Pressure (left) and density (right) contours for Model AV at t = 0.8 s. The thin black contour marks the flame surface. The red contours are highlighted to aid the eye. Each slice is 2400 km on a side. Deep in the star, where the pressure scale height is large, pressure contours lie on top of gravitational equipotentials, which are lines of constant radius. Equivalently, the lateral sound crossing time of the plume is short compared to the bubble rise so that pressure balance is maintained across the flame. At larger radii, the pressure scale height and sound speed decrease, while the bubble's lateral extent and float speed increase. These changes lead to a situation where sound waves can no longer maintain pressure balance across the flame. Lateral pressure gradients form, which accelerate the lateral expansion of the flame.