The Time of the Seasons
Did you feel it this morning - the way the air was crisp and cool, not hot and humid? Or did you notice that the color of the leaves on the trees has really started to change? Or maybe you saw another large, V-shaped flock of Canadian geese flying overhead, off on its journey south. It doesn't take a rocket scientist or a meteorologist to know what's going on. Fall is finally here.
The official autumnal equinox occurred earlier today, September 23, at 12:55 a.m. This marks the official beginning of fall in the Northern Hemisphere. (If you were living in the Southern Hemisphere, today would be the vernal equinox and would mark the beginning of your spring!) On the equinox, there are 12 hours of daylight and 12 hours of night at all locations on Earth. The word equinox means "equal night."

From now until the winter solstice on December 21, the days get will shorter. Temperatures will drop because the Northern Hemisphere is tilted away from the Sun, and thereby receives less of its solar energy.

The Fall Palette
In many places in the United States, the changing colors of the leaves marks the visible arrival of fall. This transformation is a phase in the process of photosynthesis. In photosynthesis, plants use water and carbon dioxide to produce oxygen and sugars. Photosynthesis takes place inside specialized plant cells called chloroplasts and enables plants to build new tissue for growth.

Sunlight must be present in order for photosynthesis to occur. As the days get shorter and the amount of sunlight is reduced, the chloroplasts in the leaves gradually shut down the food-making operation. As a result, chlorophyll, the pigment that gives green plants their color, begins to disappear from the leaves, making way for yellows, oranges, and browns. These more colorful pigments - orange carotene and yellow xanthophyll - are always present in leaves but aren't visible in the summer because of the abundance of green chlorophyll. Brighter oranges and fiery reds are found in the leaves of trees that are able to take carbohydrates out of the leaves and turn them into red pigments called anthcyanins.

The best conditions for producing colorful fall foliage are clear days followed by cool nights. Too much rain, or early frosts and freezing temperatures, diminish the colors. Too little rain can kill the leaves before they even have a chance to change color; instead of yellows and reds, the leaves quickly turn brown. However, "leaf peepers" and other experts generally agree that it is a combination of variables that produces vibrant fall colors.

Fall is everywhere. From the Northeast to the Plains, landscapes come to life with autumn's brilliant colors. Even coastal salt marsh grasses change color from green to golden in autumn. Mid-September to the beginning of November marks the best time to see this annual autumn phenomenon, depending on where you live, weather, and other conditions. But weather forecasters and amateur tree specialists alike admit that predicting peak viewing time is not an exact science.

In New England, viewing fall foliage is an $8 billion business. About 400,000 tourists travel to Maine annually just to see the spectacular scenery. So it's no surprise that Vermont and other New England states have fall foliage "hot lines" and Web sites where potential tourists can track the trees' changing colors on maps or see the color status on live Web cams.