A numerical model is developed for computation of the reflection of atmospheric-emitted IR radiance from a wind-roughened water body. The model assumes the Kirchhoff approximation for rough surface scattering. This allows application of the postulates of geometrical optics to determine the reflection of rays from an ensemble of wave facets. We performed the hemispherical integration with Gaussian quadrature by using an uplooking fast transmittance model. This calculation is simplified further through the concept of a reflection-diffusivity angle. The model compares favorably with observed radiance spectra obtained from the Marine-Atmospheric Emitted Radiance Interferometer during the Combined Sensor Program research cruise.