By: William R. Deedler, Weather Historian, National Weather Service Weather Forecasting Office Detroit/Pontiac Mi
Fall leaf color is basically caused by lack of sunlight and to a lesser extent is influenced by the September and October weather. Drier than normal weather in the late summer into early fall will tend to accelerate the leaf changing process, causing the leaves to fall prematurely. Likewise, a wet September and October will tend to produce fewer vivid colors and the leaves may also fall earlier due to the rain, wind and storms. The prime weather conditions which are conducive to brilliant fall colors are warm, sun dominant days and cool, crisp nights but without frosts or freezes; such as high temperatures in the 60s and 70s with lows in the mid 30s to around 50. These sharp, daily temperature swings and more importantly, the decrease in sunlight, play vital roles in the development of the leaf color. This combination of weather and lack of sunlight, creates a blocking-effect on the sugars which are manufactured in the leaves and keeps them from reaching the root system. Eventually, these sugars convert to pigments that produce the vivid and brilliant colors seen on many trees in the fall. Evidently, the green chlorophyll in the leaves begins to fade during the shorter fall days with subsequently, less sunlight. Thus, the other color pigments already in the tree leaves are exposed, come out and produce the fall color splendor. The yellow color seen in some leaves is created by the xanthophyll pigment, while the orange-red color is caused by the carotene pigment and the red-purple color can be attributed to the anthocyanin pigment.