Wildfire

A wildfire is an unplanned and uncontrolled fire occurring in an area of combustible vegetation. These fires are also known as forest fires or bushfires. While they can be destructive, many natural ecosystems actually depend on them. Certain plant species require the effects of fire for their growth and reproduction. To manage these risks, modern forest management often uses prescribed burns. These are controlled fires intended to promote natural forest cycles and reduce fire risk. However, these controlled burns can occasionally turn into wildfires by mistake.

To understand how a wildfire functions, one must look at the fire triangle. A fire requires an ignition source, combustible material, and oxygen from the ambient air. The spread of a fire is determined by the available fuels, the physical setting, and the weather. When a wildfire front approaches, it heats the surrounding air and woody material through convection and thermal radiation. This process happens in specific stages. First, heat vaporizes the water within the wood. Next, the wood undergoes pyrolysis, which is the chemical decomposition of organic material that produces flammable gases. Finally, the wood can smolder or ignite when heated sufficiently.

Wildfires are classified by how they move through different layers of vegetation. Ground fires burn subterranean organic matter, such as roots and duff on the forest floor. These fires typically smolder and can last for months. Surface fires burn low-lying matter like grass, leaves, and shrubs. These often burn at lower temperatures than crown fires. Ladder fires occur when flames consume material between low-level vegetation and the tree canopies. This includes small trees, vines, or invasive plants like kudzu. Crown fires, also called canopy or aerial fires, burn the suspended material at the top of tall trees.

Ignition can be caused by natural events or human activity. Natural causes include lightning, volcanic eruptions, rock fall sparks, and spontaneous combustion. Human activities are responsible for approximately 85% of wildfires. These include accidental ignitions from equipment like chainsaws or mowers, power line failures, and campfires. In tropical regions, farmers use slash-and-burn methods to clear fields. Arson is another human cause and may account for over 20% of human-caused fires. In California, arson generally accounts for 6% to 10% of annual wildfires.

Weather and climate significantly influence fire severity and spread. Climatic cycles that feature wet periods followed by heat and drought often precede severe fires. This is because wet periods create substantial fuels, which then dry out. Large wildfires can also create their own weather patterns through the stack effect. As air is heated, it rises, creating powerful updrafts or thermal columns. This can lead to the formation of pyrocumulus clouds. Extreme conditions may also produce fire whirls, which are spinning columns of fire with speeds exceeding 60 miles per hour.

Wildfires can spread rapidly through various mechanisms. A fire may move tangentially to form a flanking front or burn in the opposite direction by backing. They can also spread through spotting. This happens when winds carry firebrands, which are hot wood embers, through the air. These embers can jump over roads, rivers, or other firebreaks to ignite new spots downwind. In Australian bushfires, spot fires have been known to occur as far as 20 miles from the main fire front.

Wildfires have significant global and local impacts. They can affect human settlements through property destruction and direct health impacts from smoke. There is also a risk of contaminating soil and water. On a global scale, human practices have doubled the land area burned by wildfires compared to natural levels. This is linked to climate change, land-use changes, and wildfire suppression. Furthermore, the carbon released during a fire adds to carbon dioxide concentrations in the atmosphere. This contributes to the greenhouse effect, creating a climate change feedback loop.