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Gettelman has been exploring the role of Polar Mesospheric Clouds (PMCs) that form at the summer mesopause (~80km altitude) when temperatures reach below 150K. These clouds are modulated by temperature and by water vapor concentrations, and may be affected by particles acting as nuclei. The variability of these clouds provides a critical test of the coupling of dynamics and chemistry of the mesopause, and sheds important light on variability due to solar cycles, or due to long term climate changes in the upper atmosphere. Gettelman modified the cloud condensation routines in CAM & WACCM to parameterize Polar Mesospheric Clouds (PMC’s) which can serve as a sensitive diagnostic of model processes in the mesopause region strongly affecting trace gas chemistry. PMC’s are being used as a model diagnostic, as well as for historical climate simulations to look at the sensitivity of PMC’s to climate changes over the last 150 years.