πF_n and P_s as functions of α for a black planet (gray, dashed-triple-dotted line), a white planet (blue, dashed-dotted line), and a planet with a gaseous atmosphere and its surface overed by alternating black and white pixels (black, solid line). The latter curves coincide with those calculated using horizontally homogeneous planets and a weighted sum approximation (red, dashed line). All planets are cloud-free.