It's a question Sky Stephens gets a lot this time a year.

"When are the aspens going to peak?"

"I tell people, 'I have to reach for my magic eight-ball,'" said Stephens, a scientist with the Colorado State Forest Service. "It usually likes to say something like, 'Answer unclear.'"

That's how tough it is, even for experts, to predict when the aspens of Colorado's mountains will reach the zenith of their annual change from green to gold, a spectacle that draws visitors from all over the country.

Said Stephens, "We still don't have a mechanism that says if 'x,' 'y' or 'z' happens you get spectacular color or you get not-so-spectacular color."

Aspens make up about 20 percent of Colorado's forests and usually are found between 6,900 feet and 10,500 feet in elevation.

It's probably been a long time since ninth-grade science class for many readers, so here's a refresher on why the leaves change.

The golden color is actually always on the leaves, but chlorophyll production in the long days and bright sunlight keeps them green. As the days shorten and nights get cooler, chlorophyll production slows and a membrane forms between the branch and leaf stem. The color fades and the leaf eventually drops to the ground.

In some regions, such as New England, a few tree species dominate the forests and the color change tends to be rapid and uniform. But anyone who has gone leaf-peeping in Colorado knows, while one area might be ablaze in gold by the third week of September, in the next valley over the leaves might be green or already dropped. The reason is aspens are clonal, from a handful to several hundred trees all sharing the same root system. In fact, some aspen groves rank as the largest single organisms on the continent, though Stephens said there are some fungal colonies that compete for that title.

In the mountains, each peak or drainage can receive vastly different precipitation and temperatures. And, Stephens said, each grove can respond to local conditions differently each autumn.

"It really is impacted by so many factors, such as the elevation of the stand, the condition of the stand," Stephens said. "I'm sure we could pull in extra money if we could tell everyone, 'This is the week to come to Colorado.'"

Still, there are some certainties for eager leaf watchers. The temperature drops by 3 degrees for every 1,000 feet of elevation, so the higher you go, the sooner the cold temperatures will trigger the change. Stephens said some trees at higher elevations near Aspen and Vail already had begun to change last week.

As September progresses, the gold color level will drop, usually reaching a peak in the third week of the month. Or the fourth week. Or early October.

While many leafers speculate on what summer weather means for the display, such as how the dry June or wet July will impact the color change, Stephens said the overall health of a stand of aspens is much more important.

"Where stands are healthy, we're going to see good color, and where stands are less healthy, we'll probably see less magnificent color. But the trees often like to prove me wrong," she said.

There is more certainty as to what ends the color show. Early arrival of freezing temperatures, wind storms and snowfall can all hasten the leaves' demise.

Hit the peak at the right time, and it's the kind of memory and photograph you'll keep with you forever. Miss it, and, well, there's always next year.