An algorithm has been developed for using the reflection of solar radiation in the oxygen A-band to determine cloud-top altitude. Because of multiple scattering and molecular absorption inside the cloud, the reflection of clouds is substantially modified in comparison with a mirror cloud, which is assumed to have a 100% reflection. To infer true cloud-top altitude, therefore, it is necessary to accurately estimate the amount of “photon penetration.” Theoretical calculations indicate that the amount of photon penetration depends on the altitude, the scaled volume scattering coefficient, and the sealed optical thickness of the cloud. Algorithms using the reflection in the oxygen A-band to determine the cloud-top pressure have been applied to an aircraft field experiment in conjunction with CCOPE, 1981. Results of this study are very encouraging, especially for extended clouds. Abstract An algorithm has been developed for using the reflection of solar radiation in the oxygen A-band to determine cloud-top altitude. Because of multiple scattering and molecular absorption inside the cloud, the reflection of clouds is substantially modified in comparison with a mirror cloud, which is assumed to have a 100% reflection. To infer true cloud-top altitude, therefore, it is necessary to accurately estimate the amount of “photon penetration.” Theoretical calculations indicate that the amount of photon penetration depends on the altitude, the scaled volume scattering coefficient, and the sealed optical thickness of the cloud. Algorithms using the reflection in the oxygen A-band to determine the cloud-top pressure have been applied to an aircraft field experiment in conjunction with CCOPE, 1981. Results of this study are very encouraging, especially for extended clouds.