The science behind the color of autumn leaves is a mixture of bio-chemistry and meteorology.  Atmospheric conditions trigger a biological process in deciduous trees that cause the leaves to change color.  This process allows a tree to protect itself from freezing and survive the winter.

Sunlight, groundwater, and temperature all play a part in the annual phenomenon of fall foliage.  The main variable that starts the ball rolling, however, is sunlight. As the days grow shorter in autumn, less sunlight is available to power photosynthesis, the chemical process that feeds the tree by converting carbon dioxide and water into glucose and oxygen.

The reduced amount of sunlight signals the tree to stop producing food and prepare stores for the winter. The tree turns off its food producers by slowly corking the connection between leaf-stems and its branches.  This blocks the movement of sugars from the leaves to the tree as well as the flow of water from the roots to the leaves.  As a result,  the leaves stop producing chlorophyll, the agent of photosynthesis and the reason for the green color of summer foliage.  As the green fades, other chemicals that have been present in the leaves all along begin to show.  These include xanthophyll and carotene that produce yellow and orange leaves, respectively.

The change of leaves happens every autumn, but the intensity and duration vary from year to year.  This is a result of weather conditions like temperature and rainfall.  With below average temperatures, sugars trapped in the leaves react with sunlight and the cooler air to produce anthocyanin.  This gives us more red and purplish leaves.  Warmer than normal temperatures tend to produce longer displays but with less intense colors.  Drought will cause leaves to turn brownish and break off early.

It should also be noted that the bio-chemical make-up of different species of trees react to seasonal and atmospheric conditions differently. Therefore, the more diverse the forest, the wider the range of colors in Autumn.