The most vibrant fall color will occur on years with ample moisture in the summer and dry, clear weather in the fall. Extreme heat and moderate-to-severe drought may negatively affect the fall foliage season.
In summer and spring, leaves are green because they contain chlorophyll, which reflects green light more than other colors. Other colors are absorbed by chlorophyll for photosynthesis.
As temperatures begin to drop in late September, trees gradually slow down the production of chlorophyll. As the green chlorophyll disappears from the leaves, we begin to see yellow- and orange-colored leaves. These colors have been in the leaves all along; we can’t see them in the summer because of the chlorophyll. Orange colors come from carotene and the yellows from xanthophyll in the leaves. The bright red and purple colors come from anthocyanin pigments, which are made from leftover glucose trapped within the leaves of some trees (e.g. maples).
Brilliant fall colors with bright red and purple colors require conditions in which leaves can make a lot of anthocyanin pigments. The best weather conditions for this are an autumn with bright sunny days and cool, but not frosty, nights. A drab autumn has lots of cloudy days and warm nights. An early frost speeds up the fall of the leaves and brings a quick end to the fall color. Of course, too much strong winds or heavy precipitation would bring the leaves down.
Global warming is making the fall season much warmer on average. This warming trend allows chlorophyll production to continue and can delay the onset of fall colors.
If you are interested in taking a tour of the fall colors in Wisconsin, the Travel Wisconsin website provides a daily report of the fall colors (https://www.travelwisconsin.com/fall-color-report).
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
