In the Differential Absorption LIDAR (DIAL) method for the measurement of water-vapour profiles laser pulses of two different frequencies are used. One frequency is chosen in a region of high absorption cross section of the gaseous constituent under study, whereas at the second frequency the gaseous absorption should be minimal. If aerosol backscatter and extinction properties are the same for both frequencies, the ratio of the received power at online and
offline frequency only depends on the density of the species under study and the differential absorption cross section. In the case of water vapour the differential absorption cross section in dependence on atmospheric pressure is well known and the DIAL measurement yields the water vapour density. This water-vapour DIAL is operated by the Max-Planck Institute of Meteorology.