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In a video frame grab, a lightning bolt strikes a wildfire.
Nevada ranchers look on as lightning strikes and a wildfire rages in 2007 (from file footage).

Video grab by Ross Andreson, Elko Daily Free Press/AP

Ker Than

for National Geographic News

Published August 16, 2010

Intense fires have the power to generate dirty thunderclouds that may be influencing the climate, scientists have discovered.

Like volcanoes, the dirty, fire-induced thunderstorms, or pyrocumulonimbus storms (pyroCbs), can funnel smoke and particulates high into the stratosphere, experts announced Friday at an American Geophysical Union meeting in Foz do Iguaçu, Brazil.

The high-flying smoke palls created by the so-called pyrocumulonimbus storms (pyroCbs) may be having unknown effects on Earth's climate, such as warming or cooling the atmosphere, the scientists added.

And the fire-started storms aren't just figments of a climate model. Recent satellite images, for instance, show that the forest fires currently raging in Russia (pictures) are generating several pyrocumulonimbus storms of unprecedented size, according to Michael Fromm, a meteorologist at the U.S. Naval Research Laboratory in Washington, D.C.

(Also see "Planes Create Weird Clouds—And Snow, Rain Fall Out.")

Dirty Clouds Long Mistaken for Volcanic Ash

Pyrocumulonimbus clouds had long been mistaken for volcanic ash clouds, but Fromm and colleagues' new survey of satellite images dating back to 1979 has definitively linked them to especially intense forest fires called crown fires, the team says.

"It's when the fire sweeps from treetop to treetop without necessarily working its way down the trunks," said Nathaniel Livesey, an atmospheric scientist at NASA's Jet Propulsion Laboratory in California, who worked on the survey.

The intense, rising smoke and heat of a crown fire can cause storm clouds to form—clouds that are typically bigger, taller, and darker than normal thunderstorm clouds.

Like natural smokestacks, the pyrocumulonimbus clouds absorb the fire's smoke and aerosol particles and funnel the pollutants as high as 11 miles (17 kilometers) into the upper atmosphere, where they exit the clouds and drift far and wide.

"An entire hemisphere can be polluted by an individual pyroCb ... and the smoke and pollutants can linger for several months," the Naval Research Laboratory's Fromm told National Geographic News.

The fire-started clouds are abnormal in other ways too. For example, while they can produce thunder and lightning and even tornadoes, pyroCbs rarely, if ever, create rain.

"That's because of the unique ingestion of so many smoke particles into the thunderstorm," From said.

By preventing water molecules from coming together, "the smoke essentially cuts off the precipitation process, so you get very violent storms without much rain," he said.

Furthermore, unlike normal thunderclouds, the fire-born clouds don't drift. They remain tethered to the blazes from which they were born.

(Pictures: New Cloud Type Discovered?)

Fire-Started Thunderstorms Can Wreak Havoc

The new findings could force scientists to tweak their computer climate models to account for the effects of these dirty thunderclouds.

"We have found out that on several occasions stratospheric aerosol layers were assumed to be caused by volcanic eruptions," Fromm said. "Now we have shown that a number of these were in fact smoke from pyroCbs.

"The historical record of atmospheric aerosols needs to be looked at again."

That record may reveal clues to how and when the fire-induced thunderstorms appear—information with both local and global implications.

"The smoke palls that occur in the atmosphere from pyrocumulonimbus storms are important climatologically, because they may absorb radiation from the sun to warm the atmosphere and cause changes in the weather," Fromm said in a video made at the conference (watch below).

(Related: "'Brown Clouds' Contribute to Himalaya Glacier Melt?")

"They also might cool the air that we breathe significantly, or at least enough that we need to understand the effect of the smoke pall. ...

"It's important also to study the storms themselves," he added, "because they are very energetic storms that can wreak havoc on local populations."

Video: Meteorologist Michael Fromm on Fire-Started Thunderstorms

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