Wednesday, October 17, 2012
Why Are the Leaves Dull This Year?
I was driving through northeast Pennsylvania and central New York last weekend, right around the time of the fall foliage peak. The hills were colorful, but unlike other years, I wouldn't say they were ablaze with color. There was plenty of yellow and some orange, but the red was mostly missing. The colors weren't as vibrant as I was expecting, either. The natural question to ask is...why?
First, a quick primer on leaf color. Most of us remember from middle school biology that leaves are normally green due to a chemical that we call chlorophyll, but it is really a combination of chlorophyll along with another chemical called carotene that does the trick. (You might have heard of beta carotene, the chemical that makes carrots orange...same thing.) Chlorophyll absorbs red light from the incoming solar spectrum, while carotene absorbs blue-green and blue light. What is not absorbed is reflected, and so leaves normally appear to our eyes as green.
But as fall creeps in, a "corky membrane" forms between the branch and the leaf stem. This cuts off nutrient flow to the leaf, and without nutrients, chlorophyll isn't replenished. Meanwhile, the carotene, which doesn't need to be replaced, remains in the leaf. The leaf turns yellow. The membrane can also lead to the concentration of sugars in the leaf, presumably by cutting off their exit. These sugars can cause the formation of anthocyanins in some types of trees, and the anthocyanins turn the leaves red. So the key to the change in the leaves' color is apparently the formation of this "corky membrane" between the branch and the leaf stem, and the changes in chemical compounds in the leaves that result.
But as a meteorologist, I was particularly interested in why the leaves seemed so dull this year...did it have to do with the weather? Well, chlorophyll is destroyed by low temperatures (at night) and by bright sunshine. Anthocyanins - which produce those bright reds and purples that I didn't see on my drive - thrive in just-above-freezing temperatures, bright sunshine, and dry weather. So, based on this, I'd expect that the weather from September to mid-October - and especially whenever the leaves were changing color - was some combination of warm nights, cloudy, and/or rainy. But was it?
To find out, I looked at the Binghamton, NY National Weather Service office's preliminary records for Scranton-Wilkes Barre and Binghamton. At Scranton, temperatures averaged about a degree above normal in September and right around normal for the first half of October. A total of 17 days were clear, while the other 27 days were partly cloudy to overcast. Twenty-one of the 44 days had measurable precipitation. So it was relatively warm, cloudy, and rainy.
At Binghamton, the story is slightly different. Temperatures were just below normal in September and over a degree below normal through the first half of October. Only 13 of the 44 days were clear, and 24 had measurable precipitation. So Binghamton was cooler, cloudier, and rainier (it rained more often, but not as much) than Scranton during this time period.
The kicker is the stretch of weather in the first few days of October...about the time that I noticed the leaves first starting to change color in my backyard. These days (and nights) were unseasonably warm, and the warmth was accompanied by clouds and rain. For those brilliant purples and reds to form, you need a snap of cold (but not below freezing) and sunny weather...here in central New York, you need a nice Canadian air mass to drop in for a few days. But this year, we had the opposite in early October, and that's my guess as to why the leaves seemed a bit dull this year.
Sources/For Further Reading:
http://www.nws.noaa.gov/climate/index.php?wfo=bgm (weather data)