The science of wildfires
Understanding the terrifying power of the blazes
A wildfire is one of the most powerful forces in nature, as anyone who has watched its frightening fury up close can attest.
If recent warnings from scientists are anything to go by, we may see more and more ferocious wildfires that will be difficult and expensive to fight.
While North America's wildfire season typically peaks in late summer and early fall, climate change is already being blamed for a longer fire season. Some even predict the possibility of a year-round fire season.
While it doesn't make it any less awesome, taking a step back to examine the science of wildfires can at least help us understand their power and why some fires resist all efforts to quench them.
When experts explain fire, they often talk about something called the "fire triangle" and its three equal sides, representing heat, fuel and oxygen.
All three must interact for a fire to get started and to keep going. Take away any one of these ingredients and the fire is suppressed.
Heat is responsible for igniting a fire and allows it to spread.
This is how it works: heat is constantly emanating from a fire, warming the surrounding air and preheating fuel in its path. According to experts at the National Interagency Fire Center (NIFC) – the centre that helps co-ordinate firefighting efforts across the United States – heat allows fire to travel more easily by evaporating the moisture from nearby fuel.
Heat is transferred from one part of a wildfire to another by means of three different processes.
Experts at the New Zealand Fire Service describe the movement of heat using three different categories: conduction, convection and radiation.
They describe conduction as the transfer of heat through a solid, like the heat we feel when we touch the outside of a hot stove.
Convection is the transfer of heat by moving particles or liquids or gases, like the heat that flows from hot water to peas when we boil them or the heat that flows out of the kettle with steam.
Radiation is the transfer of heat by infrared electromagnetic radiation. That's the kind of heat we can feel without touching the source, like the heat from the sun or a campfire.
NIFC experts say radiation accounts for most of the preheating of fuels surrounding a fire. The temperature of these fuels can sometimes grow so high that they ignite even before they come into contact with flames.
Fuel is really just anything that is combustible. That could mean both living and dead vegetation, peat, coal and buildings. What's important to know here is that it's the moisture content of a fuel that determines how easily it will burn.
Living trees have a lot of moisture. Dead trees have very little.
But if a fire is really hot, the moisture content of a fuel means very little. When a wet fuel burns, it's because heat has converted its moisture into vapour. But in areas where the vegetation is dry, the temperature needed for ignition is much lower.
For a fuel to burn, it needs to react with oxygen from the surrounding air so that it can release heat, which in turn helps the fire to spread. The air we breathe contains roughly 21 per cent oxygen. Most fires require only 16 per cent oxygen.
Wildfires can replenish their supply if the oxygen level dips below 16 per cent. Hot flames cause the air to heat up, which in turn causes it to rise. Fresh air then rushes to fill the vacuum below, providing the fire with new oxygen.
Flashover is a fancy way to describe how a fire can suddenly spread through the air. It happens when a fire releases a smoky layer of hot gas that radiates more and more heat to surrounding fuel.
The flashover happens when the smoky gas reaches temperatures of about 600 Celsius. It's at this point that the surrounding fuel becomes so hot it appears to burst spontaneously into flame.
A variety of triggers, from lightning strikes to the human hand of arson, can spark a forest fire.
Once started, these wildfires can rage across hundreds of thousands of hectares and be devastating to human life. Australian bushfires, to take one example, were blamed for dozens of deaths across the southern state of Victoria in February 2009.
And when an inferno gets as big as the Okanagan Valley forest fire in B.C., the summer of 2003, it can create its own weather conditions that can lead to more fires.
It happens because air around a very intense fire heats and rises. The resulting vacuum pulls in more oxygen-rich air. When the fire is very large, such as the one in the Okanagan Valley, it can cause a wind pattern to emerge.
The wind pattern creates hot and dry conditions that can be felt several kilometres from the fire. Smoke from the fire also contributes to natural cloud cover.
In the former town of Centralia, Pa., coal fires have been burning underground for more than 46 years. The blaze was sparked in 1962 on the outskirts of town when a trash fire in the pit of an abandoned strip mine ignited a coal vein running near the surface.
Workers battled for decades to put out the fire, but the coal, a particularly hard coal called anthracite, is notoriously difficult to extinguish once it's burning.
The town was abandoned in the mid-1980s after an engineering study suggested the fire could burn for a century or more. Today, it continues to release thick plumes of toxic smoke throughout the town.
Some scientists believe northern Mali's superheated ground and smoking potholes are evidence not of volcanic activity, as was thought for decades, but of a layer of peat that is burning less than a metre below the desert surface of the African nation.
Unlike coal fires, the peat in Mali appears to be ignited spontaneously by heat created through bacterial activity — in the same way that a compost pile can heat up from decay. The region is dry and the ground porous, which allows oxygen into the ground and keeps it smouldering.
Sustained peat fires on the Indonesian islands of Sumatra and Borneo in 1997-1998 caused a choking haze across the region. One fire was ignited in a one-million-hectare area of peat that the government was draining for a massive rice planting project on Borneo.
The country's neighbours have grown increasingly frustrated by the fires, most of which are deliberately lit by farmers or by timber and palm oil plantation companies — some owned by Singaporeans and Malaysians — to clear land for cultivation.
Peat fires are more difficult than other forest blazes to extinguish because they can go deep underground and can burn uncontrolled and unseen for several months.