Huge clouds rising from the Pioneer Fire north of Lowman the last week of August displayed the fierce behavior of the fire that has burned out one of the largest holes of unburned timber left in the Boise National Forest.
Pyrocumulus clouds, caused by the rising heat of windblown fires burning in thick, dry fuel, could be seen over the Boise Foothills as the Pioneer Fire near Lowman grew to more than 181,065 acres as of Sunday. Cooler temperatures over the holiday weekend reduced the energy that creates these meteorological phenomena that have become more frequent across the West over the past 30 years.
The clouds form when a hot fire causes an updraft, carrying the warm, dry air up to a layer of air with water vapor, which it pushes higher to form a cumulus cloud. If fire-heated clouds rise high enough, it can become a pyrocumulonimbus, which can generate thunderstorms, rain and even lightning.
In extreme cases, including a fire in 2003 in Canberra, Australia, the storm can generate a fire tornado. Volcanoes also can create pyrocumulus clouds.
Firefighters talk about a fire creating its own weather and then later watching the convection cloud collapse on itself.
Ed Delgado, predictive services manager at the National Interagency Fire Center, kind of winces when he hears those terms.
“It’s just simple thermodynamics. Hot air rises, you get updrafts. When the fire no longer produces enough heat, you get downdrafts,” Delgado said.
The U.S. Naval Research Laboratory in Washington, D.C., monitors pyrocumulus clouds around the world by satellite and sees an average of four dozen to six dozen a year, said Mike Fromm, a meteorologist. About three dozen of those occur in the United States and Canada.
The lab has been monitoring pyrocumulus clouds since 1979. Its data collection has been good enough to hold up to vigorous review only in the past five years. Eventually, officials there hope to be able to see the trends, Fromm said.
What they also have documented is that these huge clouds that climb high into the stratosphere leave minute particles that previously were assumed to come from volcanoes.
Fire seasons have gotten longer and fire behavior more fierce across the world as global warming has made the American West, eastern Russia, Australia and other areas hotter and drier over the past 30 years.
Firefighters saw few pyrocumulus clouds prior to 1988, when the huge fires in Yellowstone National Park created frequent columns of smoke and water vapor that reporters compared to the mushroom clouds of atomic bombs.
The Yellowstone pyrocumulus clouds in August and September 1988 were among the most powerful Fromm ever saw, he said, far stronger than this year’s Pioneer Fire clouds.
“This year appears to be a pretty weak year,” said Fromm.
Perhaps the best known one between the 1910 fires in North Idaho and the 1988 Yellowstone fires was the Sundance Fire that created a huge pyrocumulus cloud Sept. 1, 1967. It burned 55,000 acres of forestland near Priest Lake that day, setting the bar for a generation of firefighters until Yellowstone in 1988.
EXTREME FIRE BEHAVIOR
Larry Sutton, Forest Service assistant director for operations at the National Interagency Fire Center in Boise, said the first pyrocumulus cloud he saw was in 1979 in Wyoming. But it was in Yellowstone when columns rose from all corners of the park in 1988 that he remembers best.
Usually such clouds or columns are tied to extreme fire behavior, where winds can drive fires to burn tens of thousands of acres in a day, Sutton said. Dense fuel and the right topography can help create such a cloud even in the absence of wind.
For instance: A fire could burn into heavy fuels as it burns uphill, heating the forest ahead of it until it gets so hot it triggers the updraft. Fire bosses now keep firefighters away from these unpredictable conditions while they are burning, Sutton said.
Because such conditions form in the afternoon, when solar radiation is the greatest, firefighters can work the areas when they are safer at cooler times, he said.
Firefighters have been using the outlines of past area fires to help contain the Pioneer Fire, which burned 29,000 acres in one day last week. Since 1990, 1,263,967 of the 2,525,020 acres of the Boise National Forest have burned, just over 50 percent.
The cold front that came through this weekend historically would have been considered a season-ending event, dramatically reducing burning activity due in part to the increasingly shorter days.
But as the Walker Fire that burned 5,440 acres northeast of Boise in October 2015 showed, Idaho’s fire season is longer now.
“Sometimes, we refer to this as the season-slowing event,” Sutton said.