This figure compares time versus height cross sections of ice abundance (expressed as the gas phase equivalent mixing ratio, Qice) from a) SOFIE observations and from b) a thermodynamic equilibrium model using SOFIE temperatures and water vapor as input.  The results are based on 3-day averages and dashed lines indicate the mesopause height.  The good agreement between SOFIE and the model suggests that on average PMCs exist in equilibrium with the surrounding environment.