The burn, called the Clear Trap Fire, is intended to clear out combustible materials that years of fire suppression have allowed to accumulate, in order to protect an increasing number of structures on private land near the park's boundary, said Jan Passek, fire manager at Zion.

Originally proposed in 2000, the fire is intended to burn 4,400 acres of the 150,000-acre park, which is located primarily in Washington County.

Smoke from perimeter fires, started Saturday night, produced a veil of smoke that hung low to the ground Sunday morning, giving the towers and cliffs in the main area of the park an eerie appearance. Later Sunday, when firefighter Earl Levanger heard about the massive column of smoke rising from the main part of the park, he suggested a new name for the burn.

"We should call it the Great Smoky Mountains Zion National Park Fire," he joked.

The northeast corner of the fire is being watched by about 80 firefighters representing a variety of federal, state and community agencies. The other boundaries of the fire are defined by natural barriers like cliffs and canyons.

Passek said managers created a fire line in the park's northeast corner by mechanically thinning vegetation and by conducting smaller burns in the area during the past two years.

"Now we'll go behind the secured black line for a mass ignition," Passek said Sunday from the burn's command post at the Zion Ponderosa Ranch and Resort, situated adjacent to a sliver of the park in Kane County.

After preparatory work by firefighters, who burned several parts of the perimeter Saturday to solidify a fire line, the main fire Sunday was ignited by a helicopter dropping small round incendiary devices, called "pingpong" balls. The white plastic balls are loaded in a machine allowing them to be dropped over designated areas by helicopter operators, said Cathy York, helicopter base manager for the park.

She said before a ball is released, it is injected with antifreeze to create a chemical reaction inside the ball causing it to burst into flames by the time it hits the ground. Operators usually drop the balls 30 to 50 feet apart to set ground fuels ablaze.

Brett Fay, the burn boss on the fire, said the plan calls for sporadic fire-starting in areas covered with ponderosa pine trees and more intense drops in interior areas of the burn, where brush is heavy.

Fay said officials opted to start the burn this weekend, after the weather created relative humidity high enough to slow the fire's spread.

He said burn managers expected to cover the entire area by today.

Passek said the fire is one of six such prescribed burns identified for the Color Country Fire Management Area in southwestern Utah. The Clear Trap Fire was proposed in 2000 and was scheduled to take place last October, but was postponed by rain, according to Zion fire information officer David Eaker.

Eaker said the fire could smolder near the burn's center for as long a month.

He said posters were put up around the towns of Springdale and Rockville warning residents and park visitors the burn had been scheduled and could be smoky. Along state Route 9 leading into the park, and on U.S. Highway 89 north of Kanab, signs informed drivers that the huge column of smoke was the product of a prescribed fire and not to report it.

Pamela Clark, marketing director for Zion Ponderosa Resort, next to the park boundary, said some people complained, or wanted to know why they were not told about the burn before they came.

"We don't know when they are going to burn," said Clark. "When they say they are going to [burn], they usually cancel."

mhavnes@sltrib.com