# Fall Colors. When and Why? **URL:** \ **Category:** Factual Questions\ **Created:** [October 30, 2009, 3:40pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730 "2009-10-30T15:40:17Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [October 30, 2009, 3:40pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/1 "2009-10-30T15:40:17Z")
Hi all, trying to roughly predict when the fall tree colors will peak in a southern Alabama town. What triggers the colors? I assume, beside the changing daylight, the weather is a major driver in moving the peak time forward or backwards a week or two. Is it a specific low temp, or a certain low temp for a sustained period of time, or daily highs below a certain temp? If those are factors, I can look at last years weather data and get a feel for whats gonna happen this year, as we were up there last year and the concensus was we were probably a week or two before “peak”. thanks for any input ! ---
**Author:** ![pan1](https://avatars.discourse-cdn.com/v4/letter/p/e495f1/32.png) [@pan1](https://boards.straightdope.com/u/pan1)\ **Post date:** [October 30, 2009, 4:35pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/2 "2009-10-30T16:35:44Z")
It’s a combination of light and temperature and the significance of each varies a little by species of tree. Short days mean less light - less reason to bother with having leaves. Sustained Temperature - in the sense that it has to actually affect the plant. But this could be a few hours at near or below freezing to start the leave’s changing. Having a cool daytime high would be a significant sign to the tree that it’s time to start thinking about hibernation. (Not that there is “thought” in a tree") And those pretty colors - they’re not just fading leaves - they change those colors to better absorb the diffferent bandwidths of light that occur as the sun is lower on the horizon (and the light is thus filtered through more atmosphere) ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [October 30, 2009, 4:51pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/3 "2009-10-30T16:51:51Z")
> [@billfish678](#): > > Hi all, trying to roughly predict when the fall tree colors will peak in a southern Alabama town. A quick perusal of the many [peak fall foliage maps](http://images.google.com/images?q=%22peak%20fall%20foliage%22%20map&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&sa=N&hl=en&tab=wi) reveals that southern Alabama doesn’t have a period of peak fall foliage. Obviously there’s something wrong with the maps. The [forest service runs a fall color](http://www.fs.fed.us/news/fallcolors/) hotline at: 1-800-354-4595. They might have the information you seek. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [October 30, 2009, 5:05pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/4 "2009-10-30T17:05:40Z")
> [@pan1](#): > > And those pretty colors - they’re not just fading leaves - they change those colors to better absorb the diffferent bandwidths of light that occur as the sun is lower on the horizon (and the light is thus filtered through more atmosphere) Do you have a cite for that? It makes no sense to me. If it were true, you’d expect trees in the Amazon to be a different color from trees in Canada, since the sun in Canada is lower in the sky even during the summer. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [October 30, 2009, 5:28pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/5 "2009-10-30T17:28:12Z")
all the pigments are present all the time to the best of my recall. there is a tendency for longer wavelengths of light every evening. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [October 30, 2009, 7:04pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/6 "2009-10-30T19:04:47Z")
> [@pan1](#): > > And those pretty colors - they’re not just fading leaves - they change those colors to better absorb the diffferent bandwidths of light that occur as the sun is lower on the horizon (and the light is thus filtered through more atmosphere) Here’s a [different, and very thorough, explanation](http://scifun.chem.wisc.edu/CHEMWEEK/fallcolr/fallcolr.html). ---
**Author:** ![Plynck](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Plynck](https://boards.straightdope.com/u/Plynck)\ **Post date:** [October 30, 2009, 7:09pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/7 "2009-10-30T19:09:32Z")
Paraphrasing from Donald Wyman’s _“Trees for American Gardens”:_ Eventually a deciduous tree in a temperate zone will stop producing chlorophyll, which masks other colors in the leaves. Yellow leaves contain carotin or xanthophyll, which become apparent when there is no chlorophyll. Similarly, red leaves contain anthocyanin, which results from the accumulation of sugars and tannins in the leaves. Warm, bright sunny days combined with cool nights (below 45 F) result in the best displays of foliage. A warm, cloudy fall with a lot of rain can restrict the formation of bright colors, especially if the nights are warm and the sugars are absorbed by the trunk and the roots. ETA: Or then again, read the explanation given above… ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [October 31, 2009, 1:52pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/8 "2009-10-31T13:52:47Z")
Thanks folks. That info was useful enough to help make the decision. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [October 31, 2009, 3:25pm UTC](https://boards.straightdope.com/t/fall-colors-when-and-why/515730/9 "2009-10-31T15:25:41Z")
Just wanted to add that I visit the Mt. Washington area in NH most years in October, mostly to fly my [glider](http://www.schempp-hirth.com/index.php?id=discus-2&L=1). The colors are always worthwhile, though they vary considerably from year to year. This year was perhaps the best I’ve ever seen them. No one tree was dramatically better than in past years, but the carpet of trees covering the hillsides, especially when seen from the air, was pretty much a caricature of stunning fall colors. Snow on the high peaks made the scene even better. I think someone who’d never seen this in person could be expected to dismiss any photo as obviously enhanced.