Weather and Fall Foliage

...FASTER FALL FOLIAGE?...

By: William R. Deedler, Weather Historian, WFO Detroit/Pontiac Mi

The Summer of 1997 in Southeast Lower Michigan will be remembered for its brevity

and unseasonably cool August. It seems as if the summer took forever to get here and

when it finally did arrive, it stayed only a month or so. The main problem was that

we couldn=t shake a fairly persistent northwesterly wind flow in the jet stream. This

pattern was actually carried over from the spring, when here too, it was the main

culprit our spring was such a late visitor. Simply put, when the dominant wind flow

is northwest across the region, one can expect cooler than normal weather for that

time period. Besides being cooler than normal, the summer also had a notable dry

period in July whereas in August, the rains reappeared but it was significantly

cooler than normal. Now, what does all this mean for the annual fall foliage display?

Actually not as much as one might think. Fall leaf color is basically caused by lack

of sunlight and to a lesser extent is influenced by the September and October weather.

Drier than normal weather in the late summer into early fall will tend to accelerate

the leaf changing process, causing the leaves to fall prematurely. Likewise, a wet

September and October will tend to produce fewer vivid colors and the leaves may

also fall earlier due to the rain, wind and storms. The prime weather conditions which

are conducive to brilliant fall colors are warm, sun dominant days and cool, crisp

nights but without frosts or freezes; such as high temperatures in the 60s and 70s

with lows in the mid 30s to around 50. These sharp, daily temperature swings and

more importantly, the decrease in sunlight, play vital roles in the development of the

leaf color. This combination of weather and lack of sunlight, creates a blocking-effect

on the sugars which are manufactured in the leaves and keeps them from reaching the

root system. Eventually, these sugars convert to pigments that produce the vivid and

brilliant colors seen on many trees in the fall. Evidently, the green chlorophyll in the

leaves begins to fade during the shorter fall days with subsequently, less sunlight.

Thus, the other color pigments already in the tree leaves are exposed, come out and

produce the fall color splendor. The yellow color seen in some leaves is created by

the xanthophyll pigment, while the orange-red color is caused by the carotene pigment

and the red-purple color can be attributed to the anthocyanin pigment.

While color peak may vary season to season across Southeast Lower Michigan,

generally the maximum leaf color occurs during the second and third week in October.

Latest indications are this years color festival is slightly ahead of schedule. Looking

ahead through October, the National Weather Service outlook for calls for temperatures

to be near normal to below and rainfall near normal to above. Generally across Southeast

Lower Michigan during peak color season or mid-October, high temperatures rise into

the lower 60s and lows fall into the lower 40s. October is usually one of the area=s drier

months with an average rainfall of just over two inches.

National Weather Service Weather Forecast Office