Autumn begins when the center of the sun crosses Earth’s equator. As Earth continues its path around the sun, days become shorter and nights become longer, with the change most noticeable for those at higher latitudes.
While Americans typically use the word “fall,” the British use the word “autumn,” though both terms date around the 16th century. Before these terms, the period was called “harvest.”
During the fall, in response to colder temperatures and less light, leaves stop producing chlorophyll, the green pigment that helps capture sunlight to power photosynthesis. As the green fades, the leave’s other pigments shine through, such as orange and yellow carotenoids and vibrant red anthocyanin.
Many birds prepare for winter migration during the fall. One of the longest migrations is the 11,000-mile journey by the Arctic Tern.
Leaves of some trees such as birches, tulip poplars, redbud and hickory are always yellow in the fall, never red.
The fall leaves of a few trees, including sugar maple, dogwood, sweet gum, black gum and sour wood are usually red but may also be yellow.
Leaves have just as much yellow pigment (xanthophyll) in July when they are green as they do in October when they are yellow. In July, the darker green pigment (chlorophyll) masks the yellow color.
Many plants stop making food in the fall. The chlorophyll goes away causing orange and yellow colors. These colors were in the leaves all summer, but the green covered them up.
The amount of rain in a year also affects autumn leaf color. A severe drought can delay the arrival of fall colors by a few weeks. A warm, wet period during fall will lower the intensity, or brightness of autumn colors.
A severe frost will kill the leaves, turning them brown and causing them to drop early. The best autumn colors come when there’s been: A warm, wet spring. A summer that’s not too hot or dry. A fall with plenty of warm sunny days and cool nights.
Plants make their own food. They take water from the ground through their roots. They take a gas called carbon dioxide from the air. They turn water and carbon dioxide into food using sunlight and something called chlorophyll. Chlorophyll is green. It gives leaves their green color.
Some leaves turn red or purple.
This color is made in the fall, from food trapped in the leaves. Brown colors are also made in the fall. They come from wastes left in the leaves.
If you cover a small section of a leaf before it changes colors, the part of the leaf without sunlight will turn yellow while the exposed part will turn red.