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Qualitative descriptions and quantitative estimates are presented of various transport processes between stratosphere and troposphere. The seasonal changes of tropopause heights account for a flux of about 10% of the mass of the stratosphere in one hemisphere during the course of 1 yr. This flux is balanced approximately by the seasonal shift of stratospheric air masses between the northern and southern hemispheres. Vertical transport through the Hadley cell transfers approximately 38% of the mass equivalent to one hemispheric stratosphere through the tropopause per year. This appears to be the most effective of all transport mechanisms. Large-scale eddies of the scale of cyclones and anticyclones transfer about 20% of stratospheric air through the tropopause per year. Small-scale and mesoscale diffusion processes at tropopause level probably account for the transfer of only 1% of stratospheric air. These mass flux estimates are in reasonable agreement with observed residence times of stratospheric pollutants.