How do wildfires start?

Wildfires are uncontrolled and unwanted fires that typically start in forested areas, although they can spread to more populated regions. They can start in many ways, but all require a few main ingredients: an ignition source, fuel and oxygen. An ignition source is anything that can provide enough heat to start a spark. Common natural igniters are lightning and lava. Most fires, however, are caused by humans: Around 85 percent of wildfires over the past two decades were started by people. Some human-caused wildfires are set intentionally, but most are accidental, with triggers such as unextinguished campfires, target shooting in extremely hot and dry environments or setting off smoke bombs . Power line failures also have started wildfires. Pay attention to fire weather alerts from your local authorities before lighting a fire. Fuel is anything flammable. Trees, grasses, shrubs, fallen pine needles and dead leaves are typical fuels for wildfires. The amount of moisture in the vegetation greatly affects how quickly a fire consumes the fuel. Vegetation with high amounts of moisture does not readily feed a fire because the fire’s heat must evaporate the water content before it consumes the plant. Dry fuel, on the other hand, can easily help start and spread a fire.

How do wildfires spread?

Once a fire has started, it can rapidly spread depending on the fuel, weather and topography. Fuel is needed to start a fire, but fuel composition also influences how fast a fire will grow. Certain plants, trees and shrubs contain oils and resins that burn more quickly and intensely. The amount of moisture contained in a fuel also matters. When moisture in vegetation is less than 30 percent of its total weight, the fuel is considered dead. Dead fuels are more susceptible to burning in warmer conditions. The U.S. Forest Service monitors fuel moisture to help determine potential fire behavior. Wind is one of the most important conditions that can rapidly enlarge a fire. Strong wind can bring a fresh supply of oxygen to the fire. It can also blow embers and sparks miles away from the initial fire and start fires in new locations, a phenomenon known as spotting. Topography also can promote or inhibit fire behavior. For instance, fires tend to move quicker uphill than downhill. This uphill sprint is partly because heat emanating from the fire rises up the slope and preheats vegetation farther up ahead of the fire, which allows it to burn more readily once the fire reaches the spot.

When is a blaze ‘active’? When is it ‘contained’? When is it ‘out’?

Wildfires can go through several stages before being extinguished or dying out on their own. “Active” blazes are any fires that are not declared “out.” While the fire burns, firefighters work to “contain” it by putting up physical barriers around it to prevent it from spreading past a certain point. But physical barriers to a fire can be natural features on the landscape, such as rivers, in addition to obstacles created by the firefighters, such as trenches. For instance, a fire that is 100 percent contained means that 100 percent of the fire is surrounded by a physical barrier. However, the blaze is still expected to burn within the perimeter, potentially for days, weeks or months. Sometimes, extreme weather can help fires jump these physical barriers and break out of containment. When the fire has been contained, firefighters aim to “control” it. Firefighters reinforce barriers, remove fuel that could help the fire spread and cool hot spots (particularly active parts of the fire). A fire is considered “out” when no hot spots can be detected within the containment lines for at least 48 hours.

How do lightning fires begin?

Most lightning fires in the country occur in the West and are caused by dry thunderstorms. Dry thunderstorms are “high elevation” storms whose rainfall largely evaporates before reaching the ground, leaving behind only dry lightning and thunder. Dry lightning in the presence of hot and dry conditions on the ground can increase the risk of wildfires. Lightning fires are particularly worrisome because lightning can strike in highly forested areas with plenty of overgrown vegetation for fuel. Although less numerous than human-caused fires, lightning fires have burned more acreage in past years than human-caused fires have.

How are heat waves connected to wildfires?

Heat waves occur when temperatures are higher than a location’s historical average for at least two or three days. Most heat waves are caused when a zone of high pressure moves into an area, causing the air to sink, compress and heat up. The sinking air also dissipates cloud cover, allowing for more direct heating of the ground by the sun. This summer, heat waves have been responsible for record-high temperatures in many parts of the western United States and Canada. These heat waves were exacerbated by drought conditions, as exceptionally dry land cannot cool itself through evaporation. Hot, dry conditions greatly increase the risk for dry thunderstorms and wildfires. One day after hitting an all-time high earlier this summer, a village in British Columbia was engulfed by a fast-moving wildfire. Extreme heat events are also responsible for many heat-related hospitalizations, illnesses and deaths each year.

Should I be concerned about wildfires hundreds of miles from me?

You don’t need to be near a wildfire to feel its effects. Wildfire smoke traveling from the West Coast can obstruct skies and, in certain weather, worsen air quality for those in the East. The smoke carries dangerous pollutants, which can be seen in the form of soot. It also contains smaller, invisible particulate matter known as PM 2.5 that can penetrate deep into our lungs and cause health problems ranging from burning eyes to lung disease. The Environmental Protection Agency, National Weather Service and local health agencies monitor air quality across the country and issue warnings when air quality reaches unhealthful levels. During periods of unhealthful air quality, limit your time outdoors.

What are red flag warnings and fire weather watches?

The Weather Service issues alerts to inform the public and fire departments of critical weather that could rapidly increase wildfire activity. A red flag warning is the highest alert and means high temperatures, low humidity and strong winds are expected to combine in the next 24 hours or already exist and can result in extreme fire behavior. Extreme fire behavior means a wildfire is likely to rage out of control. It is characterized by high speed, strong convection, fire whirls and prolific crowning (spreading via the crowns or tops of trees and shrubs) or spotting (when sparks or embers from a fire are carried away by the wind and start new fires). During a red flag warning, people must exercise extreme caution because a small spark could cause a major wildfire. Do not throw cigarettes or matches from a moving vehicle, cover all burn barrels with weighted metal lids, drown fires with water until everything is cold to the touch, and never leave a fire unattended. Local authorities may have additional specific guidance. A fire weather watch, which is one level below a warning, indicates that critical fire conditions are possible but not imminent. Watches can be issued up to 72 hours before conditions develop or before red flag warning conditions are predicted.

What are the National Wildland Fire Preparedness Levels?

The National Wildland Fire Preparedness Levels are five stages of the nation’s wildfire responses. The National Interagency Fire Center (NIFC), the country’s support center for wildland fires, establishes these levels throughout the year to make sure enough resources are available for any fires emerging around the country. Preparedness levels are determined by fuel and weather, fire activity and fire suppression resources available across the country. As the preparedness levels increase, so does the need for incident management teams and suppression resources, such as fire crews, helicopters, air tankers, and bulldozers. As of July 14, the NIFC had declared a Preparedness Level 5.

Weather

What you need to know about how wildfires spread

By Kasha Patel

July 28, 2021 at 7:00 a.m. EDT

Trees burn in the Bootleg Fire in southern Oregon on July 25. (Bootleg Fire Incident Command/AP)

High temperatures. Low humidity. Little rainfall. Dry vegetation. Fast winds.

Wildfires depend on a combination of environmental conditions to start and spread. As global temperatures rise, research shows these conditions are appearing more intensely and frequently — escalating the risk of wildfires.

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This summer, the United States has suffered through record-breaking heat, destructive fires and stifling smoke. Wildfires have burned more than a million acres in at least 12 states. Anomalously high temperatures and extremely dry conditions, particularly in the western United States, are expected to exacerbate fire conditions into the fall.

Here are a few key facts about how wildfires spread and how to protect yourself.