In the autumn months the leaves of trees change colour and eventually fall. It is a phenomenon that takes place because of the change in the chemical processes within a plant as the seasons change. The three main factors that give leaves their many fall colours are the leaf pigments, length of the night and weather.
Change of Pigments
When photosynthesis is at its peak within leaves during the summer months, chlorophyll is the dominant pigment over two other pigments called carotenoid and anthocyanin. All of the pigments contribute to a leaf's colour all year round. But when autumn comes, the dominance of chlorophyll declines as photosynthesis activity declines. It is during this time that the other two pigments give leaves their oranges, reds and other colours.
During the fall season, the days grow shorter and nights longer. This naturally leads to less light for a plant's leaves to absorb and photosynthesise. With the decline in photosynthesis comes a decline in the presence of chlorophyll. As mentioned previously, other pigments assert themselves as chlorophyll becomes less prevalent and the result we see are leaves having different colours in the fall.
The two main influences related to weather having an effect of leaves changing their colours are temperature and moisture. During the fall season, the days stay warm (but not hot) and the nights get cold (but not freezing). In addition, moisture is lessened (or visible in the form of snow instead of rain) when compared to the "wet" season of spring. The colder temperature and less moisture leads to less photosynthesis taking place, which leads to leaves turning many different colours.