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For other uses, see Wildfire (disambiguation).

A wildfire is any uncontrolled, non-structure fire that occurs in the wilderness, wildland, or bush.[1][2] Synonyms such as wildland fire, forest fire, brush fire, vegetation fire, grass fire, peat fire, bushfire (in Australasia), and hill fire are commonly used. The name wildfire was once a synonym for Greek fire as well as a word for any furious or destructive conflagration.[3] Wildfires are common in various parts of the world, occurring in cycles. They are often considered beneficial to the wilderness, as many plant species are dependent on the effects of fire for growth and reproduction. However, large wildfires often have detrimental atmospheric consequences.[4] Nine out of ten wildfires are reportedly caused by some human interaction;[5] others are caused by natural events such as lightening strikes, volcanic discharges, etc.

Prevention, detection, and suppression strategies have varied over the years, but now incorporate techniques that permit and even encourage fires in some regions. However, with extensive urbanization of wilderness, wildfires often involve the destruction of homes and other property located in the wildland-urban interface, a zone of transition between developed areas and undeveloped wilderness.[6]

Distinction between fires and wildfires

Fires start when an ignition source is brought into contact with a combustible material (e.g. peat, shrub, trees) that is subjected to sufficient heat and has an adequate supply of oxygen from the ambient air (see Fire triangle). The event that triggers ignition may be natural, such as a lightening strike, or an action of man.

File:2002 african fires nasa.png
Satellite image of wildfires on the African continent in 2002, part of a presentation on NASA's World Wind program

Wildfires differ from other fires in that they take place outdoors in areas of grassland, woodlands, bush, scrubland, peat, 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. Some of the defining characteristics of wildfires are the large area of burned land, upwards of Template:Acre to km2 to Template:Acre to km2, and higher;[7] the velocity of the burning front, which can as fast as 11 kilometres per hour (6.8 mph) in forests and 22 kilometres per hour (14 mph) in grasslands;Cite error: The <ref> tag has too many names (see the help page). and the ability of the burning front to unexpectedly change direction and to jump across fire breaks. The intense heat and smoke can lead to disorientation and loss of appreciation of the direction of the fire. These factors make fires particularly dangerous: for example, in 1959 the Mann Gulch fire thirteen Smokejumpers died when they lost their communication links and became disorientated; the fire consumed 18 km² (4500 acres).[8] In the Australian February 2009 Victorian bushfires, at least 210 people died and over 2,029 homes and 3,500 structures were lost when they became engulfed by wildfire.[9]

Even before the flames of a wildfire arrive at a particular location, heat from the wildfire 'front' can precede the flames drying and pre-heating flammable materials, due to temperatures nearing 800 °C (1,470 °F).[10][11] High-temperature and long-duration surface wildfires may encourage flashover or torching: the drying of tree canopy, the fuel, and their subsequent ignition from below.[12]

Characteristics of wildfires

Wildfire behavior is often complex and variably dependent on factors such as fuel type, moisture content in the fuel, humidity, windspeed, topology,[13] geographic location, and ambient temperature.[14] While growth and behavior are unique to each fire due to many complex variables, the basic characteristics can be described as follows:[15][16]

Physical properties

See also: Fire § Chemistry, Fire control, and Flash point
Experimental fire in Canada

A wildfire front is the portion sustaining continuous flaming combustion, where unburned material meets active flames, or the smouldering transition between unburned and burned material.[17] 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 100 °C (212 °F), then pyrolyzed around 230 °C (450 °F) to release flammable gases, then the wood will either will smolder around 380 °C (720 °F)[18] or ignite around 590 °C (1,000 °F).[19]

A high moisture content usually prevents ignition and slows propagation,[13] because higher temperatures are required to evaporate the contained water and heat the material to its flash point.[20] Dense forests usually provide more shade, resulting in lower ambient temperatures and greater humidity.[21] 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.[22] Plants continuously lose water by evaporation, but water loss is usually balanced by water absorbed from the soil, humidity, or rain.[23] When this balance is not maintained, plants dry out and are therefore more flammable, often a consequence of a long, hot, dry periods.[24][25]

Fuel Type

File:Wildfire3.jpg
A surface fire in the western Utah desert.
Charred landscape following a crown fire in the North Cascades.

Wildfires and their spread vary greatly based on the flammable material present and its vertical arrangement.[26] 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 as follows:

  • Ground: subterranean roots, duff and other buried organic matter. This fuel type is especially susceptible to ignition due to spotting (see Extremes). Ground fires typically burn by smoldering,[27] and can burn slowly for days to months,[28] such as peat fires in Kalimantan and East Sumatra, Indonesia, a result of a riceland creation project that unintentionally drained and dried the peat.[29]
  • Crawling or surface: low-lying vegetation such as leaf and timber litter, debris, grass (see grassland), and low-lying shrubbery.
  • Ladder: material between low-level vegetation and tree canopies, such as small trees, downed logs, vines, and invasive plants.[30]
  • Crown, canopy, or aerial: 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.[13][24][31][32][33][34]

Effect of Weather

Weather patterns such as heat waves, droughts, and cyclical climate changes such as El Niño can also dramatically increase the risk and alter the behavior of wildfires.[35] Years of precipitation followed by warm periods have encouraged more widespread fires and longer fire seasons.[36]

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.[37] 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.[38] Fires in Europe occur frequently during the hours of 12:00pm and 2:00pm.[39] US 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.[40]

Causes

Causes are numerous and include lightning,[41] arson,[42][43][44] volcanic activity, pyroclastic clouds, and underground coal fires.[45] Human activity plays a major role in wildfires, as fire is often considered the least expensive way to clear and prepare land for future use (see Slash-and-burn farming).[46][47] Forested areas cleared by logging encourages the dominance of flammable grasses, and abandoned logging roads overgrown by vegetation may act as fire corridors. Additionally, annual grassland fires in South Vietnam 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.[48]

Extremes

See also: Extreme weather and Firestorm

Fires in forested areas can move at speeds of 10.8 kilometres per hour (7 mph), while grass fires have been recorded at up to 22 kilometres per hour (14 mph).Cite error: The <ref> tag has too many names (see the help page). Wildfires can advance tangential to the main front to form a flanking front or burn opposite the direction of the main front by backing.[49] Wildfires may also spread by jumping or spotting, as winds and vertical convection columns carry hot wood embers (firebrands) and other burning materials through the air over roads, rivers, and other natural barriers or firebreaks.[41][50] Torching and crown fires are prone to spotting, and dry ground fuels that surround a wildfire are especially vulnerable to ignition from firebrands.[51] In Australian bushfires, spot fires have been documented "up to 10 kilometres (6 mi) ahead of the fire front."[52]

Air rises as it is heated, and large wildfires create powerful updrafts that will draw in new air from surrounding areas (see Thermal column, Stack effect).[53] Great vertical differences in temperature and humidity encourage pyrocumulus clouds and intense winds,[54] which are often 10 times faster than ambient wind (more than 50 miles per hour (80 km/h)[55] ). Extreme fire behavior includes wide rates of spread, prolific crowning and/or spotting, the presence of fire whirls, and a strong convection column.[56]

Ecology

Main article: Fire ecology
See also: Disturbance

Wildfires are common in climates that are sufficiently moist to allow the growth of trees but feature extended dry, hot periods. Such places include the vegetated areas of Australia and south east Asia, the veld in the interior and the fynbos 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.[57] Fire prevalence is also high during the summer and autumn months, when fallen branches, leaves, grasses, and scrub dry out and become more flammable.[58][59] Global warming may increase the intensity and frequency of droughts in many areas, creating more intense and frequent wildfires.[4][60][61]

Fireweed, an example of a pioneer species that quickly colonizes an area after a wildfire.

Wildfires are considered a natural part of the ecosystem of numerous wildlands,[58] where some plants have evolved to survive fires by a variety of strategies, such as fire-resistant seeds and reserve shoots that sprout after a fire (see Pioneer species). Smoke, charred wood, and heat are common fire cues that stimulate the germination of seeds.[62] Exposure to smoke from burning plants promotes germination in other types of plants by inducing the production of the orange butenolide.[63] Grasslands in Western Sabah, Malaysian pine forests, and Indonesian Casuarina forests are believed to have resulted from previous periods of fire.[64] Plants of the genus Eucalyptus contain flammable oils that can encourage fire,[65] and hard sclerophyll leaves to resist heat and drought, ensuring their dominance over less fire-tolerant species.[66]

However, 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.[67][68] Invading species such as Lygodium microphyllum and Bromus tectorum may create a positive feedback loop, increasing fire frequency even more.[30][69] Wildfires generate ash, destroy available organic nutrients, and cause an increase in water runoff, eroding away other nutrients and creating flash flood conditions.[70][71] Also, wildfires can 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.[72]

Atmospheric effects

See also: Air pollution, Atmospheric chemistry, Haze, 1997 Southeast Asian haze, 2005 Malaysian haze, and 2006 Southeast Asian haze
A Pyrocumulus cloud produced by a wildfire in Yellowstone National Park

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 8 and 13 kilometers. 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.[73] Previously, it was thought that most particles in the stratosphere came from volcanoes, but smoke and other wildfire emissions have been detected from the lower stratosphere.[74] Pyrocumulus clouds can reach 20,000 feet (6,100 m) over wildfires.[75] With an increase in fire byproducts in the stratosphere, ozone concentration was three times more likely to exceed health standards.[76] Satellite observation of smoke plumes from wildfires revealed that the plumes could be traced intact for distances exceeding 1,000 miles (1,600 km).[77] Computer-aided models (e.g. CALPUFF) may help predict the size and direction of wildfire-generated smoke plumes (see Atmospheric dispersion modeling).[78]

Wildfires can affect climate and weather and have major impacts on regional and global pollution.[79] Wildfire emissions contain greenhouse gases and a number of criteria pollutants which can have a substantial impact on human health and welfare.[80] Forest fires in Indonesia in 1997 were estimated to have released between 0.81 and 2.57 gigatonnes of CO2 into the atmosphere, which is between 13-40% of the annual carbon dioxide emissions from burning fossil fuels.[81][82] Atmospheric models suggest that these concentrations of sooty particles could increase absorption of incoming solar radiation during winter months by as much as 15%.[83]

Smoke trail from a fire seen while looking towards Dargo from Swifts Creek, 11 January 2007

Prevention

See also: Fire protection and Native American use of fire

Wildfire prevention "involves all measures that impede the outbreak of fire or reduce its severity and spread."[84] Effective prevention techniques allow supervising agencies to manage air quality, maintain ecological balances, protect resources,[85] as well as limiting the effects of future uncontrolled fires.[86] Many wilderness areas are now considered fire-dependent, and previous policies of complete suppression are believed to have upset natural cycles[87] and increased fuel loads and the amount of fire intolerant vegetation.[88] Current policies often permit fires to burn to maintain their ecological role, so long as the risks of escape onto high-value areas are mitigated.[89] 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.[90] Sources of human-caused fire may include arson, accidental ignition, or the uncontrolled use of fire in land-clearing and agriculture (for example, slash-and-burn farming in Southeast Asia).[91]

1985 Smokey Bear poster.

In the mid-1800s, explorers from the HMS Beagle observed Australian Aborigines using fire for ground clearing, hunting, and regeneration of plant food (see Fire-stick farming)[92] Such careful use of fire has been employed for centuries in the Kakadu National Park to encourage biodiversity.[93] In 1937, US 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 lastly Smokey Bear.[94]

While wildfires are a combination of factors such as topology, fuels, and weather, only fuels may be altered to affect future fire risk and behavior.[95] Current wildfire prevention programs may employ techniques such as wildland fire use and prescribed burns (controlled burns).[2][96] 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.[97] Vegetation may be burned periodically to maintain high species diversity,[98] and frequent burning of surface fuels limits fuel accumulation, thereby reducing the risk of crown fires.[99] Using strategic cuts of trees, fuels may also be removed by hand crews in order to clean and clear the forest, prevent fuel build-up, and create access into forested areas.[100] Chain saws and large equipment can be used to thin out ladders fuels and shred trees and vegetation to a mulch.[101] Multiple fuel treatments are often needed to influence future fire risks,[102] 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."[103] Additionally, while fuel treatments are typically limited to smaller areas, effective fuel management requires the administration of fuels across large landscapes in order to reduce future fire size and severity.[104]

Building codes in fire-prone areas typically require that structures be built of flame-resistant materials[105] and a defensible space be maintained by clearing flammable materials within a prescribed distance from the edifice.[106] Communities in the Philippines also maintain fire lines 5-10 metres (16-32 feet) wide between the forest and their village, and patrol these lines during summer months or seasons of dry weather.[107]

Detection

Dry Mountain Fire Lookout in the Ochoco National Forest, circa 1930.

Fast and effective detection is a key factor in wildfire fighting.[108] 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 early 1900s, and fires were reported using telephones, carrier pigeons, and heliographs.[109] 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 1988, a data station was established in West Yellowstone, permitting fire information delivery in approximately four hours.[110]

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 (see Asthenopia), 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.[111][112]

METHOD SIZE AREA RISK LEVEL DETECTION WITHIN
Aero/satellite Very large (>250,000 acres) Low 12 ha (30 acres)
Infrared/smoke scanners Medium (10,000-250,000 acres) Medium 0.01 ha (1,100 ft²) at 20 km (12 mi)[113]
Unaided lookout Medium (10,000-250,000 acres) Medium 22 m² (240 ft²) at 13 km (8 mi) and 44 m² (480 ft²) at 25 km (16 mi)[114]
Local sensor network Small (<10,000 acres) High 150 sq ft (15 m²)
Wildfires across the Balkans in late July 2007 (NASA satellite image)

A small, high risk area (thick vegetation, strong human presence or close to 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.[115][116][117] These may be battery-powered, solar-powered, or tree-rechargeable: able to recharge their battery systems using the small electrical currents in plant material.[118] Larger, medium-risk areas could 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 as well as night vision capabilities may be incorporated also into sensor arrays.[119][120][121]

The Old Fire burning in the San Bernardino Mountains (image taken from the International Space Station)

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.[122][123] Satellite-mounted sensors such as Envisat's AATSR and ERS-2's ATSR can measure infrared radiation emitted by fires, identifying hot spots greater than 39 °C (102 °F).[124][125] The NOAA's Hazard Mapping System combines remote-sensing data from satellite sources such as GOES, MODIS, and AVHRR for detection of fire and smoke plume locations (see 2006 Southeast Asian haze#Imagery).[126][127] However, satellite detection is prone to offset errors, anywhere from 2-3 km (1-2 mi) for MODIS and AVHRR data and up to 12 km (7.5 mi) for GOES data.[128] 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.[129]

Modeling

Main article: Wildfire modeling
See also: Weather forecasting
Fire Propagation Model

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, egg-shaped, and fan-shaped. 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.[130][131] However, large fires that exceed suppression capabilities are often regarded as statistical outliers in these analyses (see Extreme value theory), even though catastrophic wildfires greatly influence fire policies.[132]

Suppression

Main article: Wildfire suppression
See also: Firefighting
Tanker 910 during a drop demonstration in December, 2006

Wildfire suppression may include a variety of tools and technologies, from throwing sand and beating fires with sticks and palm fronds in rural Thailand,[133] to full-scale aerial assaults by planes and helicopters using drops of water and fire retardants.[134] 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. Wildfires in the United States are responsible for "about 95% of the total acres burned and close to 85% of all suppression costs,"[135][136] as suppression efforts and damage caused can exceed billions of US dollars annually. [137][138][139] Yearly fires in Canada consume an average of 6,200,000 acres (25,000 km2)[140] and in the US an average of 7,320,000 acres (29,600 km2).[141]

Fuel buildup can result in costly, devastating fires as more new houses and ranches are built adjacent to wilderness areas. Continued growth in fire-prone areas and rebuilding structures destroyed by fires has been met with criticism.[142] 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.[143] The ecological benefits of fire is often overridden by the economic benefits of protecting structures and lives.[139] Additionally, government policies that cover the wilderness usually differs from local and state policies that govern urban lands.[144][145]

See also

Notes

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References

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