File:Fall colors in the Northeast (MODIS 2017-09-29).jpg

English: A much-enjoyed natural event across North America is the annual color change of leaves in autumn. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite acquired a true-color image of the early color-up in the northeastern United States on September 24, 2017.

Conifers, which stay green year-round, dominate much of the northern forests but patches of yellows and oranges can be seen throughout the mostly green forests of Canada and New England. Yellows and light tans predominate in Ontario, Canada, and in Michigan and the Midwestern United States. These more southerly areas are heavily agricultural, so the autumn colors here may be heavily influenced by drying crops, such as hay and corn, which also turns yellow in the autumn.

The changing of leaf color in temperate forests involves several causes and reactions, but the dominant factors are sunlight and heat. Since temperatures tend to drop sooner and sunlight fades faster at higher latitudes, the progression of fall color changes tends to move from north to south across North America from mid-September through mid-November.

In late summer and autumn, tree and plant leaves produce less chlorophyll, the green pigment that harvests sunlight for plants to convert water and carbon dioxide into sugars. The subsidence of chlorophyll allows other chemical compounds in the leaves—particularly carotenoids and flavonoids—to emerge from the green shadow of summer. These compounds do not decay as fast as chlorophyll, so they shine through in yellows, oranges, and reds as the green fades. Another set of chemicals, anthocyanins, are associated with the storage of sugars and give the leaves of some species deep purple and red hues.