The Balloonborne Cloud Observing micrOLidar (BeCOOL) is an airborne profiler designed to be operated from the stratosphere. It weights less than 6 kg and requires on average less than 4 W to work automatically onboard a gondola attached below a stratospheric balloon. Given its sensitivity and spatial resolution, this microlidar can be used to characterize the optical properties and the life cycle of cirrus clouds, or to calibrate and validate satellite observations of clouds and aerosol layers.