Amplitude-increasing phase in the model with an envelope of 0.004 M_⊙, showing the first adiabatic exponent, the pressure-weighted volume-averaged value of the first adiabatic exponent, and the opacity. The white color corresponds to the critical value of 4/3 in panels a and b. The dashed lines give the contours containing 90, 80, 70, 60, 50, 40, 30, 20, and 10% of the envelope mass.