Coordinated flight legs of two aircraft above and below extended cirrus cloud scenes played an important part in the Tropical Composition, Cloud and Climate Coupling (TC4) Experiment (Costa Rica, 2007). The Solar Spectral Flux Radiometer (SSFR) measured up- and downward irradiance on the high-altitude (ER-2) and the low-altitude (DC-8) aircraft, which allowed deriving apparent absorption on a point-by-point basis along the flight track. Apparent absorption is the vertical divergence of irradiance, calculated by the difference of net flux at the top and bottom of a cloud. While this is the only practical method of deriving absorption from aircraft radiation measurements, it differs from true absorption when horizontal flux divergence is non-zero. Differences between true and apparent absorption are inevitable in any inhomogeneous atmospheres, in particular clouds. We show, for the first time, the spectral shape of measured apparent absorption and compare with results from a three-dimensional radiative transfer model. The model cloud field is created from optical thickness and effective crystal radius retrievals from the MODIS (Moderate Resolution Imaging Spectroradiometer) Airborne Simulator (MAS), and from reflectivity profiles from the Cloud Radar System (CRS), both onboard the ER-2. We find correlations between apparent absorption and cloud optical thickness, especially in the visible spectral range. They bring to bear a net horizontal photon transport from local maxima to minima of optical thickness within a maximum horizontal scale. Although the spectral shape is reproduced by the model calculations, the domain-averaged apparent absorption in the visible spectral range is considerably higher than the model results. This is possibly due to a net loss of photons into neighboring cirrus-free areas that are not contained within the boundaries of the model domain.