A laser radar (lidar) that can determine radial velocity (velocity toward or away from the laser) of the air by measuring the frequency shift of the returning light that was scattered from atmospheric aerosols, compared to the original transmitted frequency. Velocities of individual convective thermals and large-diameter turbulent eddies can be measured. Most lidar signals are blocked by clouds and fog, and are greatly attenuated in very polluted air. See Doppler effect.