Middle atmospheric water vapour plays a key role in the Earth's radiative budget and in several chemical processes. Recent results in the published literature indicate an increase of the middle atmospheric water vapour load. However there is a lack of understanding of the mechanisms leading to such an increase and longterm, high quality measurements of middle atmospheric water vapour are therefore urgently needed. Ground-based microwave radiometry is a well suited technique to provide such longterm, high quality measurements. The ground-based microwave radiometer MIAWARA (Middle Atmospheric Water Vapour Radiometer) measuring the 22.235 GHz emission of the rotational transition of water vapour was recently developed at the Institute of Applied Physics, University of Bern (Switzerland) where it is currently operated. Middle atmospheric water vapour profiles can be retrieved from the pressure broadened line in the range of 20-70 km using a retrieval algorithm based on the Rodgers optimal estimation technique.

The instrument has started its operation in autumn 2001. In the future it is planned to operate it in the frame of the Network for the Detection of Stratospheric Change (NDSC). Results of the first operational period of the instrument will be presented.