To a first approximation, an example of radiative exchange equilibrium is in the exchange of non-window wavelength thermal radiation between the land-and-sea surface and the lowest atmosphere, when there is a clear sky. As a first approximation (W. C. Swinbank 1963, G. W. Paltridge and C. M. R. Platt 1976, pages 139–140), in the non-window wavenumbers, there is zero net exchange between the surface and the atmosphere, while, in the window wavenumbers, there is simply direct radiation from the land-sea surface to space. A like situation occurs between adjacent layers in the turbulently mixed boundary layer of the lower troposphere, expressed in the so-called "cooling to space approximation", first noted by C. D. Rodgers and C. D. Walshaw (1966).