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Weather Almanac for July 2004
As I walked through the forest, my footsteps crunched with each stride across the desiccated litter. Not a good sound, even for our normally dry summer, for the preceding winter had also been dry and rather mild. Such was the condition of many of the forests in British Columbia and other western regions of North America during the Summer of 2003. Here in southern Vancouver Island, the fire danger potential was high, but we are fortunately blessed with few lightning storms that set many remote fires ablaze. Such was not the case, however, elsewhere in this corner of the continent.
In mid-summer, thousands of fires had sprung up across this Canadian province, set by both human carelessness and lightning, to produce one of the most devastating fire years in memory. At one time the danger hit close to home as my daughter's family was trapped between major wildfires, cutting access out of their town and sending dense smoke into the broad valleys in eastern BC. An evacuation convoy ushered them out the only open road, and they spent several weeks with us until the danger lessened.
Weather plays a major role in the conditions leading to high forest and grass fire dangers and also becomes a factor once a fire begins. Fire weather is most strictly defined to mean weather conditions during a burn that affect the fire's behaviour. But in the longer term, weather sets the stage for most of the fires by withholding needed moisture from the vegetation across an area. Such conditions may last for weeks, months and even years prior to a fire outbreak, increasing the danger with each passing day.
As a result, most major forested areas today have instrumented weather stations that monitor several weather factors including humidity, precipitation, wind speed and temperature which are used by fire control centres across the continent to assess and monitor fire potential. Such data are combined to form a fire weather index (See Canadian Fire Weather Index) designed to alert the public and those charged with fire suppression to the potential dangers in the forest.
Wildfire is the general term used to describe fires burning forest and grass lands, the latter of which includes dry chaparral, prairie grasses and the grasses of the Everglades. In 2003 and early 2004, wildfires were not confined to British Columbia nor the northwestern region of the continent. Southern California and the American southwest also had wildfires that swept into the news headlines for days on end, destroying many homes and threatening thousands more. In the extreme opposite end of the continent, grassfires burned in southern Florida.
The Fire Triangle
Whatever vegetation is burning, the process and the impacts of weather are basically the same. In a nutshell, fires of all kinds require three prime ingredients, known to fire professionals as the fire triangle because removing one will collapse the fire like the removal of a leg on a three-legged stool. The components are fuel, heat and oxygen. Fuel is the wood, grass or other vegetation. Heat is the initial spark that ignites the fire, usually coming from lightning or human actions, intended or accidental. Oxygen comes from the air and is replenished by the wind. Firefighters strive to remove one or more of the three components to douse an active fire.
Weather affects all three legs of the fire triangle and is a very important factor in the initiation stage. And once a fire begins, weather combines with fuel availability and topography to affect the burn and all fire-fighting operations to suppress it.
The main fuels of wildfires are ground litter; trees, shrubs and bushes; and grasses, but in the extreme case, even the soil may burn. The drier the fuel, the greater the likelihood it will burn and burn hot. When fuel is wet, energy that might go into the combustion process first goes into ridding the fuel of moisture. (Water boils at 100 oC (212 oF) while wood burns from 232 to 325 oC (450 to 617 oF) depending on its moisture content. To boil off the moisture requires a great deal of energy the latent heat of evaporation (454 cal/g of water) in addition to the heat used to raise the water to the boiling temperature heat that would otherwise go into heating the wood to combustion. That is why a dry log burns so much better and hotter than a wet one in a campfire or fireplace.
Ground litter is composed mostly of dead leaves/needles and branches that have fallen from the surrounding vegetation. Ground litter is often the tinder that starts the fire because it is dead tissue and therefore cannot replenish its own moisture and must rely on precipitation or runoff to remain damp. When long-term weather conditions are very dry such as during droughts or regular dry seasons the litter and the then living plant tissues are dried by sun, air heat and wind. Dryness in the fuel increases the wildfire danger, all that is needed is the spark of heat.
Wherever fuels are present, the potential for fire exists when given the spark of ignition. Though the percentage varies with location, lightning and human activities are the main causes of wildfires. Some human-initiated fires are deliberately set: arson or control fires that escape. Others are accidental, carelessly tossed smoking materials, undoused campfires, careless burning of trash, sparks from railroads, etc.
Lightning is a major natural ignitor of wildfires, particularly in remote terrain. From 10 to 85 percent of forest fires are ignited by lightning. A lightning bolt striking a tree raises its temperature to as high as 28,000 oC (50,000 oF) in a split second. Such temperatures can easily ignite even the wettest wood. In many instances, this fledgling fire is extinguished by the rains accompanying the lightning. But in many others, this is the spark that ignites the inferno.
Earth's surface layer of atmosphere contains about 21 percent oxygen in its mix. If the oxygen content was too low (around 15 percent), no fires would burn. If too high (above 25 percent), fires would start spontaneously even in wet wood, and little land vegetation would survive.
Thus, to continue burning, a fire needs a regular supply of oxygen. When wind speeds are strong, they can fan the flames by introducing a steady supply of oxygen to the burning region. At first, these winds arise from the general overall weather patterns, or perhaps local wind circulations such as mountain and valley winds. But when fires grow large, they begin to produce their own wind fields, sucking fresh air toward the fire core to replace the hot air rising above the flames.
While weather conditions are important factors in the initiation of all wildfires, fire experts use the term fire weather to describe the meteorological conditions that promote the spread of wildfires. The prevailing weather then combines with hydrological, topographical and vegetation factors to determine the spread of fire. One example of topographical influences is hilly terrain. When all other factors are equal, fire will spread faster uphill than downhill.
The weather conditions most important to the rapid spread of fires include: low humidity, high air temperatures (pre-heated fuels burn more rapidly than cold fuels), moderate to strong winds, and the presence of thunderstorms, particularly dry ones (those having lightning without rain). Once a large fire is burning, the vertical temperature, humidity and windfields of the surrounding air mass become important factors.
Thunderstorms have the potential to douse the fire with their falling rain and cooling temperatures. But often in those created by the fire, any rainfall is quickly evaporated in the dry air above the fire zone and never reaches the ground. Thus, pyro-thunderstorms are feared because of the rapid changes they can make in the nature and direction of the fire. Inflow and outflow winds associated with the storm add oxygen to the mix and can suddenly push the flames in a different direction causing them to jump ahead as burning embers are carried to regions not currently burning. Such conditions can trap firefighters and is a prime cause of death and injury among the firefighters.
The worst fire weather condition is the firestorm. Firestorms combine the elements of the hottest of fires with the dangerous aspects of severe dry thunderstorms to form conditions equivalent to Hell. Some spawn fire tornadoes, whirlwinds blowing in excess of 250 km/h (150 mph) that are capable of lifting large logs and producing gaseous explosions around them. Under naturally generated firestorms, trees ahead of the storm appear to explode into flames as if torched by an unseen hand. (For more on such severe firestorms, see accounts of the Great Peshtigo Fire in "The Great Fires of October 1871.")
A Final Word
Entering late spring of 2004, the fire danger in much of the western region of the continent presaged a repeat of 2003 devastation. But, the return of rainier conditions in the last month before the Summer Solstice gave hope that the number of fires would be lower than originally expected. Wildfires will still burn across the continent, but perhaps not with the numbers of previous years.
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