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.[11][12] Growth and behavior are unique to each fire due to many complex variables, but each wildfire exhibits several basic characteristics.[12][13]
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.[2] 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.
Wildfires have a rapid forward rate of spread (FROS) when fueled by dense uninterrupted vegetation, particularly in wooded areas with canopies.[14] They can escalate as fast as 10.8 kilometers per hour (6.7 mph) in forests and 22 kilometers per hour (14 mph) in grasslands.[15] 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 Montana, USA, thirteen smokejumpers died when they lost their communication links and became disorientated; the fire consumed 18 square kilometers (4,400 acres).[16] 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.[17]
While wildfires may be categorized as large, uncontrolled disasters that burn through 0.4 to 400 square kilometres (100 to 100,000 acres) or more, they can be as small as 0.0010 square kilometers (0.25 acres) or less.[18][19][20] 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.[21][22][23]
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.[3][11] Dense forests usually provide more shade, resulting in lower ambient temperatures and greater humidity.[12] 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.[24] Plants continuously lose water by evapotranspiration, but water loss is usually balanced by water absorbed from the soil, humidity, or rain.[25] When this balance is not maintained, plants dry out and are therefore more flammable, often a consequence of a long, hot, dry periods.[26][27]
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.[28] 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). Next, the pyrolysis of wood at 230 °C (450 °F) releases flammable gases. Finally, wood can smolder at 380 °C (720 °F) or, when heated sufficiently, ignite at 590 °C (1,100 °F).[29][30] 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 800 °C (1,470 °F) and drying and pre-heating flammable materials.[24][31] High-temperature and long-duration surface wildfires may encourage flashover or torching: the drying of tree canopies and their subsequent ignition from below.[32]
Wildfires can advance tangential to the main front to form a flanking front, or burn opposite the direction of the main front by backing.[33] 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.[34][35] Torching and fires in tree canopies encourage spotting, and dry ground fuels that surround a wildfire are especially vulnerable to ignition from firebrands.[36] In Australian bushfires, spot fires have been documented 10 kilometers (6 mi) from the fire front.[37]
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.[38] 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 80 kilometers per hour (50 mph).[39][40][41] Wide rates of spread, prolific crowning and/or spotting, the presence of fire whirls, and strong convection columns signify extreme conditions.[42]
Fuel type
The spread of wildfires varies based on the flammable material present and its vertical arrangement.[43] 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:[note 1]
- 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 Sumatra, Indonesia, which resulted from a riceland creation project that unintentionally drained and dried the peat.[44][45]
- Crawling or surface fires are fueled by low-lying vegetation such as leaf and timber litter, debris, grass, and low-lying shrubbery.[46] Human-ignited ground-clearing fires can spread to the Amazon rain forest, damaging ecosystems not particularly suited for heat or arid conditions.[47]
- 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.[48]
- 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.[49]
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.[50] Years of precipitation followed by warm periods have encouraged more widespread fires and longer fire seasons.[51] 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.[52]
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.[53] 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.[54] Fires in Europe occur frequently during the hours of 12:00 p.m. and 2:00 p.m.[55] 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.[56]
Causes
The four major natural causes of wildfire ignitions are lightning, volcanic eruption, sparks from rockfalls, and spontaneous combustion.[57] 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, Pennsylvania, Burning Mountain, Australia, and several coal-sustained fires in China.[58][59] However, many wildfires are attributed to human sources such as arson, discarded cigarettes, sparks from equipment,[60] and power line arcs (detected by arc mapping).[61] 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.[62][63] 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 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.[64]
Ecology
Wildfires are common in climates that are sufficiently moist to allow the growth of vegetation but feature extended dry, hot periods.[5] Such places include the vegetated areas of Australia and Southeast 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.[65] Fire prevalence is also high during the summer and autumn months, when fallen branches, leaves, grasses, and scrub dry out and become more flammable.[66][67] Global warming may increase the intensity and frequency of droughts in many areas, creating more intense and frequent wildfires.[4][68][69]
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.[70][71] Invasive species such as Lygodium microphyllum and Bromus tectorum may create a positive feedback loop, increasing fire frequency even more.[48][72] 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.[73] 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 NASA's Earth Observatory.[74][75] Wildfires generate ash, destroy available organic nutrients, and cause an increase in water runoff, eroding away other nutrients and creating flash flood conditions.[43][76] A wildfire in the North Yorkshire Moors, on September 17, 2003, destroyed some 2.5 square kilometers (600 acres) 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.[77] 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.[78]
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.[66][79][80] 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.[5] 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.[81] Exposure to smoke from burning plants promotes germination in other types of plants by inducing the production of the orange butenolide.[82]
Grasslands in Western Sabah, Malaysian pine forests, and Indonesian Casuarina forests are believed to have resulted from previous periods of fire.[83] 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.[84][85] Chamise deadwood litter is low in water content and flammable, and the shrub quickly sprouts after a fire.[5] Sequoia rely on periodic fires to reduce competition, release seeds from their cones, and clear the soil and canopy for new growth.[86] 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.[87]
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 8 and 13 kilometers (5 and 8 mi). 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.[88] 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.[89] Pyrocumulus clouds can reach 6,100 meters (20,000 ft) over wildfires.[90] With an increase in fire byproducts in the stratosphere, ozone concentration was three times more likely to exceed health standards.[91] Satellite observation of smoke plumes from wildfires revealed that the plumes could be traced intact for distances exceeding 1,600 kilometers (1,000 mi).[92] Computer-aided models such as CALPUFF may help predict the size and direction of wildfire-generated smoke plumes by using atmospheric dispersion modeling.[93]
Wildfires can affect climate and weather and have major impacts on regional and global pollution.[94] Wildfire emissions contain greenhouse gases and a number of criteria pollutants which can have a substantial impact on human health and welfare.[95] 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.[96][97] Atmospheric models suggest that these concentrations of sooty particles could increase absorption of incoming solar radiation during winter months by as much as 15%.[98]
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