The statistics of tracer variability on small scales (<200 km) is investigated using high resolution aircraft measurements of ozone in the northern winter middle/high latitudes. Conditioning based on potential temperature is used to isolate the statistics of filamentation on isentropic surfaces from spurious variability due to cross-isentropic motion of the platform. The distribution of isentropic increments Δr&khgr; in the tracer field &khgr; across a horizontal scale r have non-Gaussian tails and are consistent with stretched exponential functions of the form P(Δr&khgr;)~exp(-a|Δr&khgr;|p), where a is a scale-dependent parameter and the exponent p increases overall with r. A scale break in the second order structure function suggests a dissipation scale rd~20 km during northern winter 91--92, but the scale break is closer to 100 km during northern winter 88--89. Possible reasons for this are discussed. ¿ 2001 American Geophysical Union