Lidar (light detection and ranging) is a technique, similar to radar, that works by beaming pulses of laser light through the atmosphere and detecting the light reflected by dust and other small particles in the air called aerosols. The time between the pulse and the echo determines the distance, and the shift in color of the light determines the velocity of particles along the line of sight. True wind speed and direction, even in clear air, can be found in this way. Long-used for ground-based observations of the atmosphere, lidar is now being carried by a new generation of Earth observing satellites.