Water vapor measurements are presented which were performed with a newly developed airborne water vapor differential absorption lidar (DIAL). The transmitter of this DIAL system consists of an injection-seeded optical parametric oscillator at a signal wavelength of ≈940 nm capable of measuring very low values of water vapor (10 ppmv). The instrument was tested in an experiment where the aircraft crossed a potential vorticity streamer at about 12 km altitude. The downward looking DIAL can scan vertical cross sections in the atmosphere up to a range of 6 km, with a resolution of about 30 km in the horizontal and 250 m in the vertical. The measured data show significant water vapor structures of 5--100 ppmv within the stratospheric air of the streamer. The measured cross section is compared with fields simulated by a mesoscale model and analyzed from operational data. The comparison of the three data fields shows good agreement in water vapor structures and magnitudes. In addition, the streamer exhibits a clear structure in the aerosol backscatter lidar scan. ¿ 1999 American Geophysical Union