Satellite measurements of the sunlight reflected from clouds are often used to retrieve cloud water content. Current retrievals are based on one-dimensional (1D) theory, and so they do not consider the three-dimensional (3D) radiative effects caused by horizontal variations in cloud properties. Theoretical studies have long suggested that the 1D approximation can cause uncertainties in the retrievals, but there were no observations available on how frequently 3D effects are important.

The MODIS instrument on board the Terra satellite has provided new opportunities to examine the radiative processes that occur in clouds. This study analyzes some MODIS observations to obtain statistical information on the occurrence and magnitude of 3D radiative effects. 3D effects are detected as they cause an asymmetry in retrieved cloud properties with respect to solar illumination: cloud surfaces tilted toward the sun reflect more sunlight than surfaces tilted away from the sun. The results indicate frequent 3D effects for a wide variety of situations and cloud types, and give new insights into the spatial and temporal distribution of 3D effects. For example, 3D effects were found to be similar in marine and continental clouds but to have strong seasonal changes.