On top of Warm Springs Mountain, the conditions are dry, the temperature is 61, the winds are light and the humidity is low.

It is a good day to start a forest fire.

And that is just what The Nature Conservancy and the U.S. Forest Service did Thursday.

“This is great,” Sam Lindblom, fire manager for the conservancy, said after climbing out of a helicopter from which he had just surveyed a test fire set on nearby Brushy Mountain.

“It’s doing exactly what we want.”

Although an environmental group and the public caretaker of national forestland might seem an unlikely duo to be setting the woods on fire, what happened in Bath County on Thursday was actually a controlled burn aimed at improving the health of the forest.

By clearing out brush and creating open spots in the forest canopy, a controlled burn can lead to a greater diversity of trees and improved habitat for wildlife.

It is, in other words, a case of burning some trees to save the forest.

Thursday’s blaze, which covered about 1,500 acres, was the first strike in a plan to burn 5,877 acres owned by the Forest Service and The Nature Conservancy – one of the largest controlled burns of a mountainous area east of the Mississippi River.

Unlike most forest fires, which are started by an act of God (a sudden bolt of lightning) or a careless act of man (an unattended campfire or a tossed cigarette), this one involved incendiary plastic balls dropped from a helicopter that buzzed the treetops.

As dramatic as that was, it came only after careful planning and preparation by a small army of about 70 forest service and Nature Conservancy workers.

“When you do a prescribed burn, it’s an exercise in containment,” said Judy Dunscomb, a senior conservation scientist for The Nature Conservancy.

That means that long before the first fire was set, crews were using leaf blowers, shovels, rakes and bulldozers to clear fire lines designed to make sure the controlled burn did not become a wildfire.

Even when the lines of engagement were drawn, there was much more to be considered before the helicopter crew was given a green light to rain fire.

Wind, temperature, humidity, cloud formations and the condition of the forest floor all had to be just right for forest service officials to declare a “window” for burning. So precise is the system that sticks placed in strategic parts of the forest are weighed to determine their moisture content.

“There are close to a dozen and a half parameters that have to be checked before we can call it a burn window,” said Pat Sheridan, head ranger for the Warm Springs and James River district of the George Washington and Jefferson National Forests.

After a test fire was set on Brushy Mountain about 9:30 a.m. – the one that Lindblom was so enthusiastic about after his flyover – several complications arose.

First, it was light rain that passed through, just enough to delay ignition for several hours. Then there was word, sent by radio to where the helicopter sat waiting at the top of Warm Springs Mountain, that some of the fire lines at the base of the mountain needed reinforcement.

Finally, at about 4 p.m., Lindblom and his team got the clearance they were waiting for.

With its doors removed for greater visibility, the helicopter took off carrying a device that contained about 4,000 plastic spheres, about the size of pingpong balls, that contained a highly flammable powder.

Just before the balls were dropped from the helicopter, a needle in the machine injected a dose of antifreeze into each one, sparking a chemical reaction that caused them to burst into flames within 30 seconds.

By then, the balls of fire were on the forest floor, starting countless little blazes that quickly joined forces and spread across the mountainside.

As the helicopter made repeated passes and drops – clearing the treetops by about 100 feet as it methodically traversed the landscape – wisps of smoke grew into huge plumes that soon obscured the mountainside.

Starting a fire from above is the best way to handle remote terrain that Dunscomb described as “wicked steep.”

As the air assault ramped up, crews on the ground stood guard at the fire lines.

Earlier in the day, they had extended the roads, trails and creeks that served as fire lines into larger, burned-out areas by setting smaller fires with drip torches, which pour a mixture of gasoline and diesel fuel over a burning wick at the spout.

The effect, assuming things worked as planned, was to form strips of burned-out areas too wide for the raging fire above them to cross as it crept downhill. The fire was expected to burn through the night before reaching the perimeters and sputtering out today.

By 5 p.m., it appeared all was going well from the ridgetop where Greg Sanders, fire management officer for the national forest, was watching.

Years ago, there were many more fires – both natural and man-made – that did the work of this controlled burn, Sanders said. So why did the number decline to the point where controlled burns are now on the rise?

“Smokey Bear,” Sanders said, referring to a mindset that took hold in the middle of the last century that every forest fire needed to be put out as quickly as possible.

“We don’t talk about Smokey Bear in a bad way,” Sanders said. “We don’t want people being careless on windy days.”

But at the same time, forest management experts are coming to the conclusion that more controlled burns are necessary to preserve the ecosystem.

Which is why, when a patch of pine trees bursts into flames more than a hundred feet high, Sanders had the following reaction from his vantage point:

“That’s wonderful.”