The fluid-filled long cavity with differentially heated end walls is a common and simple model for flows both in the environment and in technical applications. A delineation of the flow regime showing confined flowing layers from those showing cavity-filling counter currents is provided. Theoretical considerations yield a description of the flowing layer thickness and an approximate value for the longitudinal temperature gradient in the core of the tank. This way, a good description of the major features in the long cavity is achieved, which is confirmed by experimental data.