Seasonal variations in the color statistics of natural images.

We examined how the distribution of colors in natural images varies as the seasons change. Images of natural outdoor scenes were acquired at locations in the Western Ghats, India, during monsoon and winter seasons and in the Sierra Nevada, USA, from spring to fall. The images were recorded with an RGB digital camera calibrated to yield estimates of the L, M, and S cone excitations and chromatic and luminance contrasts at each pixel. These were compared across time and location and were analyzed separately for regions of earth and sky. Seasonal climate changes alter both the average color in scenes and how the colors are distributed around the average. Arid periods are marked by a mean shift toward the +L pole of the L vs. M chromatic axis and a rotation in the color distributions away from the S vs. LM chromatic axis and toward an axis of bluish-yellowish variation, both primarily due to changes in vegetation. The form of the change was similar at the two locations suggesting that the color statistics of natural images undergo a characteristic pattern of temporal variation. We consider the implications of these changes for models of both visual sensitivity and color appearance.