The bright colors become visible in the fall as plants begin to shut down for the winter and slow their production of chlorophyll. Chlorophyll produces the bright green pigment that colors leaves throughout the spring and summer. During the spring and summer months, the green pigment masks the red, yellow, and orange pigments also present in leaves. As chlorophyll production ceases, the green disappears and the other pigments become visible, according to state foresters. Weather plays a large role in determining the brilliance of the colored leaves and the length of time they hang on the trees before dying.
Foresters point to three criteria required for optimum viewing conditions.
First, the trees must enter the fall healthy, which the abundance of precipitation this year 29 inches at Baltimore-Washington International Airport through July, over 20 percent higher than normal has ensured. Second, the days in September and October must be warm and sunny and the nights cool and crisp without a frost. Lastly, a spell of hot weather in late September could drastically shorten the prime viewing period. Chlorophyll production increases, meaning the leaves stay greener longer. But the viewing season does not extend.