The CS135 is an elastic lidar. It emits short pulses of near infra-red light into the atmosphere from a semiconductor laser. These are scattered back by aerosols including cloud droplets. The time between transmission of the pulse and the return signal gives the range, and therefore height, of the scattering aerosols. The variation in the strength of the backscattered light signal with height gives a profile of scatter coefficients and allow cloud bases to be identified. If significant scattering is detected but there is no defined cloud base then a vertical visibility can be calculated.