'Dry lightning' sparks Western wildfires

ATLANTA (CNN) -- Tinder dry fields and forests in Western states have provided the fuel for dozens of large wildfires. And lightning has provided the spark to ignite the flames in some cases, which can set off a vicious circle in an exceptionally dry year such as this one.

Fires sparked by lightning generate their own weather, spawning more lightning, which can start even more fires.

Here's how it works:

VIDEO CNN's David George explains how one begets the other Play video (QuickTime, Real or Windows Media)     GALLERY Click here for images of the wildfires     RESOURCES Click here to view the National Interagency Fire Center's map of large wildland fires Click here to view satellite images of the wildfires (NOAA Web site)    WEATHER FORECASTS Montana   • Big Sky   • Bozeman   • Butte   • Hamilton   • Helena   • Missoula Wyoming   • Yellowstone National Park Idaho   • Boise     ALSO U.S. wildfires outpace supply of firefighters A prosperous town anxiously waits as flames approach Wildfires spoil vacationers' plans out West Smokey may have overdone it     MESSAGE BOARD Nature's wrath

  •  A big fire sends heat and smoke as high as six miles up into the sky.

  •  The heat encounters cooler air, producing a cloud.

  •  The cloud generates little localized rain, as well as plenty of what is called "dry lightning."

"In other words, it may be raining here, and the bolt would shoot out five miles (eight kilometers) from where the rain is and hit the ground where there is no rain. And we see a lot of that this year in the West," said Fire Team Section Chief Ray Johnston.

A bolt of lightning is about as wide as a human finger.

Negatively charged lightning, the most common kind of lightning in thunderstorms, lasts about 41 microseconds or 41-millionths of a second.

But almost every positively charged lightning bolt, the kind most often associated with "dry lightning, lasts up to a full half a second longer -- zapping the ground with a 30,000 degree burst of what scientists call "continuing current."

"So the positives are more effective at starting fires," said Don Latham of the United States Department of Agriculture's Fire Sciences Laboratory. "Not because they're positive, necessarily, but rather because they have a continuing current, which not all negatives do."

By studying pictures from space shuttles, scientists have learned that lightning can occur up to 50 times a minute in a big storm.

Officials began mapping fires and "hot spots" in the 1960s. Currently the USDA Forest Service at the National Interagency Fire Center uses three aircraft: a Sabreliner 80 Jet, a Super King Air 200 and a King Air B-90.

With infrared cameras aboard these huge flying laboratories, it is possible to scan 1 million acres per hour and detect an eight- to 12-inch "hot spot" from 10,000 feet above the ground. The information helps create computer animations that enable firefighters to plot the likely course of fires caused by lightning.

But with a record number of acres going up in wildfires so far this year, scientists still don't have a way to break nature's vicious cycle of fires started by lightning, generating more lightning, which starts more fires.

Copyright 2000 CNN.com

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Infrared mapping
National Interagency Fire Center
Yellowstone National Park
GORP - Bitterroot National Forest
USDA Forest Service
Smokey Bear

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