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.
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 10.8 kilometers per hour (6.7 mph) in forests and 22 kilometers per hour (14 mph) 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 Montana, USA, thirteen smokejumpers died when they lost their communication links and became disorientated; the fire consumed 18 square kilometers (4,400 acres). 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 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. 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
See also: Fire, Fire control, Extreme weather, and FirestormWildfires 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.
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 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). 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. 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 10 kilometers (6 mi) 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 80 kilometers per hour (50 mph). 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 Sumatra, Indonesia, which resulted from a riceland creation project that unintentionally drained and dried the peat
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