Trees exposed to the sun may continue photosynthesis and turn red while others in the shade may be yellow. A single tree may even have branches with different colored leaves due to one leaf shading another. Best Weather for Autumn Color Fall weather conditions affect anthocyanins more than the other pigments involved in autumn color. The shades of yellow and brown always appear, but it is the brilliant reds and purples mixed with them that impart the awesome beauty to fall landscapes. Fall weather conditions favoring formation of bright red autumn color are warm sunny days followed by cool, but not freezing, nights.
Photosynthesis still occurs in leaves even while chlorophyll is declining.
Rainy or cloudy days with their reduced sunlight near the time of peak coloration decrease the intensity of autumn colors by limiting photosynthesis and the sugars available for anthocyanin production. There is an old wives' tale that claims rain washes the color out of leaves. It is not true, but the overcast conditions do reduce light intensity, and heavy rains and high winds can sweep the leaves off trees early. The common belief fostered by legends that a hard frost is necessary for good autumn color isn't true either. Freezing temperatures greatly reduce the brilliance of autumn color by killing or severely injuring the leaves, preventing physiological processes before the pigments reach their maximum development.