Autumn Arrives

The days are getting shorter, the nights are getting cooler, and getting into your car after a day of work is no longer like stepping into a blast furnace—all this adds up to one thing: autumn has arrived here in Oklahoma. This year, autumn began on September 22 at 10:44am, when the fall equinox occurred. Well, astronomical autumn, that is—meteorologists often define northern hemisphere autumn to be the season consisting of the entirety of September, October and November. Many eastern cultures, using amount of insolation as their guide, traditionally define the equinox as the middle of autumn—by this reckoning, autumn began on August 7 and will last through November 7.

But I digress… whichever definition you prefer, autumn is here, bringing with it cool northerly winds, drier airmasses, pumpkins, bonfires, and brilliant multicolored leaves. It’s the lattermost item in that list that we’ll be looking at in depth this month. Though you may not realize it, meteorology and climate play a large role in whether we will see blazing red and orange trees or a drab brown landscape when we walk down the street this fall.

The root cause of the brightly colored leaves of autumn is, like many things in meteorology, the sun. Leaves use chlorophyll to generate energy from sunlight, but as summer transitions to autumn, less and less solar radiation reaches the leaves each day, meaning less and less energy to be extracted. Eventually the cost to the tree of maintaining the leaves outweighs the energy the tree gains from photosynthesis, and the tree essentially cuts off the supply of chlorophyll to the leaves and prepares to go dormant for the winter. With the bright green chlorophyll gone, other pigments present in the leaves are free to show through, producing the yellow, orange, and red trees of autumn that we are all familiar with.

Still, all autumns are not created equal—climate and weather greatly affect the process. Even within the United States, the time of the year when autumn leaf color peaks varies by nearly two months, as shown by the graphic from Wikipedia below.

Note that the areas where leaves change color the earliest correspond with significant climate and meteorological trends. In the Rocky Mountain states, leaves change earlier because of the higher elevation—due to the lapse rate of the atmosphere, the air becomes cooler as you move farther above sea level. This cooler air at higher elevation essentially brings an early start to autumn, and the leaves change nearly a full month earlier as a result. In the eastern United States, leaves change later and later as you move south. Early in autumn, cool air filtering down from Canada is often confined to the northeastern United States—this is where leaves change the earliest. As autumn continues, storm tracks shift to the south and cold fronts penetrate farther into the Midwest and Southeast, with the peak in autumn leaf color following along. Also notice the subtle elevation effects of the Appalachians.

But wait—those are all climatological effects—the weather within a single year can have a profound impact on autumn leaf color too! The biggest influences: temperature and moisture. The pigments that color leaves red (called anthocyanins, in case you were curious) are produced in the greatest abundance on warm, sunny days, with cool, clear nights, while yellow pigments (called carotenoids) are likely to be present whatever the temperature. Rain, which affects soil moisture, also plays a strong role—brilliant leaf colors are favored in years with wet springs and drought-free summers.

For those of you checking off the boxes, central Oklahoma had a moist spring and summer this year, and a stretch of warm fall days and cool fall nights in recent weeks. With this in mind, keep your eyes on the trees, because barring a mid-autumn heatwave or drought, we could see some spectacular autumn colors later this month!



Story is © Nate Snook, 2008