by Brian Boone
It’s autumn, so that means the leaves are changing color and falling from trees, and there’s a palpable chill in the air. Here’s exactly how and why all those festive and fascinating fall things happen, according to a pumpkin spice latte-fueled Uncle John.
Why do trees’ leaves change color in the fall?
Carotenoids are pigments in plant cells that create the fall colors, but they’re present in leaves year-round. During the spring and the rest of the growing season, those colors are there but overpowered by the intense green of chlorophyll. When chlorophyll production slows and halts toward the end of the summer, the carotenoid colors of yellow, orange, and red become visible and prominent.
Why do leaves fall off of trees in the fall?
Leafy, or deciduous trees, part with their ever-changing leaves in the early weeks of fall, leaving a slow but consistent cascade of descending colorful items and big piles for kids and dogs to play in. This is all due to a weakening abscission layer. As the leaves grow out of the tree back in the spring, cells form between the stem and the branch to keep it in place. The abscission layer also works as a channel — the leaves generate energy via photosynthesis, and that makes its way into the branches and trunk through that efficient and open part. When temperatures cool off in the fall, the tree slows down production of the hormone auxin, which ordinarily keeps the abscission layer strong and working correctly. Without it, the abscission layer can’t strongly bind leaf to tree any longer and as it weakens, it makes the leaves fall off the tree.
Get Uncle John's Action-Packed Bathroom Reader Today!
Why does the air get so chilly and crisp in the fall?
That distinctive flavor comes about from a combination of astronomical and meteorological phenomena. As the season of fall sets in, the Earth tilts in such a way that the Northern Hemisphere slightly shifts away from the sun, making for cooler temperatures then what we receive in the summer. Less exposure to the sun makes for shorter days and longer nights, and so that makes it colder, too. As the overall day-to-day temperatures fall because of all that, the humidity level drops, too. Colder air can’t retain as much moisture as hot summer air can, resulting in a drier, easy to breathe air that one might categorize as crispy.