Autumn: Why Leaves Change Color...

Autumn Returns
Courtesy Reid Wolcott, NWS

Autumn officially arrives September 22nd, 2012 at 8:49 AM MDT.

Have you ever wondered why leaves change color during Autumn?

It's all caused by the earth's orbit around the sun and its effect on the weather. During the summer months, the northern hemisphere (where we live) is tilted towards the sun and incoming energy from the sun hits at a more direct angle, increasing the temperature more rapidly. While in the winter the northern hemisphere is tilted away, reducing the angle of the suns energy, and limiting the amount of heating at the surface. Autumnal Equinox marks the halfway point between the summer and winter solstices and usually signals a turn to cooler weather.


American Meteorological Society

This 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.

Centennial Park
Courtesy Reid Wolcott, NWS
Interesting Note: 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.

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

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 (this is why the Pacific Northwest isn't know for their fall colors, even though they have many varieties of deciduous trees).

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, visit the U.S. Forest Service Fall Colors website or call their Fall Colors Hotline at 1-800-354-4595.

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