Using high resolution spectroscopy of the atmospheric oxygen A-band rotational absorption lines (760 - 780 nm), the probability distribution of optical pathlengths were derived for clear and cloudy skies. A large enhancement of the mean optical pathlength and a systematic change in the form of the probability distribution is found for the cloudy sky. Our observations are compatible with Gamma type distributions. The measurements are compared with a radiative transfer model in order to validate the treatment of the multiple Mie-scattering in clouds. Our measurements show a significant discrepancy compared to a model (DISORT) assuming plane parallel homogeneous cloudiness. Larger cloudy sky optical paths may contribute to explain the intensely debated anomalous cloud absorption. These measurements can serve as direct input for atmospheric heating calculations.