An efficient method for computing the transmission function in the 15-μm CO2 and the 9.6-μm O3 bands is presented. An inhomogeneous atmospheric path is treated as homogeneous by applying simple pressure and temperature scaling approximations. The transmission functions are then derived from small precomputed tables. Because the atmospheric cooling rate is primarily contributed from adjacent layers, the simple scaling approximations can be used to accurately compute transmission functions in both the middle and lower atmosphere. Applying the parameterization to vastly different atmospheric conditions, the difference with line-by-line calculations is small. In the region between 0.01 mbar and the Earth's surface, the cooling rate difference is <0.3 ¿C/d in the 15-μm CO2 band and <0.1 ¿C/d in the 9.6-μm O3 band. ¿ American Geophysical Union 1991