The distribution of extratropical potential vorticity (PV) on isentropic surfaces that transect the tropopause is a key feature of planetary-scale teleconnection patterns, synoptic-scale weather systems, and mesoscale stratosphere-troposphere exchange. Here a Northern Hemisphere January and July climatology is presented for the mean and variability patterns of the PV using the so-called European Centre for Medium-Range Weather Forecasts reanalysis-15 (ERA-15) data set. It is derived taking into account the strong seasonal cycle of the tropopause height and the sharp quasi-latitudinal gradient of PV on the isentropic surfaces together with the accompanying bimodality in the scale and amplitude of positive and negative anomalies. The bimodality is both emphasized and circumvented by comparing conventional standard deviation measures of the variability with those of separate depictions of the probability density function structures of positive and negative anomalies. It is shown that in winter the zonal heterogeneity is pronounced and the variability (e.g., storm track) patterns exhibit a rich spatial structure with marked differences between the Pacific and Atlantic. In contrast, in summer the heterogeneity on the lower stratospheric portion of the isentropic surfaces is much weaker, but there remain regions of high variability over oceanic regions on the tropospheric portion of the surfaces. The results relate directly to the structure and dynamics of storm tracks and their spatial and seasonal variation.