When obliquely observed from an airplane, gray clouds near the horizon are sometimes observed to overlap with white clouds. Photographic observation from an airplane and simulations using a three-dimensional radiative transfer model are conducted to understand why such clouds appear gray. From observations, the brightness depression rate of gray clouds relative to surrounding whitish clouds is about 25%, whereas in simulations, it is as high as about 30%. Conditions necessary for the observation of gray clouds are as follows: (1) two clouds at different altitudes do not overlap, but the higher cloud overlaps with the lower cloud along the line of sight when these clouds are observed in near-horizontal direction, and (2) the higher cloud is optically thin in the vertical direction, but optically thick along the line of sight.