Ground-based lidar measurements of ozone mixing ratio and aerosol backscatter profiles above Fritz Peak Observatory near Boulder, Colorado (39.9 ¿N, 105.3 ¿W), are used in conjunction with nearby radiosonde profiles and GOES-9 satellite water vapor imagery to describe the mixing of a dry, ozone-rich streamer of stratospheric air into the upper troposphere over the western United States on June 30 and July 1, 1997. The satellite images show that the streamer extended over more than 2000 km from the eastern Pacific Ocean to the central United States beneath the subtropical jet with a width of ~200 km over Colorado. The streamer persisted with little change for more than 6 hours of lidar observations and then abruptly disappeared to be replaced by a region of unusually low ozone. The disappearance of the streamer is attributed to localized convective activity induced by the incursion of a moist, ozone-poor subtropical air mass between the streamer and the convective boundary layer. This activity disrupted the streamer and, together with shear-induced mixing beneath the descending subtropical jet, merged ~6.5¿1031 molecules of ozone into the free troposphere over Colorado between 8 and 10 km. These results illustrate the effectiveness of small-scale processes for the irreversible transfer of stratospheric air into the upper troposphere. ¿ 1998 American Geophysical Union