How Weather Affects Fall Colors

Nothing says autumn quite like a lazy drive through the countryside with the sun illuminating oranges, reds, and yellows in the treetops. But before planning a day of leaf-peeping, its a good idea to check local and regional weather forecasts—and not simply for travel weather purposes. Weather conditions such as temperature, precipitation, and amount of sunlight, actually determine how vibrant (or not) fall colors will be. Leaf Pigment Leaves have a functional purpose for trees: They produce energy for the entire plant. Their broad shape makes them good for capturing sunlight. Once absorbed, the sunlight interacts with carbon dioxide and water within the leaf to produce sugars and oxygen in a process known as photosynthesis. The plant molecule responsible for this process is called chlorophyll. Chlorophyll is responsible for giving a leaf its trademark green color. But chlorophyll isnt the only pigment residing within leaves. Yellow and orange pigments (xanthophylls and carotenoids) are also present; these remain hidden for most of the year because chlorophyll masks them. Chlorophyll is continually depleted by sunlight and is replenished by the leaf through the growing season. Only when chlorophyll levels subside do the other pigments  become visible. Why Leaves Change Color While a number of factors (including weather) influence the brilliance of leaf color, only one event is responsible for  triggering  the decline of chlorophyll:  the shorter daylight and longer overnight hours associated with the change in season from summer to fall. Plants depend on light for energy, but the amount they get changes through the seasons. Beginning on the summer solstice, Earths daylight hours gradually decrease and its nighttime hours gradually increase. This trend continues until the shortest day and longest night is reached on December 21 or 22 each year (the winter solstice). As the nights progressively lengthen and cool, a trees cells begin the process of sealing off its leaves in preparation for winter. During winter, temperatures are too cold, sunlight too dim, and water too scarce and susceptible to freezing to support growth. A corky barrier is formed between each branch and each leaf stem. This cellular membrane blocks the flow of nutrients into the leaf, which also stops the leaf from making new chlorophyll. Chlorophyll production slows and eventually stops. The old chlorophyll begins to decompose, and when its all gone, the leafs green color lifts. In the absence of chlorophyll, the leafs yellow and orange hues dominate. As sugars become trapped inside the leaf by the trees sealant, red and purple (anthocyanins) pigments are also created. Whether by decomposition or by freezing, all of these pigments eventually break down. After this happens, only browns (tannins) are left. Effects of Weather According to the U.S.  National Arboretum, heres how the following weather conditions at each stage of the leaf growing season work to the benefit or detriment of  foliage come September, October, and November: During spring, a wet growing season is ideal.  Drought conditions during the spring (the beginning of the leaf growing season)  can cause the sealing barrier between leaf stem and tree branch to form earlier than normal. This, in turn, can lead to an early shutdown of leaves: Theyll drop before theyve had a chance to develop fall coloration.From summer into early autumn, sunny days and cool nights are desirable.  While adequate moisture is good during the early growing season, it works to mute colors in the early fall. Cool temperatures and abundant sunshine cause chlorophyll to be destroyed more rapidly (recall that chlorophyll breaks down with exposure to light), thus allowing yellows and oranges to be revealed sooner, and also promoting the formation of more anthocyanins.  While cool is best, too cold is detrimental. Freezing temperatures and frosts can kill thin and fragile leaves.During autumn, calm days prolong viewing opportunities.  Once the autumn season arrives, l eaves need time for the buildup of chlorophyll to entirely fade and their dormant pigments to fully take over. Gusty winds and hard rains can cause leaves to fall before their full color potential is reached. The conditions that make for spectacular autumn color displays are a moist growing season followed by a dry autumn with warm, sunny days and cool (but not freezing) nights.