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Wildfires |
What shapes a wildland fire?
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| What shapes a wildland fire? |
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Fire is a chemical reaction, combustion, requiring three components:
Removing any "side" of this fire triangle stops a fire. The availability of fuels, heat, and oxygen to a wildland fire is strongly influenced by the interplay of climate patterns, recent weather, and the topography of the landscape. Fuels The amount and type of fuel play a key role in determining the strength and extent of a fire. Fire managers use the term fuel loading to describe the amount of burnable material in a given area, by weight. Vegetation offering a large surface area relative to its volume, such as grasses, leaves, and ground litter (dead branches, fallen leaves and needles), will ignite at lower temperatures than large, smooth-barked tree trunks. Trees with high oil or resin content, such as eucalyptus or fir, burn more readily than hardwoods. Climate plays a key role in fuel availability. The Mediterranean climate found not only in that region but also along the coasts of California and Chile, the east coast of Australia, and elsewhere, provides a setup for wildfire vulnerability. The typical pattern is a wet season that spurs plant growth, followed by a dry season in which the new vegetation withers, becoming tinder in abundance. Seasonal climate and recent weather also shape fuel moisture content, the key factor determining how readily fuels burn. Live plants and trees can hold as much as three times their weight in moisture during a healthy growing season. Dead fuels hold far less moisture, topping out at about 30%. Moisture levels of dead fuels can change daily. Fuels wet from the spring runoff of melting snow pack or recent precipitation are slow to ignite and produce lower-temperature, smoldering fires compared to fuels dried out from summer heat or prolonged drought. Fuel buildup as litter on forest floors and in overcrowded stands of thin-trunked trees is a hazardous, unintended consequence of U.S. policy to intervene in the natural cycle of wildland fire. The fuel glut created by 100 years of fire suppression has prompted scientists and policy makers to call for changes in forest management. A debate is now under way on the merits of increased logging versus reintroduction of fire to reduce fuel loads (see, for example, NIFC: Wildland Fire Policy and Resource Management Planning). In 2002 President Bush proposed a Healthy Forests Initiative, including proposals for reducing hazardous fuel buildup that have received both praise and criticism. For more on public policy issues, see A people problem: subdivisions in the forests, below. Topography Hills and mountains don't just hinder fire crews, they also influence the direction and speed of wildfires. Valleys on the lee side of a mountain ridge, the side away from the prevailing wind, are vulnerable to hot, dry downslope winds. The prevailing winds rise along the windward side and shed their moisture as snow or rain as they reach cool temperatures at ridge tops. Once over the top, they are heated by increasing pressure as they descend to lower altitudes on the lee side of the ridge. Called foehn in the Alps and chinook in the Rocky Mountains, these downslope winds increase fire danger first by drying out the fuels in their path and then by steering the fire into new fuels once started. California's Santa Ana winds are another, particularly notorious example. Along mountain valleys, solar heating and nighttime cooling bring changes in wind direction. Fires driven upslope by daytime heating will typically settle down after sunset. This nocturnal laydown happens as decreased temperatures and raised relative humidity cause the moisture content in dead fuels to rise. However, nighttime doesn't always ensure the cooling and increased humidity firefighters hope for. For example, temperature inversions can leave midslope areas much warmer and drier, and thus more vulnerable to fire, than other parts of the mountainside or valley. Flames at the base of a slope heat the vegetation higher up, releasing combustible gases that burst into new flames, spreading fire uphill. Typically, this heating can move a blaze up a hillside at speeds up to 15 miles per hour. But observers have seen acceleration as fast as 100 mph during a crown fire in treetops on a steep slope, when hot gasses suddenly leaped out from the treetops, igniting ground fuels farther up the slope. This threat makes firefighting on steep slopes especially dangerous. Weather Temperature, humidity, and winds influence fire development. Cold fronts and other regional features bring moist air that slows fires down with cooler temperatures and raised humidity. But the strong winds of a passing front can feed fresh oxygen to stir up a fire that has died down, steer a fire in new directions, or accelerate its progress, making cold fronts a mixed blessing. Dry thunderstorms with lightning and winds are major culprits in starting and spreading wildfires. A wet thunderstorm can offer relief if it brings sufficient rain. Lightning is the ignition source for most wildfires in remote areas of the western United States. But wherever they live close to forests, people now cause the majority of wildland fires. Besides the weather brought in by regional cold fronts or local thunderstorms, wildland fires create their own weather. The heated air in a raging wildfire rises, sending water vapor released during combustion into the atmosphere. This buoyancy also produces intense updrafts and horizontal winds that shape and drive the fire line itself, possibly triggering sudden changes in direction and intensity that can threaten firefighters' lives. NCAR meteorologist Janice Coen heads a collaborative team developing the NCAR atmosphere-fire model, a computer model that takes this fire-generated weather into account. Water vapor released by the heat of a fire is sometimes lifted high enough to form pyrocumulus clouds (Latin, pyro = fire). Cumulus clouds form when rising warm air encounters cooler air aloft. When conditions are right, a line of pyrocumulus clouds may rise along the same path as a large smoke plume from a major wildfire. Can a wildfire ever put itself out? In certain situations, a large fire might make a localized area prone to thunderstorm development. However, the scale of the fire is usually far greater than the amount of rain—if any—that falls from pyrocumulus clouds. Further reading The COMET Introduction to Fire Behavior illustrates fuel, topography, and weather issues. More on fire's physical processes can be found at NIFC: The Science of Wildland Fire.
Smoke and toxins | Top |
There are many pollutants in forest fire smoke that the U.S. Environmental Protection Agency would regulate if the source were human industrial activity, according to Chris Geron, an environmental scientist in the EPA's Air Pollution Prevention and Control Division. While there is no method to control these emissions, Geron has been working with colleagues at NCAR to get a better idea of what is in the smoke. "We're most concerned about fine particles and gaseous emissions," says Geron. "Many of them are carcinogens," he notes. Fine particles of invisible soot and ash are too small for the human respiratory system to filter out. Their effects range from irritation to an increased risk of contracting cancer. The gases found downwind in the smoke plume of a burning fire include
When air quality warnings are issued by the U.S. Department of Agriculture's Forest Service or other agencies, the EPA recommends staying indoors and filtering air through an air conditioner, wearing a respirator rated for fine particles, or leaving the area until the fire is suppressed or the winds have shifted. Heavy metals may reach such high temperatures that they turn into gases in the heat of wildfires. NCAR researcher Hans Friedli and colleagues have found significant amounts of one metal, mercury, in laboratory burns and in research flights over wildland fires. Atmospheric mercury (from both natural and human-generated sources) is absorbed by forest vegetation when it falls or rains out onto leaves or needles. During a wildfire, the stored mercury is released back into the atmosphere, only to fall or rain out again. This redistributed mercury can enter watersheds (water sources), where interaction with microbes converts it into methyl mercury, a neurotoxin. Wildfires also contribute large quantities of carbon monoxide (CO) to the atmosphere. Inhaling too much CO can be fatal, and this gas poses the greatest threat to wildland firefighters, who must watch for signs of lightheadedness or disorientation. An urban area downwind from a wildfire might see levels of CO exceeding EPA standards. Large, intense fires add such sizeable amounts of CO to the atmosphere that this gas can be measured hundreds or thousands of miles away—even from space. And because CO persists in the atmosphere for several weeks, it can be used to trace the path of pollution plumes above wildfires drifting for thousands of miles. The MOPITT instrument aboard NASA's Terra satellite detects plumes of carbon monoxide as they hover about 2 miles (3 kilometers) above Earth's surface. MOPITT tracked a sizeable pollution plume from the fires in Southern California during October 2003.
Climate change | Top | Scientists study the global "budget" of carbon to determine where it is being stored and where it is being released into the atmosphere as carbon dioxide (CO2), the major greenhouse gas implicated in increasing global temperatures. Research at NCAR and elsewhere shows that the policy of suppressing fires over the past century has locked up about 25% of Earth's carbon budget in forest vegetation. "We haven't seen as much global warming yet as we could have because forests have been storing all that carbon," says NCAR scientist David Schimel, who is studying the effect of wildfires on the global carbon budget. Scientists brought together by the United Nations and the World Meteorological Organization to assess the current and future condition of Earth's climate foresee a likely increase in summer drying over the U.S. interior and the interiors of other midlatitude continents during the 21st century. The associated drought, the researchers note, brings increased risk of forest fire ("Summary for Policy Makers," Climate Change 2001: Impacts, Adaptation and Vulnerability, Intergovernmental Panel on Climate Change). If more North American forests burn, whether from wildfires or increased prescribed burning, the carbon now stored in forests could be released back into the atmosphere as CO2.
People and development | Top | The U.S. Department of Agriculture (USDA) reports a 10% rate of population increase in nonmetropolitan areas during the 1990s. The recent spurt in development within and at the edges of western forests poses new challenges for forest and town managers. Because trees and undergrowth have different fuel characteristics than buildings, the methods for fighting fires in forests and subdivisions are different. Those differences pose serious problems for fire managers working at the urban-wildland interface, according to Jack Cohen, a research scientist in the Fire Sciences Laboratory of the USDA Forest Service in Missoula, Montana. The federal government's National Fire Plan has made recommendations for reducing the risks of living in forested areas, but these recommendations are not without controversy. Conflict arises, for example, when a prescribed burn to thin a once-isolated forest threatens a new subdivision with exposure to smoke levels that violate EPA standards. Other potential conflicts involve jurisdiction. If, for example, land managers alter the kind and amount of vegetation on public lands but lack authority to do so on private property within nearby communities, the overall risk of wildfire may not be reduced. The National Wildland/Urban Interface Fire Program sponsors Firewise, an information resource for homeowners, local governments, and others designed to help people live with wildland fire. Strategies for living in wildfire-prone areas are offered, such as ways property owners can design and create defensible space that is more likely to survive a wildfire. One effort to help states and local communities examine risks and plan prevention measures is under way at NCAR and UOP. A team including Robert Harriss is designing a computer-aided toolkit employing geographic information systems, the NCAR fire-atmosphere model, and computer visualization techniques. With the toolkit, planners will be able to create alternative scenarios and see how different land-use decisions and other actions play out in virtual time and space for their own communities.
Backgrounders provide supplementary information and should not be considered comprehensive sources. All Backgrounders have been reviewed by a scientific expert. Opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any of UCAR's sponsors. UCAR is an Equal Opportunity/Affirmative Action employer.  Wildfire Web links | Top | News updates National Interagency Fire Center
National Park Service Fire News
USDA Forest Service Fire & Aviation Newsroom
Putting the current season in context
NIFC Billion Dollar U.S. Weather Disasters since 1980
Glossary NIFC Glossary of Wildland Fire Terms Science and policy COMET Introduction to Fire Behavior: Topography, Fuels, and Weather
International Association of Wildland Fire
USDA Forest Service Fire Science Page
U.S. National Fire Plan Reports
Wildland Fire R&D Collaboratory
Impacts Colorado Department of Public Health and Environment
NCDC Billion Dollar U.S. Weather Disasters since 1980
USDA Forest Service Fire Science Page
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