In Situ Sensors -- Thin Film Capacitive Sensors -- Balloon-Borne Frostpoint-Hygrometry -- Application of Fluorescence Method -- Microwave Radiometry -- Fourier Transform Infrared Spectrometry -- Lidar -- Role of Networks and Long-Term -- Overview of Satellite Sensors -- Cominging Observation by Different Techniques -- Survey of Intercomparisons of Water Vapour Measurements.

Atmospheric water plays a key role in climate. Water vapour is the most abundant greenhouse gas and its condensed forms exert a profound influence on both incoming solar and outgoing infrared radiation. Unfortunately, accurate, height-resolved global-scale measurements of atmospheric humidity are difficult to obtain. The change in concentration of five orders of magnitude from the ground to the stratosphere means there is no standard instrument that will measure everywhere. This has led to different measuring techniques, all with strengths and weaknesses. This book assesses all presently available techniques that are used in monitoring networks. Special weight is given to the different technical concepts, the accuracy of different sensor types, calibration issues and retrieval aspects.