A statement of the conservation of energy applied to a given surface. The main terms involved include the vertical fluxes of energy into or out of the surface due to net radiation, sensible heat, and latent heat, as well as the net horizontal fluxes of energy that may take place below the surface (e.g., due to ocean currents). Any nonzero residual flux is typically applied as a storage term, increasing or decreasing the internal energy below the surface, usually resulting in an associated change of surface temperature.