassess the role of sonic thermometry in boundary-layer studies. The sonic temperature signal, when corrected for crosswind velocity contamination, very closely approximates the virtual temperature of air. This variable is needed for many boundary-layer calculations. We describe preliminary tests with a new sonic anemometer-thermometer that performs the velocity correction in real time. Our test results offer new insights into the nature of the velocity error on temperature standard deviations and fluxes. They also draw attention to the high noise threshold that appears as an f+1- rise in the fS(f) spectrum when spectral levels drop below 10-4°C2.