During September and October, chilly, but not freezing, nights and sunny days are needed to bring out the best colors. These exacting conditions are not met every year, so some years are better than others. Long periods of rainy and cloudy fall weather result in drab colors because of the low light intensity. On the other hand, trees under drought stress will often drop their leaves without showing much color. Temperature also affects fall color.
Warm temperatures encourage late season chlorophyll production that keeps the leaves green.
A belowfreezing cold snap will kill the leaves before they turn to their fall colors. Even though the colors may be bright, heavy rain or high winds during the peak of color will cause the leaves to drop early. The usefulness of the leaves does not end when their colors fade. Fallen leaves contain large amounts of minerals, such as nitrogen and phosphorus. Decomposition of the leaves returns these nutrients to the soil while adding a water-absorbing layer of humus to the forest floor.