Fall is here and the leaves are hitting the ground, but their colors aren't really changing just yet.
That's because some of the processes by which leaves change their colors haven't come into play due to the heat (highs this week are expected to hit in the mid-80s), Chattanooga city forester Gene Hyde said Monday.
Shorter days and cooler - but not freezing - nights help those biochemical processes start changing the leaves' colors. For example, some trees start to make less chlorophyll - the substance that gives the leaf its green color - and they start to turn shades of yellow.
Unlike the carotenoids - the substance that gives the leaf its yellow color - reds and purples aren't present in the leaf from the very beginning. They are produced by anthocyanin pigments, which are only formed in the presence of very bright sunlight, Hyde said.
"You almost have to have the fall weather where the temperatures drop down into the 40s and 50s at night, and you have really clear, cool nights and those days where you have the electric blue skies," he said. "When that happens, that's when you get some of those amazing, electric, dazzling colors."
The amount of rainfall a region gets also dictates when colors will change and how intense they'll be. A mild drought will favor big red colors, but late-summer droughts can delay the onset of fall color by a few weeks.
Too much rain near the peak of color season can also decrease the intensity of colors, and the Chattanooga area recently experienced flash flooding after a frontal boundary stalled over the Tennessee Valley, National Weather Service meteorologist Derek Eisentrout said.
So far this year, Chattanooga is "well above the annual normals" at 47 inches of rain, Eisentrout said. The average is about 40 inches.
"And with temperatures above normal, those trees are probably staying green a bit longer," he said. "It certainly doesn't feel like fall out there."
The Chattanooga area's fall display of colors usually peaks at around late October and early November. But with the warm weather lingering, Hyde said, that peak could be pushed into mid-November.
"It's hard to say, but that's what I'm anticipating," he said.
Hyde has some trees in his yard that help him judge where the season is.
"They either haven't turned color yet, or they're dropping dead leaves. They're not dropping colorful leaves," he said. "So if we have a fall season this year, it's going to be short to say the least. I hope I'm wrong."
While the reality of the warm weather isn't too bright, Hyde still has hope.
"I'd like to think that this cool weather they're forecasting for the fall of the year is going to work its magic and we're going to get some real nice color," he said. " I'm not sure how it's going to look in terms of brilliance of color. That is still being worked out between the trees and mother nature."
"All I can say is: stay tuned," he said with a chuckle.
Contact staff writer Rosana Hughes at rhughes@timesfreepress.com or 423- 757-6327 with tips or story ideas. Follow her on Twitter @HughesRosana.