 Sodar systems are used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere.Radar and lidar are not passive because both use electromagnetic radiation to illuminate a specific portion of the atmosphere.Weather satellites along with more general-purpose Earth-observing satellites circling the earth at various altitudes have become an indispensable tool for studying a wide range of phenomena from forest fires to El Niño.