Fall is in full swing and many trees are starting to show their colors. But why exactly does this happen? Well, it has to do with weather and how trees respond to the approaching winter season. Let's investigate.
Leaves actually have two different ways that they form colors: The green color is formed in the summer when the production of chlorophyll is high due to abundant sunshine. The fall colors begin creeping in when the sun starts to fade and the production of chlorophyll decreases. This is when the other, more vibrant color pigments that are in the leaf start to show.
The two worst weather factors for vibrant fall foliage is extreme heat and drought, because they put a lot of stress on the tree. This could cause the leaves to fall off prematurely before the color pigments can fully show, or it can cause the leaves to not look as bright and vivid. On the opposite side, a wetter than normal summer with somewhat cooler temperatures will help the colors turn out brighter and also help the trees retain their leaves for a longer period of time.
The summertime isn't the only time of year when the weather affects fall foliage. The early fall is just as important to the foliage because the best colors from the leaves result from cool & clear days and night, but not freezing. Once the fall sets in, you don't want a lot of rain or wind as this could knock the leaves off of the trees before they reach peak color.
So how well will the colors show through October? Well, the Northeast saw mild temperatures throughout this summer, only slightly warmer than normal with no prolonged stretches of extreme heat. Meanwhile, significant rainfall occurred during the summer (especially in August and September) due to several major weather systems, allowing precipitation totals to come in several inches above normal. Because of this, we're expecting conditions to be prime for impressive colors in leaves this fall across the Northeast that may also last longer into the season than usual. Fall foliage is definitely expected to be better than last fall, as extended drought conditions across the Northeast in the late summer of 2020 resulted in early leaf loss and duller colors in autumn.
For the Midwest, Ohio and Indiana saw, on average, slightly wetter and warmer than normal conditions throughout this past summer. We expect fall foliage here to be rather typical or slightly above average, with a good amount of bright colors and decent longevity of leaves. Conditions differ though for northeastern IL, where the summer months featured rather dry and warm conditions. This dry and warm pattern was not as drastic towards the Chicago metro, which is currently just "abnormally dry" according to the U.S. drought monitor. Here, fall foliage will probably look typical for autumn, though leaves may be slightly less bright or could even fall off a bit sooner than normal. Meanwhile, farther north in Lake and McHenry counties, the summer months were plagued with much warmer than normal temperatures, with precipitation totals well below normal (especially in Lake County). Current drought conditions here are considered to be "moderate" in McHenry and "severe" to even "extreme" in Lake, likely resulting in more dull fall colors with leaves expected to fall off trees sooner than usual.
Current drought conditions in IL, courtesy of U.S. Drought Monitor
Nevertheless, here's a link to the dates for the peak of autumn color across the United States in case you are thinking of a scenic road trip: https://smokymountains.com/fall-foliage-map/
