Why Do Leaves Change Color in Autumn?

Albert Camus once said, "Autumn is a second spring when every leaf is a flower." It's easy to agree with such a sentiment if you've ever taken a scenic autumn drive in New England or the Rockies, but what's the science behind those breathtaking fall colors?

There are several reasons why leaves change color in the fall, but the most significant contributing factors are shorter daylight hours and longer nighttime hours, and how those factors affect the chemical process inside each leaf.

It all comes down to biological pigments (also known as "biochromes"), which are molecular substances that manifest in living things as specific colors by absorbing or reflecting wavelengths of light.

You might already know a little something about chlorophyll — it's the green pigment produced by plants during the photosynthesis process. Some of the other pigments found in plants are carotenoids, which are responsible for oranges, and anthocyanins, which yield red and purple leaves. While chlorophyll and carotenoids are present throughout the growing season, most anthocyanins are produced exclusively in late summer and early autumn.

As the days become shorter and the nights become longer, the amount of light required for photosynthesis wanes, and chlorophyll production gradually comes to a halt. Without any new chlorophyll being produced, the leaves' characteristic green color begins to break down and vanish. This mechanism essentially "unmasks" the colors of the carotenoids and anthocyanins that were lurking beneath.

While the waning hours of sunlight are the most significant contributing factor to affect changing foliage colors, temperature, and humidity can also play a role in the intensity of these seasonal displays. For example, warm, sunny days coupled with cool, mild nights are a particularly potent recipe for brilliance.

As the National Forest Service explains: "During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions — lots of sugar and lots of light — spur production of the brilliant anthocyanin pigments, which tint reds, purples and crimson."

The elevation and type of tree species are two other factors that affect the timing of fall foliage. Trees in higher mountain elevations are likely to change colors more quickly than their valley counterparts within the same latitude.

If you'd like to keep tabs on how the year's leaf-peeping season is shaping up, be sure to check out the Fall Foliage Prediction Map, which is a great resource and tool for timing your autumn road trips to coincide with peak foliage colors.