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The Science Behind Fall Foliage

September 10, 2020
By WeatherBug Meteorologists
The season is changing across the U.S. and the spotlight shines on autumn, a season renowned for its cooler temperatures and fall foliage. Let`s take a look at what makes the leaves change color. 

First of all, why are leaves green during the spring and summer? One word: chlorophyll. This green pigment is abundant in leaf cells during the growing season. As a result, the chlorophylls` green color masks out any other pigments in the leaf. 

In order to understand the changing fall colors, we must learn how the leaf works. In addition to giving leaves their green color, chlorophyll also captures the sun`s energy. Chlorophyll utilizes this energy to split water molecules, brought into the leaf via the roots, into hydrogen and oxygen atoms. Meanwhile, tiny pores on the surface of the leaf, called stomata, take in carbon dioxide from the air. Once the carbon dioxide reacts with sugars in the plant, it too is broken down to its simplest parts: carbon and oxygen. 

After all of the compounds have been simplified, the leaf begins the reconstruction process. Individual atoms of carbon, hydrogen and oxygen are joined together to make glucose, an energy-rich sugar that nourishes the plant. For every molecule of glucose that is produced, there are six molecules of oxygen leftover. The stomata release this gas into the air we breathe.

This process, called photosynthesis, tends to destroy the chlorophyll. But the warm temperatures and strong incoming solar radiation during the spring and summer helps to create more chlorophyll to replace what is lost. Because the chlorophyll is constantly replenished, the green pigment is always present, helping the leaf to maintain its green color.

That`s all well and good, but what gives the leaves that unmistakable fall color? During the autumn season, the Northern Hemisphere starts to tilt away from the sun. This shortens the window of daylight, which in turn lowers the temperature. As this cooling takes place, a thin layer of cells, located at the junction of the leaf and stem, begin to swell and form a cork-like substance. This swelling cuts off the supply of water from the roots, thereby taking away one key ingredient of photosynthesis.

Another important part of photosynthesis, the sunlight, is also in short supply due to the Earth`s tilt. The lack of water and sunlight halts the photosynthetic process and therefore ceases chlorophyll production. With this absence of chlorophyll, other pigments in the leaves finally have a chance to come forward. 

One such pigment, carotenoid, is responsible for the yellows, oranges, browns and hues in between. Like chlorophyll, the carotenoids are present in the leaf cell during the entire growing season. The similarities stop there, however, as the concentration of carotenoids is much less than that of chlorophyll. Some species of trees that have higher amounts of carotenoids include hickories, birch, poplar, sugar maple and black maple.

The reds and purples come from a pigment called anthocyanin. In contrast to chlorophyll and carotenoids, anthocyanins are not always present in the leaf cell. Their formation is dependent on low levels of phosphate in the leaf. Because there is a high level of phosphate during the growing season, the production of anthocyanins must hold off until that level drops, usually around late summer. A few trees that produce red and purple leaves in the fall include the dogwood and the red maple.

Weather plays a very large role determining how vibrant the fall color will be. A warm, wet period during the autumn will decrease the brightness of the foliage. A severe frost early in the season will likely kill the leaves, turning them brown and causing them to fall from the tree. Severe drought could delay the fall colors by two to three weeks. 

So what is the ideal weather for great fall foliage? While there is no magic recipe, the best colors tend to be seen when there is a warm, rainy spring, a summer that is not brutally hot and a fall season with sunny days and crisp, cool nights. 

If you want to plan your fall foliage drive, the attached map gives the average peak time for fall foliage across the U.S. mainland. For those who have never driven through the changing colors, you should make an attempt this autumn as the trip is truly scenic.