The assumption of dynamically balanced flow allows one to completely encase the dynamics of extratropical cyclones in a potential vorticity (PV) framework. This approach offers a conceptually simple interpretation of dynamics because PV is a conserved quantity (in the absence of heating and friction) from which the flow itself can be deduced (the property of invertibility). The conservation law allows one to identify developments significantly influenced by heating and friction, and the invertibility property can be used to quantitatively measure such effects. We develop a diagnostic system based on the relative smallness of the irrotational part of the horizontal wind, which allows us to calculate the balanced flow given the three-dimensional distribution of Ertel's PV. The close agreement between the observed and balanced flows, even for intense cyclones, illustrates the practical utility of the PV approach. Furthermore, we present a technique for determining the flow associated with individual... Abstract The assumption of dynamically balanced flow allows one to completely encase the dynamics of extratropical cyclones in a potential vorticity (PV) framework. This approach offers a conceptually simple interpretation of dynamics because PV is a conserved quantity (in the absence of heating and friction) from which the flow itself can be deduced (the property of invertibility). The conservation law allows one to identify developments significantly influenced by heating and friction, and the invertibility property can be used to quantitatively measure such effects. We develop a diagnostic system based on the relative smallness of the irrotational part of the horizontal wind, which allows us to calculate the balanced flow given the three-dimensional distribution of Ertel's PV. The close agreement between the observed and balanced flows, even for intense cyclones, illustrates the practical utility of the PV approach. Furthermore, we present a technique for determining the flow associated with individual...