In the stratosphere, turbulence is confined within shallow, localized patches. The resulting vertical mixing of tracers is highly intermittent and difficult to quantify. Here a new technique is employed to estimate from high-resolution, lower stratospheric mid-latitude balloon data a well-defined vertical diffusivity associated with this mixing. The technique, which is based on a stochastic model of the distribution of turbulent patches, emphasises the dependence of the diffusivity on the typical patch lifetime. Assuming a lifetime of a few hours, the diffusivity is found to be in the range 0.01-0.02 m2 s-1. This value is an order of magnitude smaller than those previously derived from radar measurements. The potential of the technique for the analysis of ocean microstructure measurements is noted. ¿ 2000 American Geophysical Union