As you may know, fall brings more than just cold temperatures to the Northern Hemisphere. Prior to leaves falling off of trees, we get to witness a miraculous changing of colors. This month’s Free For All will help you understand why this happens and provide a temporal guide to where and when these changes occur in the United States.
If you refer to the first image to the right, you will see a representation of senescence, which is the scientific term for the deterioration of a leaf that happens due to photoperiodism. Photoperiodism is the change in the amount of light due to seasonal changes. In other words, as the amount of daylight decreases (i.e. days get shorter in the fall), the leaves are unable to photosynthesize as well and they slowly die off, causing a change in color as the molecule chlorophyll breaks down. Since some carotenoid pigment is maintained, the leaves will appear to change from green to yellow-orange throughout the season. A small percentage of leaves will even produce anthocyanin pigments just before falling from a tree, resulting in a red color (refer to this guide for more details on the process of leaf color change).
The second image to the right is an animation showing the progression of autumn leaf change as it moved through the United States in 2019. You can see that the higher latitudes are affected first, starting in early September, and gradually changes occur vertically until the end of December. The animation was featured in an article last October in The Old Farmer’s Almanac, a periodical about weather that started in 1792. For a list of dates and destinations to check out the fall colors in 2020, visit this article here. For a more detailed list, including foliage hotlines and tourism websites, refer to this article written by Tripsavvy earlier in the summer.
As we are based in Boulder, Colorado, we are lucky enough to witness the phenomenal color changes of aspens and cottonwoods throughout the month of September. We encourage all our readers to take advantage of this strange time by spending more time outdoors, particularly witnessing the natural beauty of fall leaves as they change colors.
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