Pronounced filamentation of tropospheric and stratospheric air masses was observed on a flight in the vicinity of the polar jet stream during the STREAM 1998 campaign in Timmins, Canada. High-spatial resolution O3 and H2O measurements showed highly variable structures for the investigated region. Reconstructing high-resolution potential vorticity (PV) fields using the reverse-domain-filling (RDF) technique reproduce the in-situ O3 measurements qualitatively. Measurements and RDF results show the intrusion of stratospheric air penetrating deep into the troposphere. The observed intrusions occur on smaller horizontal scales than generally assumed for tropopause folding events. Moreover, fine-scale structures of tropospheric air masses being lifted up into the stratosphere could be reconstructed by the RDF technique using the tropospheric tracer specific humidity in addition to the more conventional calculations for the stratospheric tracer PV.