Water vapor is one of the most important species in the middle atmosphere. It plays many roles in chemical and radiative processes. However, measurements of stratospheric and mesospheric water vapor are very sparse in time and in space. This makes it difficult to estimate long-term changes, especially since the variation of water vapor can become very large in certain geographic or altitude regions.

For several years, the Institute of Applied Physics (IAP) has provided measurements of water vapor in the middle atmosphere on a campaign basis with an airborne microwave radiometer. This radiometer observes a strong water vapor emission line at 183 GHz which is ideal for these airborne measurements. Altitude profiles of stratospheric H₂O in the range of 20-60 km along the flight track can be derived from the measured spectra. Since 1998, the instrument has taken part in campaigns like THESEO 1999 or THESEO 2000/SOLVE on a yearly basis. A typical campaign took about one week and covered almost all of the northern latitudes over Europe from the tropics to the arctic. The flights took place during all seasons.

Our data set allows us to look at changes in stratospheric water vapor over a large geographic area over a period of several years. Flights in summer show mostly climatological distributions of stratospheric H₂O which are very good to estimate long-term changes. In contrast to that, the flights in winter and spring show a high variability of the water vapor distribution, both in altitude and horizontally. The profiles show interesting dynamic features especially in the arctic stratosphere. Downward motion of air masses and evidence of the polar vortex are visible up to an altitude of more than 50 km.