Physical limnology is the study of physical processes within lakes. This study focuses on water circulation, mixing within lakes and the thermal structure of lakes, including ice formation, water temperature and stratification. These processes are, in turn, driven by atmospheric processes – heat fluxes, momentum transfer – at the surface of a lake, and constrained by the morphology – size, depth, shape, sediment – of a lake. River inflow/outflow also influences the physical processes of lakes, most noticeably the residence time of the water. Whilst of intrinsic interest to the lake physicist these processes are also of fundamental importance to the ecology and chemistry of lake systems.