In a first part, results from a recently developed Lagrangian climatology of extratropical troposphere-to-stratosphere transport (TST) are presented for the ECMWF 15-year reanalysis period (1979-1993). It is shown that TST is most frequent in the Arctic and in the northern parts of the oceanic storm tracks. TST events occur typically in regions where the tropopause has been slightly lifted above its climatological altitude and is therefore some 5 K colder than normal. This reduces significantly the upper bound for the amount of water vapour that can be transported into the extratropical lowermost stratosphere via TST. Furthermore, the temperature variations associated with TST events provide a reasonable measure for the high variability of water vapour mixing ratios in the lowermost stratosphere. In a second part, high-resolution measurements of H2O from three aircraft campaigns in the framework of the SPURT (trace gas transport in the tropopause region) project are investigated. Backward trajectories indicate episodes of recent TST that are associated with enhanced water vapour mixing ratios. The detailed findings from the observations are compared with the general results from the long-term exchange climatology; and the relationship between tropopause temperature, time since the exchange and measured H2O mixing ratios is analyzed for the observed TST events.