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Recipe for Wildfires

Updated September 12, 2012

By WeatherBug Meteorologists

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Wildfires can cause devastating destruction while burning valuable forests, wildlands and even people`s homes and businesses.

A wildfire is an uncontrolled fire often occurring in woodland and other rural areas. It can destroy houses, businesses and crops. Wildfires often begin unnoticed, but can spread very quickly, igniting brush, trees and homes. Common causes of wildfires include lightning, human carelessness and arson, volcanic eruptions, winds, drought and heat waves. Cyclical climate changes such as El Nino and human development can also further the risk of wildfires. The following are the most influential wildfire ingredients and triggers:

Weather

The weather, in particular temperature, humidity and rainfall can affect the conditions contributing to both spark and spread of fires. Extremely high temperatures can cause fuels like shrubbery, underbrush and dead leaves to dry out faster, making fire ignition and spreading much easier. Low humidity and low rainfall amounts can both help fires spread. Drier air evaporates moisture from fuels, making it easier for combustion.

Lightning

When lightning strikes, it can easily spark a fire. Lightning often strikes trees, power lines and the open ground. Lightning can explode the item it strikes and leave a glassy residue as the heat melts on the ground or vegetation. A fire may not start immediately after a lightning strike, but can smolder for some period of time before becoming a wildfire.

Drought

Long-term periods of drought can play a major role in the number and intensity of wildfires. Any huge deficit of rainfall, especially when coupled with very warm temperatures, can allow forest fuels such as dead leaves and branches to become unusually dry. Dry forest fuels are very susceptible to fire, especially when relative humidities are extra low and wind speeds are high.

Wind

Wind plays a major role in fire spread and can change the course of a burning fire. The wind can influence the direction of fire spread, accelerate the flame onto new fuels, further the evaporation of moisture and fuels in the advance of the fire, and carry embers and other flaming material aloft, depositing them on unburned areas. The direction and intensity of wind is influenced by global wind patterns, pressure differences, solar heating, topography and even the fire itself.

Special Case: The Santa Ana Winds

California is well known for wildfires, especially during the summer and fall. A key factor in igniting California wildfires are the Santa Ana winds. The Santa Ana winds are blustery, dry and warm winds that blow out of the desert. Even though the winds are often hot, the Santa Anas develop when the desert is cold and are therefore most common during the cool season stretching from October to March. However, Santa Ana conditions can exist anytime in which the Great Basin tends to be cooler than Southern California.

The typical scenario for Santa Anas to exist is when high pressure builds over the Great Basin, allowing the cold air there to sink. This cooler air is forced downslope which compresses and warms as it descends. As the air descends and the temperature rises, the relative humidity drops and winds at sea level end up much drier. The air can also speed up as it is channeled through passes and canyons. Once started, the Santa Ana winds cause vegetation to dry out, increasing the danger of wildfires. If fires are ignited, the winds can fan the flames and help them spread.

Santa Anta in Action

In the fall of 2007, the largest wildfire evacuation in U.S. history occurred in San Diego due to the Santa Ana winds. In San Diego County alone, more than 500,000 residents were told to evacuate. The fast blowing Santa Ana winds fueled 16 major fires along California`s coast. During the autumn of 2007, these dry, warm winds made the coastal areas warmer then the deserts and drove the humidity down to less than 15 percent. The combination of wind, heat and dryness turned the low lying vegetation into sparks for wildfires. With gusts up to 70 mph, the winds made it nearly impossible to battle the fires. In addition to the Santa Ana`s, the 2007 wildfires were also shaped by an extended drought and extremely high temperatures.

Human Helped

Campfires, especially if left unattended or improperly extinguished, can spread to neighboring fuels and start major wildfires. Discarded cigarettes can also ignite wildfires, if the conditions are favorable for ignition. Power transmission lines can also be a common source of ignition for wildfires if trees fall on power lines, or electrical transformers malfunction. Another human cause is fireworks; fireworks can ignite dry vegetation with sparks and hot debris. Arsonists have also been blamed for several big, destructive wildfires over the years.

Natural Disasters

Natural disasters such as volcanoes can also have an effect on wildfires. Volcanic ash can be thousands of degrees and when in contact with fuel, can ignite intense wildfires.

Things You Can Do...

It is important to be aware of an increased threat for wildfires when any of the above conditions exist. In case wildfires do ignite, there are a number of safety tips to remember to help protect life and property from wildfires. First, always remember to check with local fire authorities to obtain current fire restrictions.

  • If planning a camping trip, clear the campfire sites down to bare soil, never leave a campfire site unattended, and always keep a bucket of water nearby.

  • In residential areas, rake surface fuels such as leaves and tree branches away from homes in wooded areas. Remember to follow local regulations about clearing and burning brush and shrubbery from your land.
  • If you get rain, don`t assume the drought is over. Fuels dry out quickly with higher temperatures and increased winds.
  • Keep lawnmowers and agricultural equipment in proper working condition and avoid rocks and other materials which might cause a spark.
  • Connect hoses to outside water faucets to help protect your home in the event of a wildfire. Your household water source can prove to be vital in protecting your home in some circumstances.
  • And lastly, the most common cause of wildfires is humans: Careless debris burning, unattended campfires, dumping hot charcoal or hot ashes, driving on a flat tire, parking or driving in dry grass, dragging chains on the road and welding can all cause fires.
  • ---

    Fire safety tips, in part, courtesy of the Texas Forest Service

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