It is well-established that the air in the tropical stratosphere remains somewhat isolated from that in the midlatitudes over relatively long time scales. We examine the seasonal and interannual variability of the subtropical tracer gradients that mark the boundaries between tropical and extratropical air. We use probability distribution functions (PDFs) of satellite measurements of long-lived tracers to identify the air in the transition region between the tropics and the extratropics in each hemisphere. This region is commonly called the subtropical edge region, and it appears as a valley in the tracer PDF. We provide a method to identify a single characteristic latitude for these edge regions as well as, when possible, their areal extent. We identify the subtropical edges in more than six years of measurements at eight pressure levels on quasi-monthly time scales. We correlate the variability of the subtropical edges with variability in several transport parameters and thus shed light on the mechanisms of tropical isolation from a diagnostic standpoint. We also discuss the implications of our results in terms of large-scale tracer transport in the context of simple models of tracer budgets.