The goal of this work is to explore relationships between polar mesospheric clouds (PMCs), temperature, and water vapor and to understand the extent that bulk thermodynamic equilibrium can explain observed PMC characteristics. We use observations from the Solar Occultation for Ice Experiment (SOFIE) and employ a simple PMC model which assumes that ice exists in thermodynamic equilibrium with the local temperature and water vapor. Model results using SOFIE temperatures and water vapor are found to reproduce the observed ice layer altitudes, ice frequency versus time and altitude, ice mass density (expressed as the gas phase equivalent contained in the ice phase, Q ice) versus time and altitude, and the vertical column abundance of ice (or ice water content (IWC)). The differences (model - SOFIE) for July 2008 were -0.1 km in the altitude of peak ice mass density (Z max), 16% in Q ice at Z max, and 35% in IWC. These results suggest that on average, PMCs can exist in equilibrium with the surrounding environment, and the results also imply that ice nucleation may occur throughout the PMC altitude range. Good correlations were found between ice abundance and temperature (or saturation ratio), although knowledge of both water vapor and temperature is required for a quantitative prediction of observed ice characteristics. Our results indicate that the seasonal dependence of ice abundance is generally controlled by temperature and that in a broad sense, changes in water vapor are a result of changes in ice. We also find that lower temperatures are associated with higher ice mass density, higher ice concentration, and slightly smaller particle radii. This finding indicates that the increase in ice mass density is due to the nucleation of more particles rather than the growth of existing ice, and this finding points to nucleation as an important factor in determining PMC variability.