The balanced atmospheric response to a squall line as a moving heat source is computed. Specifically, we consider the permanent modifications to the large-scale balanced flow (geostrophy) rather than the transient gravity-inertia wave motion.The potential vorticity anomaly, horizontal and vertical wind shears in terms of a dimensionless parameter £\ and swept-through distance of the squall line are presented. The physical meanings of the parameter £\ are discussed. Observational cases from mid-latitude, subtropics and tropics are given in terms of the squall line speed and £\. This classification is based on the balanced atmospheric response that the squall line induced. Namely, we emphasize the concept of potential vorticity and balances dynamics to classify the squall lines. The £\-c classification will provide a measure of the squall line force in dynamic models. The computed balanced solutions give a reference base to monitor the geostrophic adjustment processes. Moreover, the £\-c observations enable us to interpret the model result related to different observational squall lines. The invertibility computations from squall lines during Taiwan Area Mesoscale Experiment (TAMEX) are shown. The implication of the results and future research are discussed.