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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.
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. In recent years, policy debates have focused on the relative merits of increased logging versus reintroduction of fire to reduce fuel loads.
For more on public policy issues, see People and development, below.
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 fires spread as fast as 100 mph during a crown fire traveling uphill, when narrow streams of flame suddenly shoot upslope along the ground. This threat makes firefighting on steep slopes especially dangerous.
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, an expert on the interactions between fire and weather, heads a collaborative team that developed NCAR's Coupled Atmosphere–Wildland Fire–Environment model (CAWFE), a computer model that takes 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, pyrocumulus clouds may rise above the smoke 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.
The Introduction to Fire Behavior training module from UCAR's MetEd illustrates fuel, topography, and weather issues.
This MOPITT image shows plumes of carbon monoxide streaming from Alaskan fires across North America and the Atlantic during mid-July 2004. (Image courtesy the NCAR MOPITT Team.)
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 Risk Management - Air Research division. For example, fires can increase ground-level ozone concentrations to the point where they violate federal health standards.
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 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 has tracked sizeable pollution plumes from fires in Southern California, Alaska, and western Canada, among others.
Concerns have been raised about the negative impact of the fire retardant used to fight wildfires in hard-to-reach areas. The red-tinged retardant, a combination of water and fertilizer, is typically dropped by "slurry bomber" aircraft to assist with wildfire containment. Environmental scientists are studying whether the ammonium-based fertilizer poses a threat to forest wildlife and plants.
Because fire is a complex product of many factors, as described above, a changing climate could affect the frequency and severity of wildfires in a variety of ways. Temperature is one of the most important. In a warmer atmosphere, water is more likely to evaporate from vegetation, making the landscape more fire prone. Moreover, many parts of the globe are already seeing rain and snow falling in heavier bursts, but less often, with longer periods of dryness in between.
In a 2011 special report on the risk of extreme weather and climate events, the Intergovernmental Panel on Climate Change examined wildfire risk in a warming world. The report found that evidence of climatic influence in forest fires throughout the American West and Canada and that severe wildfires are projected to increase in many areas. Wildfire activity in the western United States increased substantially in the late 20th century, and that increase is caused by higher temperatures and earlier snowmelt, according to the IPCC report.
Another climate concern is the effect of fires themselves on the atmosphere over the long term. 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 David Schimel. Schimel, who has studied the effect of wildfires on the global carbon budget, is a former NCAR scientist now serving as chief scientist for NEON, the National Ecological Observatory Network.
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. But there's still some uncertainty regarding the overall effect of more fires on global warming.
The U.S. Department of Agriculture (USDA) reports a 4.5% rate of population increase in nonmetropolitan areas from 2001 to 2011. 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 researchers in the Missoula Fire Sciences Lab of the USDA Forest Service in Missoula, Montana.
In the early 2000s, the federal government's National Fire Plan made recommendations for reducing the risks of living in forested areas, but these recommendations were not without controversy. More recently, the Wildland Fire Leadership Council has been tasked with directing the development of a national Cohesive Wildfire Management Strategy. The council is a cooperative effort between the U.S. departments of Interior and Agriculture and their land management agencies. Past and current research and policy efforts are described in detail at the council's Forests and Rangelands website.
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 the Firewise Communities Program, 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.
National Interagency Fire Center page with daily statistics, current wildfires, news releases
Current information on specific geographic areas
For the latest on fires within U.S. national parks.
Information on fire management on public lands, including current fire activity, fire weather reports, closures and restrictions, and fire news.
Nationwide projections of fire-conducive conditions for the next one to eight days.
An expert assessment (map and description) of drought risk for the next three months, updated on the first and third Thursday of each month.
Putting the current season in context
National Interagency Fire Center page with a trove of links to current and historical data, from lightning vs. human-caused fires to acreage burned to significant wildland fire events, and more.
From the National Wildfire Coordinating Group, this is one of several useful lexicons available on the Web.
USFS Fire Danger Rating System (Adjective Class Rating)
Chart explaining adjectives (low to extreme) and color codes (dark green to red) for rating fire danger.
Science, training, and policy
MetEd has developed extensive training material for fire fighters, meteorologists, emergency managers, and others interested in understanding fire behavior. Two distance learning courses and more than 30 individual training modules at three levels of expertise are available free of charge (site registration required). Skill levels range from an introduction to fire behavior to advanced fire-weather forecasting and support for all-hazards incidents, hazardous material releases, and incidents of national significance.
IAWF is an organization of global wildland fire professionals. The site includes Wildfire magazine and the Journal of Wildland Fire.
This page provides links to research on natural fire regimes for different ecosystems, the effects of fire on landscape, firefighting technology, fire behavior, and other studies fostered by the U.S. Department of Agriculture Forest Service.
The U.S. Geological Survey sponsors research on fire management support, such as "green mapping" of vegetation cover during the growing season; studies of postfire effects, such as debris-flow hazards and threats to water supplies; and other topics in fire history and ecology.
Forests and Rangelands Resources (Wildland Fire Leadership Council)
Supporting research, background, and archival information for federal wildland fire management initiatives is collected on this website from the Wildland Fire Leadership Council. The WFLC is a cooperative effort between the U.S. departments of Interior and Agriculture and their land management agencies.
See the section on air quality during wildfires, available in English and Spanish.
This site offers online fact sheets and downloadable files on reducing the risks to people and property from wildfires.
This site offers information for people who live or vacation in fire-prone areas of the United States, provided by a consortium of wildland fire agencies, including the USDA Forest Service, the Department of Interior, the National Association of State Foresters, the U.S. Fire Administration, and the National Fire Protection Association.
The National Climatic Data Center, "the nation's scorekeeper," maintains this chronological list of major events, including qualifying wildfire season damages, acreage, and deaths.
Statistics from the National Interagency Fire Center on the name, date, location, acres burned, and impacts of important wildfires, from a fire recorded by Lewis and Clark in 1804 to the present.
This page includes links to research on the effects of fire on landscape.
This page includes links to information on postfire effects, such as debris-flow hazards and threats to water supplies.
UCAR Digital Image Library (search "fire")
USDA Forest Service Active Fire Maps (includes satellite imagery)
Scientific advisers: Janice Coen, Hans Friedli
Last updated : June 2012
Backgrounders provide supplementary information and should not be considered comprehensive sources.
The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.