ESA’s Aeolus satellite carries the Atmospheric Laser Doppler Instrument (Aladin). A lidar uses the phenomenon of light scattering and the Doppler effect to acquire data on wind. Its lidar works by emitting a short, but powerful, light pulse from a laser through the atmosphere and then collects light that is backscattered from particles of gas and dust and droplets of water in the atmosphere. The time between sending the light pulse and receiving the signal back determines the distance to the ‘scatterers’ and thus the altitude above Earth. Rayleigh scattering refers to the scattering of light from molecules of air, such as oxygen and nitrogen. Rayleigh scattering is most efficient at scattering light at shorter wavelengths, i.e. blue and violet – which is why the sky looks blue. Mie scattering describes light scattering from particles larger that the wavelength of light. For more information see: Rayleigh and Mie Scattering. As the scattering particles are moving in the wind, the wavelength of the scattered light is shifted by a small amount as a function of speed. The Doppler wind lidar measures this change so that the velocity of the wind can be determined.