An acoustic radar (sodar) that can determine radial velocity (velocity toward or away from the sodar) of the air by measuring the frequency shift of the returning sound waves that were scattered from regions of turbulent fluctuations of air temperature, compared to the original transmitted frequency. Velocities within individual surface-layer plumes and the bottom of convective thermals can be measured. Most sodar signals are blown away in strong winds, and the sodar also requires acoustic shielding from outside noise such as from roads, towns, and rustling tree leaves. See Doppler effect, acoustic sounder, sodar; compare RASS.