Boundary conditions (such as surface drag against the surface) and body forcings (such as heating caused by infrared radiation divergence within the air) that are imposed on the atmosphere from outside the domain of interest. The exact definition depends on the scale context of the problem. For example, boundary layer meteorologists might consider geostrophic wind to be imposed as an external forcing at a single weather station, while general circulation dynamicists might calculate geostrophic wind as an internal variable based on the global differential heating imposed on the earth as an external forcing by the sun.