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Wildfire

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(Redirected from Forest fire)
Fire in  Mountains (image taken from the )
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Fire in San Bernardino, California Mountains (image taken from the International Space Station)

A wildfire, also known as a forest fire, vegetation fire, grass fire, or bushfire (in Australasia), is an uncontrolled fire in wildland often caused by lightning; other common causes are human carelessness and arson.

Drought and the prevention of small forest fires are major contributors to extreme forest fires.

The word "wildfire" originally meant a medieval European weapon substance somewhat like napalm; the word got its present meaning by people misunderstanding the expression "spread like wildfire".
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Forestfire5.jpg
A massive forest fire

Contents

Background

Green Knoll Wildfire in
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Green Knoll Wildfire in Jackson, Wyoming

Wildfires are common in many places around the world, including much of the vegetated areas of Australia, forest areas of the United States and Canada, where the climates are sufficiently moist to allow the growth of trees, but feature extended dry, hot periods when fallen branches, leaves, and other material can dry out and becomes highly flammable. Wildfires are also common in grasslands and scrublands. Wildfires tend to be most common and severe during years of drought and occur on days of strong winds. With extensive urbanization of wildlands, these fires often involve destruction of suburban homes located in the wildland urban intermix.

Today it is accepted that wildfires are a natural part of the ecosystem of wildlands, where, at the least, plants have evolved to survive fires by a variety of strategies (from possessing reserve shoots that sprout after a fire, to fire-resistant seeds), or even encourage fire (for example eucalypts contain flammable oils in the leaves) as a way to eliminate competition from less fire-tolerant species. In 2004, researchers discovered that exposure to smoke from burning plants actually promotes germination in other types of plants by inducing the production of the chemical butenolide. Most native animals, too, are adept at surviving wildfires.

On occasions, wildfires have caused large-scale damage to private property, particularly when they have reached urban-fringe communities, destroying many homes and causing deaths.

Slash (small, rotten, mis-shapen, or otherwise undesirable wood discarded during logging) has historically provided the fuel for devastating fires such as the fires in Michigan in the 19th century.

The aftermath of a wildfire can be as disastrous if not more so than the actual fire itself. A particularly destructive fire burns away all the plants and trees which prevented erosion. If heavy rains occur after such a fire, landslides, ash flows, and flash floods are to be expected. Not only does this result in severe property damage for those living in the immediate fire area, but it also affects the quality of the local water supply.

There have been wildfires long before Man evolved. One main component of Carboniferous north hemisphere coal is charcoal left over by forest fires.

Behavior

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Deerfire.jpg

When the water reserves in the soil are between 100 and 30%, the evaporation of water in plants is balanced by water absorbed from the soil. Below this threshold, the plants dry out, releasing flammable essences to keep some moisture. A consequence of a long hot and dry period is that the air contains flammable essences and plants are drier and highly flammable.

The propagation of the fire has three mechanisms:

  • "crawling" fire: the fire spreads via low level vegetation (e.g. bushes)
  • crown fire: a fire that "crowns" (spreads to the top branches of trees) can spread at an incredible pace through the top of a forest
  • "jumping" fire: burning branches and leaves are carried by the wind and start distant fires; the fire can thus "jump" over a road, river, or even a firebreak

The Nevada Bureau of Land Management identifies several different wildfire behaviors. For example, extreme fire behavior includes wide rates of spread, prolific crowning and/or spotting, the presence of fire whirls, or a strong convection column. Extreme wildfires behave erratically and unpredictably.

In southern California, under the influence of Santa Ana winds, wildfires can move at tremendous speeds, up to 40 miles (60 km) in a single day, consuming up to 1,000 acres (4 km²) per hour. Dense clouds of burning embers push relentlessy ahead of the flames crossing firebreaks without pause.


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Propagation_model_wildfire.png
Propagation of the fire with a characteristic shape of a "pear"

The powerful updraft caused by a large wildfire will draw in air from surrounding areas. These self-generated winds can lead to a phenomenon known as a firestorm.

French models of wildfires dictate that a fire's front line will take on the characteristic shape of a pear; the major axis being aligned with the wind. In the case of the fires in South-Eastern France, the speed of the fire is estimated to be 3 to 8% of the speed of the wind, depending on the conditions (density and type of vegetation, slope). Other models predict an elliptical shape when the ground is flat and the vegetation is homogeneous.

Prevention

For many decades the policy of the United States Forest Service was to suppress all fires, and this policy was epitomized by the mascot Smokey Bear and was also the basis of parts of the movie Bambi. The policy began to be questioned in the 1960s, when it was realized that no new sequoias had been grown in the redwood forests of California, because fire is an essential part of their life cycle. This produced the policy of controlled burns to reduce underbrush. This clears much of the undergrowth through forest and woodland areas, making travel and hunting much easier while reducing the risk of dangerous high-intensity fires caused by many years of fuel buildup.
A forest fire's charred remains
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A forest fire's charred remains

The previous policy of absolute fire suppression in the United States has resulted in the buildup of fuel in some ecosystems such as dry ponderosa pine forests. However, this concept has been misapplied in a "one-size-fits-all" application to other ecosystems such as California chaparral. Fire suppression in southern California has had very little impact over the past century. The amount of land burned in 6 southern California counties has been relatively unchanged. In fact, fire frequency has been increasing dramatically over the past century in lock step with population growth. Urbanization can also result in fuel buildup and devastating fires, such as those in Los Alamos, New Mexico, East Bay Hills, within the California cities of Oakland and Berkeley, between October 19 and 22, 1991, all over Colorado in 2002, and throughout southern California in October, 2003. Homes designed without considering the fire prone environment in which they are built have been the primary reason for the catastrophic losses experienced in wildfires.

On average, wildfires burn 4.3 million acres (17,000 km²) in the United States annually. In recent years the federal government has spent $1 billion a year on fire suppression. 2002 was a record year for fires with major fires in Arizona, California, Colorado, and Oregon.

The risk of major wildfires can be reduced by reducing the amount of fuel present. In wildland, this can be accomplished by either conducting "controlled burns" - deliberately setting areas ablaze under less dangerous weather conditions in spring or autumn - or physical fuel removal by removing some trees as is conducted in many American forests. Such techniques are best used within the wildland/urban interface where communities connect with wild open space. Prescribed burns in the backcountry, away from human habitations, are not particularly effective in preventing large fires. All the large catastrophic fires in the United States have been wind driven events where the amount of fuel (trees, shrubs, etc.) has not been the most important factor in fire spread.

People living in fire-prone areas typically take a variety of precautions, including building their homes out of flame-resistant materials, reducing the amount of fuel near the home or property (including firebreaks - their own miniature control lines, in effect), and investing in their own firefighting equipment.

Rural farming communities are rarely threatened directly by wildfire. These types of communities are usually located in large areas of cleared, usually grazed, land, and in the drought conditions present in wildfire years there is often very little grass left on such grazed areas. Hence the risk is minimized. However, urban fringes have spread into forested areas, for example in Sydney and Melbourne, and communities have literally built themselves in the middle of highly flammable forests. These communities are at high risk of destruction in bushfires.

Fire suppression

An Air National Guard C-130 Hercules drops fire retardant on wildfires in Southern California]]Most fire-prone areas have large firefighter services to help control bushfires. As well as the water-spraying firetrucks most commonly used in urban firefighting, bushfire services use a variety of alternative techniques. Typically, forest fire fighting organizations will use large crews of 20 or more people who travel in trucks to the fire. These crews use heavier equipment to construct firebreaks, and are the mainstay of most firefighting efforts. Other personnel are organized into fast attack teams typically consisting of 5-8 people. These fast attack teams are helicoptered into smaller fires or hard to reach areas as a preemptive strike force. They use portable pumps to douse small fires and chainsaws to construct firebreaks or helicopter landing pads if more resources are required. Hand tools are commonly used to construct firebreaks and remove fuels around the perimeter of the fire to halt its spread, including shovels, rakes, and the pulaski, a tool unique to wildland firefighting. In the eastern United States, portable leaf blowers are sometimes used. In the western United States, large fires often become extended campaigns, and temporary fire camps are constructed to provide food, showers, and rest to fire crews. These large fires are often handled by 20 person hand crews, known as "hotshot" crews, specially organized to travel to large fires around the west.

Fast attack teams are often considered the elite of firefighting forces, as they sometimes deploy in unusual ways. If the fire is on a particularly steep hill or in a densely wooded area, they may rappel or fast-rope down from helicopters. If the fire is extremely remote, firefighters known as smokejumpers may parachute into site from fixed-wing aircraft. In addition to the aircraft used for deploying ground personnel, firefighting outfits often possess helicopters and water bombers specially equipped for use in aerial firefighting. These aircraft can douse areas that are inaccessible to ground crews and deliver greater quantities of water and/or flame retardant chemicals. Managing all of these various resources over such a large area in often very rugged terrain is extremely challenging, and often the Incident Command System is used. As such, each fire will have a designated fireboss who oversees and coordinates all the operations on the fire. This fireboss is ultimately responsible for the safety of the firefighters and for the success of firefighting efforts.


Large fires are of such a size that no conceivable firefighting service could attempt to douse the whole fire directly, and so alternative techniques are used. In alternative approaches, firefighters attempt to control the fire by controlling the area that it can spread to, by creating "control lines", which are areas that contain no combustible material. These control lines can be produced by physically removing fuel (for instance, with a bulldozer), or by "backburning", in which small, low-intensity fires are started, using a device such as the driptorch, or pyrotechnic flares known as "fusees", to burn the flammable material in a (hopefully) controlled way. These may then be extinguished by firefighters, or, ideally, directed in such a way that they meet the main fire front, at which point both fires run out of flammable material and are thus extinguished.


Unfortunately, such methods can fail in the face of wind shifts causing fires to miss control lines or to jump straight over them (for instance, because a burning tree falls across a line, burning embers are carried by the wind over the line, or burning tumbleweeds cross the line).

The actual goals of firefighters vary. Protection of life (those of both the firefighters and "civilians") is given top priority, then private property according to economic and social value and also to its "savability" (for example, more effort will be expended on saving a house with a tile roof than one with a wooden-shake roof). In very severe, large fires, this is sometimes the only possible action. Protecting houses is regarded as more important than, say, farming machinery sheds, although firefighters, if possible, try to keep fires off farmland to protect stock and fences (steel fences are destroyed by the passage of fire, as the wire is irreversibly stretched and weakened by it). Preventing the burning of publicly-owned forested areas is generally of least priority, and, indeed, it is quite common (in Australia, at least) for firefighters to simply observe a fire burn towards control lines through forest rather than attempt to put it out more quickly - it is, after all, a natural process. Ensuring the safety of firefighters takes priority over fire suppression when a situation becomes dangerous. When arriving on a scene a fire crew will establish a safe zone, known as an "anchor point," which they can retreat to if necessary, and are trained to keep aware of escape routes and designate lookouts (known by the acronym LCES - for lookouts, communications, escape routes, safety zones). They carry portable fire shelters that can be deployed if all else fails. This emphasis on safety is reinforced with a list of 18 "watch out situations" for firefighters to be aware of, which warn of potentially dangerous conditions.

French strategy

Forest covers about 28% of France (189,000 km?, 46.7 million acres); the biggest forest is the for괠des Landes de Gascogne, a pine forest which covers over 10,000 km? (2.5 million acres), mostly part being in the Landes (6,193 km?, 1.53 million acres).

This relatively small area allows for excellent forest management. Preventive logging (used for the wood and paper industries) and mandatory clearing of undergrowth for private landowners are enforced. The law forbids the construction of isolated homes in fire-prone areas. These measures usually eliminate the need to send teams to remote zones that are not accessible from the ground; airborne teams are seldom used (there are no smokejumpers), but teams are occaisionally deployed via helicopter (e.g. on the top of a hill). However, the reglementation is not strictly applied, partly due to conflicting interest: undergrowth are interesting for hunters (an important lobby in rural area) because they hide boars, mayors allow the construction of isolated homes to attract rich citizens, pines are preferred to other species because they are more cost-effective (but burn much easier than, for example, cork oak)… It is symptomatic to note that there are less fires in the biggest for괠des Landes de Gascogne than in the massif des Maures, which is much smaller (334.85 km?, 82,741 acres).

The firefighting system is called DFCI (d馥nse des for괳 contre l'incendie, literally "Forest defense against fire"). The system used for the localisation is called "DFCI coordinates" (coordonn饳 DFCI) and the access tracks are called "DFCI tracks" (chemins DFCI).

Three different forces are involved in fire prevention and firefighting:

  • the police forces (Police nationale, Gendarmerie and foresters), for the prevention (patrols, information of wanderers, possibly sanction of dangerous acts);
  • d鰡rtement employees (sapeurs forestiers, forest sappers) and volunteers construct preventive firebreaks and keep a firewatch;
  • sapeurs-pompiers (french firefighters); local firefighters are reinforced in summer by columns from lower risk d鰡rtements and by military troops assigned to the civile defense (UIISC, Unit頤'instruction et d'intervention de la s飵rit頣ivile).

M鴩o France, the French national meteorology institute, computes an index for 40 locations in France (3 are in the Landes), the "forest meteo index" (indice for괠m鴩o, IFM). This index ranges from 0 to 20, the higher the index, the higher the risk. This index helps in prepositionning men and vehicles.

A strategy pioneered in France dictates that firefighters attack the sides of the fire to make the head as small as possible and limit its propagation until it dies. This attack can be performed by land teams or by air units. Excellent coordination is required between air and land teams: dropping tons of water on ground-based firefighers can cause severe injuries and death. Usually, land teams attack one side and air teams attack the other. To prevent accidents, land-based firefighters will raise their firehose and shoot water into the air if they hear an aircraft. The water jet is easy to see from the air and allows air crews to avoid those firefighters' positions.

French firefighters do not use backburning. The firebreaks are made preventively and are not aimed at stopping the fire. Rather, they are used as access lines and withdrawal routes.

Land teams use four wheel drive fire engines called "tank engines for wildfire" (CCF, camion-citerne pour feu de for괧'), with three (sometimes four) firefighters and 2,000 to 4,500 L of water. These engines have a protection system that sprays water around the truck in case it is surrounded by the fire; in such a case, the firefighters lock themselves in the truck. For this reason, a minimum water level is always kept (300 to 500 L). Ondivual equipment also includes a filtering hood (similar to a gas mask) and a protective poncho in case the firefighter could not reach the truck. Unfortunately, this is sometimes not sufficient and several deaths occur each year in Southern France and Corsica.

Maintaining an adequate water supply to the front lines is very difficult. Inflatable tanks are often prepositionned in strategic places downwind of a fire. They also use 30 tons tank trucks, called "large capacity tank trucks" (CCGC, camion citerne de grande capacit駧).

As in other jurisdictions, the main concern is the protection of people and houses. The population is always evacuated beforehand. The protection of a house consists on:

  • parking the car inside the garage;
  • closing the shutters;
  • spraying water on the house to prevent the temperature rising via radiation;
  • making a line of wet ground to direct the fire around the building.

Protecting a house usually involves four fire engines; isolated houses are the greatest problem.

Atmospheric effects

Most of the Earth's weather and air pollution reside in the troposhere, 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 in the area of a large wildfire can have its vertical lift enhanced to boost smoke, soot and other particles as high as the lower stratosphere. (Wang, 2003)

Perviously, it was thought that most particles in the stratosphere came from volcanoes or were generated by high-flying aircraft. Collection of air samples from the stratosphere in 2003 led to detection of carbon monoxide and other gasses related to combustion at a level 30 times higher than can be accounted for by commercial aircraft.

Satellite observation of smoke plumes from wildfires revealed that the plumes could be traced intact for distances exceeding 5,000 kilometers. This observation suggests that the plumes were in the stratosphere above weather conditions that would have brought the plume back to earth.

Atmospheric models suggest that these concentrations of sooty particles could increase absorbtion of incoming solar radiation during winter months by as much as 15%. (Baumgardner,et al., 2003)

Statistics

Every year, the burnt surface represents about:

  • France: 30,000 hectares (300 km?, 12,140 acres), 0.04% of the country
  • Portugal:
    • 1991 : 182,000 ha (1,820 km?, 73,650 acres), i.e. 2% of the territory
    • 2003 : 424,900 ha (4,249 km?, 298,000 acres), i.e. 4.6% of the territory; 20 deaths ;
    • 2004 : 120,530 ha (1,205.3 km?, 1.05 million acres), i.e. 1.3% of the territory
  • United States: 1.74 million hectares (17,400 km?, 4.3 million acres) i.e. 0.18% of the territory

See also

References

  • Baumgardner, D., et al. 2003. Warming of the Arctic lower stratosphere by light absorbing particle. American Geophysical Union fall meeting. Dec. 8-12. San Francisco.
  • Fromm, M., et al. 2003. Stratospheric smoke down under: Injection from Australian fires/convection in January 2003. American Geophysical Union fall meeting. Dec. 8-12. San Francisco.
  • Wang, P.K. 2003. The physical mechanism of injecting biomass burning materials into the stratosphere during fire-induced thunderstorms. American Geophysical Union fall meeting. Dec. 8-12. San Francisco.


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