The Color of Autumn

Each Autumn we are treated to a spectacular display of color by Mother Nature. As Autumn arrives, less sun and cooler temperatures allow the green leaves of Summer to change. Have you ever wondered what processes lead to the yearly occurrence?

In the Autumn, the decrease in the intensity and duration of sunlight, as well as cooler temperatures, causes the leaves to stop their food-making process (photosynthesis). The chlorophyll in the leaves breaks down, causing the green color to disappear, and the yellowish pigments, known as carotenoids, become visible. The Autumn foliage of some trees, including aspen, birch and ash, are mostly yellowish colors. Also, the reddish pigments, the anthocyanins, become more visible in the leaf veins and cells in some leaves, such as maples.



eaves 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.


Autumn weather conditions favoring the most brilliant colors are warm, sunny days and cool (but not freezing) nights.

Autumn starts September 22.  The official time of the Atumnal Equinox, when the geometric center of the Sun’s disk crosses the equator, is 8:49 A.M. Mountain Daylight Time.

During this kind of weather, the leaf produces quite a bit of sugar. However, the cooler weather results in the closing of the leaf veins, thus preventing the sugar to leave.

A few hard frosts can cause the leaves to wither and fall prematurely. The degree of color may also vary from variety to variety, and from tree to tree. Leaves directly exposed to the sun tend to turn red, while those on the shady side of the same tree may be yellow. When there is much warm, cloudy and/or rainy weather in the Fall, the leaves will have less red coloration.




Mt. Werner above Fish Creek (near Steamboat)
(photo courtesy of Art Judson)


 Carotenoids are always present in leaves, allowing the yellow and gold colors to remain until the leaf falls.

As Winter approaches, there will not be enough light or water for photosynthesis. The leaf veins will continue to close, eventually leading to the separation from its base. The tree will then become dormant  and live off the food it stored internally during the summer.


For the latest leaf report, call the U.S. Forest Service Fall Color Hotline:


Or, check their website:




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