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A California wildfire on September 5, 2008


A wildfire is any uncontrolled fire that occurs in the countryside or a wilderness area. Reflecting the type of vegetation or fuel, other names such as brush fire, bushfire, forest fire, grass fire, hill fire, peat fire, vegetation fire, and wildland fire may be used to describe the same phenomenon. A wildfire differs from other fires by its extensive size, the speed at which it can spread out from its original source, and its ability to change direction unexpectedly and to jump gaps, such as roads, rivers and fire breaks. Wildfires are characterized in terms of their physical properties such as speed of propagation; the combustible material present; the effect of weather on the fire; and the cause of ignition.

Wildfires occur on every continent except Antarctica. Fossil records and human history contain accounts of wildfires, which can be cyclical events. Wildfires can cause extensive damage, both to property and human life, but they also have various beneficial effects on wilderness areas. Some plant species depend on the effects of fire for growth and reproduction, although large wildfires may have negative ecological effects.

Strategies of wildfire prevention, detection, and suppression have varied over the years, and international wildfire management experts encourage further development of technology and research. Current techniques may permit and even encourage smaller fires in some regions to minimize or remove sources of flammable material from any wildfire that might develop. While some wildfires burn in remote forested regions, they can cause extensive destruction of homes and other property located in the wildland-urban interface: a zone of transition between developed areas and undeveloped wilderness.

Characteristics

Wildfire behavior is often complex and variably dependent on factors such as fuel type, moisture content in the fuel, humidity, windspeed, topography, geographic location, and ambient temperature. Growth and behavior are unique to each fire due to many complex variables, but each wildfire exhibits several basic characteristics.

Distinction from other fires

The name wildfire was once a synonym for Greek fire as well as a word for any furious or destructive conflagration. Wildfires differ from other fires in that they take place outdoors in areas of grassland, woodlands, bushland, scrubland, peatland, and other woody materials that act as a source of fuel, or combustible material. Buildings are not usually involved unless the fire spreads to adjacent communities and threatens these structures.

Distribution of wildfires on the African continent in 2002


Wildfires have a rapid forward rate of spread (FROS) when fueled by dense uninterrupted vegetation, particularly in wooded areas with canopies. They can escalate as fast as in forests and in grasslands. The ability of a wildfire's burning front to change direction unexpectedly and jump across fire breaks is another identifying characteristic. Intense heat and smoke can lead to disorientation and loss of appreciation of the direction of the fire. These factors make fires particularly dangerous: in 1949 the Mann Gulch fire in Montanamarker, USA, thirteen smokejumpers died when they lost their communication links and became disorientated; the fire consumed . In the Australian February 2009 Victorian bushfires, at least 173 people died and over 2,029 homes and 3,500 structures were lost when they became engulfed by wildfire.

While wildfires may be categorized as large, uncontrolled disasters that burn through or more, they can be as small as or less. However, even though smaller events may be included in wildfire modeling, most do not earn press attention, which can be problematic because the way the media portrays catastrophic wildfires influences public fire policies more than small fires do.

Physical properties

Wildfires occur when the necessary elements of a fire triangle intersect: an ignition source is brought into contact with a combustible material such as vegetation, that is subjected to sufficient heat and has an adequate supply of oxygen from the ambient air. A high moisture content usually prevents ignition and slows propagation, because higher temperatures are required to evaporate any water within the material and heat the material to its fire point. Dense forests usually provide more shade, resulting in lower ambient temperatures and greater humidity. Less dense material such as grasses and leaves are easier to ignite because they contain less water than denser material such as branches and trunks. Plants continuously lose water by evapotranspiration, but water loss is usually balanced by water absorbed from the soil, humidity, or rain. When this balance is not maintained, plants dry out and are therefore more flammable, often a consequence of a long, hot, dry periods.

Experimental fire in Canada


A wildfire front is the portion sustaining continuous flaming combustion, where unburned material meets active flames, or the smoldering transition between unburned and burned material. As the front approaches, the fire heats both the surrounding air and woody material through convection and thermal radiation. First, wood is dried as water is vaporized at a temperature of . Next, the pyrolysis of wood at releases flammable gases. Finally, wood can smolder at or, when heated sufficiently, ignite at . Even before the flames of a wildfire arrive at a particular location, heat transfer from the wildfire front can precede the flames, warming the air to and drying and pre-heating flammable materials. High-temperature and long-duration surface wildfires may encourage flashover or torching: the drying of tree canopies and their subsequent ignition from below.

Wildfires can advance tangential to the main front to form a flanking front, or burn opposite the direction of the main front by backing. They may also spread by jumping or spotting as winds and vertical convection columns carry firebrands (hot wood embers) and other burning materials through the air over roads, rivers, and other barriers that may otherwise act as firebreaks. Torching and fires in tree canopies encourage spotting, and dry ground fuels that surround a wildfire are especially vulnerable to ignition from firebrands. In Australian bushfires, spot fires have been documented from the fire front.

Especially large wildfires may affect air currents in their immediate vicinities by acting as natural chimneys. In an occurrence termed stack effect, air rises as it is heated, and large wildfires create powerful updrafts that will draw in new, cooler air from surrounding areas in thermal columns. Great vertical differences in temperature and humidity encourage pyrocumulus clouds, strong winds, and fire whirls with the force of tornadoes at speeds of more than . Wide rates of spread, prolific crowning and/or spotting, the presence of fire whirls, and strong convection columns signify extreme conditions.

Fuel type



The spread of wildfires varies based on the flammable material present and its vertical arrangement. Fuel density is governed by topography, as land shape determines factors such as available sunlight and water for plant growth. For example, fuels uphill from a fire are more readily dried and warmed by the fire than those downhill, yet burning logs can roll downhill. Overall, fire types can be generally characterized by their fuel:

  • Ground fires are fed by subterranean roots, duff and other buried organic matter. This fuel type is especially susceptible to ignition due to spotting. Ground fires typically burn by smoldering, and can burn slowly for days to months, such as peat fires in Kalimantan and Eastern Sumatramarker, Indonesiamarker, which resulted from a riceland creation project that unintentionally drained and dried the peat.
  • Crawling or surface fires are fueled by low-lying vegetation such as leaf and timber litter, debris, grass, and low-lying shrubbery. Human-ignited ground-clearing fires can spread to the Amazon rain forest, damaging ecosystems not particularly suited for heat or arid conditions.
  • Ladder fires consume material between low-level vegetation and tree canopies, such as small trees, downed logs, and vines. Invasive plants such as Kudzu and Old World climbing fern that scale trees may also encourage ladder fires.
  • Crown, canopy, or aerial fires devour suspended material at the canopy level, such as tall trees, vines, and mosses. The ignition of a crown fire is dependent on the density of the suspended material, canopy height, canopy continuity, and sufficient surface and ladder fires in order to reach the tree crowns.


Effect of weather

Heat waves, droughts, cyclical climate changes such as El Niño, and other weather patterns can also increase the risk and alter the behavior of wildfires dramatically. Years of precipitation followed by warm periods have encouraged more widespread fires and longer fire seasons. Since the mid 1980s, earlier snowmelt and associated warming has also been associated with an increase in length and severity of the wildfire season in the Western United States.

Fire intensity also increases during daytime hours. Burn rates of smoldering logs are up to five times greater during the day due to lower humidity, increased temperatures, and increased wind speeds. Sunlight warms the ground during the day and causes air currents to travel uphill, and downhill during the night as the land cools. Wildfires are fanned by these winds and often follow the air currents over hills and through valleys. Fires in Europe occur frequently during the hours of 12:00 p.m. and 2:00 p.m. U.S. wildfire operations revolve around a 24-hour fire day that begins at 1000 hours due to the predictable increase in intensity resulting from the daytime warmth.

Causes

A tree struck by a lightning
The four major natural causes of wildfire ignitions are lightning, volcanic eruption, sparks from rockfalls, and spontaneous combustion. The thousands of coal seam fires that are burning around the world can also flare up and ignite nearby flammable material such as those in Centralia, Pennsylvaniamarker, Burning Mountain, Australiamarker, and several coal-sustained fires in China. However, many wildfires are attributed to human sources such as arson, discarded cigarettes, sparks from equipment, and power line arcs (detected by arc mapping). In societies experiencing shifting cultivation where land is cleared quickly and farmed until the soil loses fertility, slash and burn clearing is often considered the least expensive way to prepare land for future use. Forested areas cleared by logging encourages the dominance of flammable grasses, and abandoned logging roads overgrown by vegetation may act as fire corridors. Annual grassland fires in Southern Vietnammarker can be attributed in part to the destruction of forested areas by herbicides, explosives, and mechanical land clearing and burning operations during the Vietnam War.

Ecology



Wildfires are common in climates that are sufficiently moist to allow the growth of vegetation but feature extended dry, hot periods. Such places include the vegetated areas of Australia and Southeast Asia, the veld in the interior and the fynbosmarker in the Western Cape of South Africa, and the forested areas of the United States and Canada. Fires can be particularly intense during days of strong winds and periods of drought. Fire prevalence is also high during the summer and autumn months, when fallen branches, leaves, grasses, and scrub dry out and become more flammable. Global warming may increase the intensity and frequency of droughts in many areas, creating more intense and frequent wildfires.

Many ecosystems are suffering from too much fire, such as the chaparral in southern California and lower elevation deserts in the American Southwest. The increased fire frequency in these areas has caused the elimination of native plant communities and have replaced them with non-native weeds. Invasive species such as Lygodium microphyllum and Bromus tectorum may create a positive feedback loop, increasing fire frequency even more. In the Amazon Rainforest, drought, logging and cattle ranching practices, and slash-and burn agriculture damage fire-resistant forests and promote the growth of flammable brush, creating a cycle that encourages more burning. Fires in the rainforest threaten its collection of diverse species, produce large amounts of CO2, and threaten to "clear or severely damage 55 per cent of the Amazon rainforest by the year 2030" according to Rebecca Lindsay of NASAmarker's Earth Observatory. Wildfires generate ash, destroy available organic nutrients, and cause an increase in water runoff, eroding away other nutrients and creating flash flood conditions. A wildfire in the North Yorkshire Moorsmarker, on September 17, 2003, destroyed some of heather and the underlying peat layers. Wind erosion stripped the ash and the exposed soil, revealing archaeological remains dating back to 10,000 BC; however continuing erosion of the burnt moorland threatened these remains. The burnt moorland was stabilized by sowing ryegrass and heather seeds to allow the heather to regenerate. Wildfires can also have an effect on climate change, increasing the amount of carbon released into the atmosphere and inhibiting vegetation growth, which affects overall carbon uptake by plants.

Plant adaptations



Some wilderness areas are now considered fire-dependent, especially those in North America. Previous policies of complete suppression are believed to have upset natural cycles and increased fuel loads and the amount of fire-intolerant vegetation. In the absence of human intervention, certain organisms in these ecosystems survive through adaptations to fire regimes. Such adaptations include physical protection against heat, increased growth after a fire event, and flammable materials that encourage fire and may eliminate competition. Dense bark, shedding lower branches, and high water content in external structures may protect the organisms from rising temperatures. Fire-resistant seeds and reserve shoots that sprout after a fire encourage species preservation, as embodied by pioneer species. Smoke, charred wood, and heat are common fire cues that stimulate the germination of seeds in a process called serotiny. Exposure to smoke from burning plants promotes germination in other types of plants by inducing the production of the orange butenolide.

Grasslands in Western Sabahmarker, Malaysian pine forests, and Indonesian Casuarina forests are believed to have resulted from previous periods of fire. Plants of the genus Eucalyptus contain flammable oils that encourage fire and hard sclerophyll leaves to resist heat and drought, ensuring their dominance over less fire-tolerant species. Chamise deadwood litter is low in water content and flammable, and the shrub quickly sprouts after a fire. Sequoia rely on periodic fires to reduce competition, release seeds from their cone, and clear the soil and canopy for new growth. Caribbean Pine in Bahamian pineyards have adapted to and rely on low-intensity, surface fires for survival and growth. An optimum fire frequency for is every 3 to 10 years. Too frequent fires favor herbaceous plants, and infrequent fires favor species typical of Bahamian dry forests.

Atmospheric effects



Most of the Earth's weather and air pollution reside in the troposphere, the part of the atmosphere that extends from the surface of the planet to a height of between . A severe thunderstorm or pyrocumulonimbus in the area of a large wildfire can have its vertical lift enhanced to boost smoke, soot and other particulate matter as high as the lower stratosphere. Previously, prevailing scientific theory held that most particles in the stratosphere came from volcanoes, but smoke and other wildfire emissions have been detected from the lower stratosphere. Pyrocumulus clouds can reach over wildfires. With an increase in fire byproducts in the stratosphere, ozone concentration was three times more likely to exceed health standards. Satellite observation of smoke plumes from wildfires revealed that the plumes could be traced intact for distances exceeding . Computer-aided models such as CALPUFF may help predict the size and direction of wildfire-generated smoke plumes by using atmospheric dispersion modeling.

Wildfires can affect climate and weather and have major impacts on regional and global pollution. Wildfire emissions contain greenhouse gases and a number of criteria pollutants which can have a substantial impact on human health and welfare. Forest fires in Indonesia in 1997 were estimated to have released between 0.81 and 2.57 gigatonnes (0.89 and 2.83 billion short tones) of CO2 into the atmosphere, which is between 13%–40% of the annual carbon dioxide emissions from burning fossil fuels. Atmospheric models suggest that these concentrations of sooty particles could increase absorption of incoming solar radiation during winter months by as much as 15%.



History

Human involvement

Wildfires have been mentioned in human history, from minor allusions in the Bible to classical writers such as Homer, although less focus was placed on uncultivated lands where wildfires occurred. Wildfires were also used in battles throughout human history as early thermal weapons. From the Middle ages, accounts were written of occupational burning as well as customs and laws that governed the use of fire. In 14th century Sardinia, firebreaks were used for wildfire protection. In the Atlantic Ocean on the island of Madeiramarker, fire was used to clear the land of Laurisilvamarker (laurel forest) in 1419. In Spain during the 1550s, sheep husbandry was discouraged in certain provinces by Phillip II due to the harmful effects of fires used in transhumance. In the countries bordering the Baltic Seamarker, fire in land use systems was typical during the Neolithic period until World War II. As early as the 1600s, Native Americans were observed using fire for many purposes including cultivation, signaling, and warfare. Scottish botanist David Douglas noted the native use of fire for tobacco cultivation, to encourage deer into smaller areas for hunting purposes, and to improve foraging for honey and grasshoppers. Charcoal sedimentary data off the Pacific coast of Central America also suggests that more burning occurred in the 50 years before the Spanish colonization of the Americas.

Wildfires typically occurred during periods of increased temperature and drought. An increase in fire-related debris flow in alluvial fans of northeastern Yellowstone National Parkmarker was linked to the period between AD 1050 and 1200, coinciding with the Medieval Warm Period. However, human influence caused an increase in fire frequency. Dendrochronological fire scar data and charcoal layer data in Finlandmarker suggests that, while many fires occurred during severe drought conditions, an increase in the number of fires during 850 BC and 1660 AD can be attributed to human influence. Charcoal evidence from the Americas suggested a general decrease in wildfires between 1 AD and 1750 compared to previous years. However, a period of increased fire frequency between 1750 and 1870 was suggested by charcoal data from North America and Asia, attributed to human population growth and influences such as land clearing practices. This period was followed by an overall decrease in burning in the 20th century, linked to the expansion of agriculture, increased livestock grazing, and fire prevention efforts.

Prevention



Wildfire prevention refers to the preemptive methods of reducing the risk of fires as well as lessening its severity and spread. Effective prevention techniques allow supervising agencies to manage air quality, maintain ecological balances, protect resources, and to limit the effects of future uncontrolled fires. North American firefighting policies may permit naturally-caused fires to burn to maintain their ecological role, so long as the risks of escape onto high-value areas are mitigated. However, prevention policies must consider the role that humans play in wildfires, since, for example, only 5% of forest fires in Europe are not related to human involvement. Sources of human-caused fire may include arson, accidental ignition, or the uncontrolled use of fire in land-clearing and agriculture such as the slash-and-burn farming in Southeast Asia. Landholders with flammable investments such as orchards and tree crops may encourage neighboring landowners to reduce fire risks.

In the mid-1800s, explorers from the HMS Beagle observed Australian Aborigines using fire for ground clearing, hunting, and regeneration of plant food in a method called fire-stick farming. Such careful use of fire has been employed for centuries in the lands protected by Kakadu National Parkmarker to encourage biodiversity. In 1937, U.S. President Franklin D. Roosevelt initiated a nationwide fire prevention campaign, highlighting the role of human carelessness in forest fires. Later posters of the program featured Uncle Sam, leaders of the Axis powers of World War II, characters from the Disney movie Bambi, and the official mascot of the U.S. Forest Service, Smokey Bear.

A photograph of a small fire on the slope of a hill.
The hill features small, green shubbery and some trees.
A person in light-colored clothing in seen in the background, some distance from the flames.


Wildfires are caused by a combination of factors such as topography, fuels, and weather. Other than reducing human infractions, only fuels may be altered to affect future fire risk and behavior. Wildfire prevention programs around the world may employ techniques such as wildland fire use and prescribed or controlled burns. Wildland fire use refers to any fire of natural causes that is monitored but allowed to burn. Controlled burns are fires ignited by government agencies under less dangerous weather conditions. Vegetation may be burned periodically to maintain high species diversity, and frequent burning of surface fuels limits fuel accumulation, thereby reducing the risk of crown fires. Using strategic cuts of trees, fuels may also be removed by handcrews in order to clean and clear the forest, prevent fuel build-up, and create access into forested areas. Chain saws and large equipment can be used to thin out ladder fuels and shred trees and vegetation to a mulch. Multiple fuel treatments are often needed to influence future fire risks, and wildfire models may be used to predict and compare the benefits of different fuel treatments on future wildfire spread. However, controlled burns are reportedly "the most effective treatment for reducing a fire’s rate of spread, fireline intensity, flame length, and heat per unit of area" according to Jan Van Wagtendonk, a biologist at the Yellowstone Field Station. Additionally, while fuel treatments are typically limited to smaller areas, effective fire management requires the administration of fuels across large landscapes in order to reduce future fire size and severity.

Building codes in fire-prone areas typically require that structures be built of flame-resistant materials and a defensible space be maintained by clearing flammable materials within a prescribed distance from the edifice. Communities in the Philippinesmarker also maintain fire lines wide between the forest and their village, and patrol these lines during summer months or seasons of dry weather.



Detection



Fast and effective detection is a key factor in wildfire fighting. Early detection efforts were focused on early response, accurate day and nighttime use, the ability to prioritize fire danger, and fire size and location in relation to topography. Fire lookout towers were used in the United States in the early 1900s and fires were reported using telephones, carrier pigeons, and heliographs. Aerial and land photography using instant cameras were used in the 1950s until infrared scanning was developed for fire detection in the 1960s. However, information analysis and delivery was often delayed by limitations in communication technology. Early satellite-derived fire analyses were hand-drawn on maps at a remote site and sent via overnight mail to the fire manager. During the Yellowstone fires of 1988marker, a data station was established in West Yellowstonemarker, permitting fire information delivery in approximately four hours.

Currently, public hotlines, fire lookouts in towers, and ground and aerial patrols can be used as a means of early detection of forest fires. However, accurate human observation may be limited by operator fatigue, time of day, time of year, and geographic location. Electronic systems have gained popularity in recent years as a possible resolution to human operator error. These systems may be semi- or fully-automated and employ systems based on the risk area and degree of human presence, as suggested by GIS data analyses. An integrated approach of multiple systems can be used to merge satellite data, aerial imagery, and personnel position via GPS into a collective whole for near-realtime use by wireless Incident Command Centers.

A small, high risk area that features thick vegetation, a strong human presence, or is close to a critical urban area can be monitored using a local sensor network. Detection systems may include wireless sensor networks that act as automated weather systems: detecting temperature, humidity, and smoke. These may be battery-powered, solar-powered, or tree-rechargeable: able to recharge their battery systems using the small electrical currents in plant material. Larger, medium-risk areas can be monitored by scanning towers that incorporate fixed cameras and sensors to detect smoke or additional factors such as the infrared signature of carbon dioxide produced by fires. Brightness and color change detection and night vision capabilities may be incorporated also into sensor arrays.



Satellite and aerial monitoring can provide a wider view and may be sufficient to monitor very large, low risk areas. These more sophisticated systems employ GPS and aircraft-mounted infrared or high-resolution visible cameras to identify and target wildfires. Satellite-mounted sensors such as Envisat's Advanced Along Track Scanning Radiometer and European Remote-Sensing Satellite's Along-Track Scanning Radiometer can measure infrared radiation emitted by fires, identifying hot spots greater than . The National Oceanic and Atmospheric Administration's Hazard Mapping System combines remote-sensing data from satellite sources such as Geostationary Operational Environmental Satellite (GOES), Moderate-Resolution Imaging Spectroradiometer (MODIS), and Advanced Very High Resolution Radiometer (AVHRS) for detection of fire and smoke plume locations. However, satellite detection is prone to offset errors, anywhere from for MODIS and AVHRR data and up to for GOES data. Satellites in geostationary orbits may become disabled, and satellites in polar orbits are often limited by their short window of observation time. Cloud cover and image resolution and may also limit the effectiveness of satellite imagery.

Suppression



Wildfire suppression may include a variety of tools and technologies, including throwing sand and beating fires with sticks and palm fronds in rural Thailandmarker, using silver iodide to encourage snow fall in China, and full-scale aerial assaults by ALTUS II unmanned aerial vehicles, planes, and helicopters using drops of water and fire retardants. Complete fire suppression is no longer an expectation, but the majority of wildfires are often extinguished before they grow out of control. While more than 99% of the 10,000 new wildfires each year are contained, escaped wildfires can cause extensive damage. Worldwide damage from wildfires is in the billions of euros annually. Wildfires in Canada and the US consume an average of per year.

Fuel buildup can result in costly, devastating fires as new homes, ranches, and other development are built adjacent to wilderness areas. Continued growth in fire-prone areas and rebuilding structures destroyed by fires has been met with criticism. However, the population growth along the wildland-urban interface discourages the use of current fuel management techniques. Smoke is an irritant and attempts to thin out the fuel load is met with opposition due to desirability of forested areas, in addition to other wilderness goals such as endangered species protection and habitat preservation. The ecological benefits of fire is often overridden by the economic benefits of protecting structures and lives. Additionally, government policies that cover the wilderness usually differs from local and state policies that govern urban lands.

Modeling

Fire Propagation Model


Wildfire modeling is concerned with numerical simulation of wildfires in order to understand and predict fire behavior. Wildfire modeling can ultimately aid wildfire suppression, namely increase the safety of firefighters and the public, reduce risk, and minimize damage. Using computational science, wildfire modeling involves the statistical analysis of past fire events to predict spotting risks and front behavior. Various wildfire propagation models have been proposed in the past, including simple ellipses and egg- and fan-shaped models. Early attempts to determine wildfire behavior assumed terrain and vegetation uniformity. However, the exact behavior of a wildfire's front is dependent on a variety of factors, including windspeed and slope steepness. Modern growth models utilize a combination of past ellipsoidal descriptions and Huygens' Principle to simulate fire growth as a continuously expanding polygon. Extreme value theory may also be used to understand large wildfires. However, large fires that exceed suppression capabilities are often regarded as statistical outliers in standard analyses, even though fire policies are influenced by catastrophic wildfires more so than small fires.

See also

Notes



References

Bibliography

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