This is an important economic question in northern Virginia,

This is an important economic question in northern Virginia, where the tourist trade is so significant.

The answer is a complex one. It involves the interaction of rainfall, temperature, frost and biological responses to the shortening day lengths of late summer.

Chlorophyll is the pigment that gives summer leaves their greenness. Its function is to aid in the miraculous transformation of the summer sun’s radiant energy into plant food. As leaf fall becomes imminent in autumn, plants cease manufacture of food and chlorophyll is no longer useful. The green pigment fades away, unmasking other pigments that have been present all along.

At first, these secondary pigments aesthetically complement the green chlorophyll. Eventually, they overpower it.

We perceive these pigments as a striking array of golds, purples, reds, flaming oranges and clear yellows. Our bright autumnal landscape is accentuated by the deep, everlasting greens of pines, cedars and Canada hemlocks.

Thousands of seasonal visitors, known locally as “leafers,” travel the ridges of the Shenandoah National Park each fall to see and photograph the forested mountain slopes. There’s always a great deal of interest among Park visitors in the predicted “peak day” for fall colors.

“There really isn’t a peak day,” maintains Dave Haskell, resource management specialist for the Shenandoah National Park. “There’s a period of time — usually about a wreek — when if the people voted, a majority would say the colors are at their peak.

“It’s a personal preference,” Haskell continued. “A lot of people like to see the trees when there’s the most foliage and the most variety of colors. Others like it when the leaves are half gone. ”

In an average year, what Haskell calls the “peak week” is around the middle of October. “The second or third weekend of the month is almost always the busiest for us,” he said.

Haskell explained that shorter