CALL IT better viewing through chemistry: The fall glories celebrated by the poets are, in fact, the result of chemical reactions. Some colors are manufactured; others, once masked, are liberated.

During the summer, a tree makes chlorophyl and supplies the bright green compound to the leaves, but as days get shorter and nights cooler, consumption outstrips production, and the green begins to fade.

As it does, the yellows, browns and oranges -- which have been there all along -- are revealed.

The reds, blues and purples, on the other hand, are produced when certain chemicals retreat from the leaves to the branches, altering the sugar-manufacturing process.

In some trees, such as dogwood and sumac, the two processes combine to produce fiery reds, deep oranges and bronze.

"A dry fall with cool nights usually produces the most intense colors," says Dr. Stanley Krugman of the U.S. Forest Service. "But long periods of warm, cloudy and rainy fall weather mute the colors, especially reds." While frost can accelerate the leaf changes, it does not cause them, and "a hard frost actually can speed up the deterioration of color and the loss of leaves," Krugman says.

He won't even guess about the colors we'll get this fall. This has been a very peculiar year, very wet earlier, then very dry in September, and then very wet again.

Nor will he predict when the colors may be at their "peak." He used to do that but "quit with a 100 percent batting average -- 100 percent wrong," he says with a wry smile. "You have to understand that it all depends on the local environment, the particular combination of light, temperature and moisture."

In other words, the peak for colors is different for different locations. "The best way to tell locally," he says, still smiling, "is just to look outside."