A UV Raman lidar system (BASIL) is operational at DIFA-Univ. of Basilicata (Potenza-Italy). The system was recently involved in LAUNCH 2005 – the International Lindenberg campaign for assessment of humidity and cloud profiling systems and its impact on high-resolution modelling - held from 12 September to 31 October 2005. During this period BASIL collected approx. 250 hours of measurements distributed over 13 Intensive Observation Periods (IOPs) and 25 days. One specific IOP was continuously run between 1-3 October 2005, covering a dry stratospheric intrusion episode associated with a tropopause folding event and the subsequent onset of perturbed weather conditions that leaded to the development of clouds and precipitations. Tropopause folds are the dominant and most efficient mechanism of stratosphere-troposphere exchange (STE) in the middle latitudes [1]. Intruding stratospheric air forms filamentary features in ozone and water vapour profiles [2]. The use of water vapour to trace stratospheric air intrusion allowed to clearly identify a dry structure (approx. 1 km thick) originated in the stratosphere and descending in the free troposphere down to ~ 3 km. A similar feature was present in the temperature field, with lower temperature values observed within the dry air tongue. Relative humidity measurements revealed values as small as 0.5-1 % within the intruded air. The stratospheric origin of the observed dry layer has been verified by the application of a Lagrangian trajectory model. Lidar measurements have been also compared with forecasts from a MM5 mesoscale model. Comparisons in term of water vapour reveal the capability of the model to forecast the deep penetration in the troposphere of the dry intruded layer. Results from the comparison of lidar and model data will be discussed at the conference.