The transition from incoherent to coherent buoyancy-driven gas flow is investigated in two-dimensional tanks filled with glass beads using a high-resolution optical-gravimetrical setup. Both a grain-size (d _k )- and flow rate (Q)-dependent transition are observed in the gas flow pattern. Standard quasistatic criteria do not explain the experimental results, since they do not take into account the competition between stabilizing friction forces and destabilizing capillary and gravitational forces. Conceptualizing the steady state tortuous gas flow as core-annulus flow and applying Hagen-Poiseuille flow for a straight capillary, we propose a flow rate and grain-size-dependent stability criterion that accounts for the experimental results and is used to classify the experiments in a d _k -Q diagram.