What is fire?
We define fire as a chemical reaction that breaks down organic matter, such as cellulose in trees and shrubs, into inorganic matter, such as carbon dioxide, water and black soot (mostly carbon), and in the process emit a large amount of heat.
What factors control a fire?
There are five critical factors in starting and maintaining a fire: a fuel source, oxygen, temperature, wind and humidity.
- Oxygen
Oxygen is what ignites and maintains the fire as it gets used in the chemical reaction. - Fuel
A fuel is anything that can ignite or sustain combustion in the presence of oxygen and heat. For example, long dry grass, trees with a high oil content in their leaves and bark, and dead leaves are all examples of fuel because they can be easily ignited. - Temperature
Temperature is the means of bringing closer the fuel closer to the point of ignition in the presence of oxygen. - Humidity
Humidity makes it either harder or easier to ignite the fuel depending on the amount of water in the air and/or in the fuel itself. - Wind
Wind will either dries or adds moisture to the fuel, and increases the amount of oxygen added to the fuel when it burns.
Essentially, the optimum conditions for starting and maintaining an outdoor fire are a prolonged period of dry, hot winds over dry land on a clear day (when air pressure is high and oxygen levels at ground level are at its maximum) and there is plenty of fuel on the ground to burn.
How do we control a fire?
Fuel, heat, oxygen, humidity and wind all contribute to starting and maintaining a fire. Reducing the amount of fuel, lowering the temperature of the fuel, increasing the humidity or amount of water in the air and/or fuel, reducing the wind created by the flames as they absorb oxygen, or cutting off the oxygen supply will stop a fire.
Generally, there is little we can do about the oxygen levels in the air other than minimising wind speed or using other means to prevent the flames from absorbing oxygen from the air.
You will sometimes see firefighters using blankets to smother a fire. This has the effect of reducing the amount of oxygen reaching the flames and, therefore, increases the success rate of putting out the fire.
We can choose lower temperature conditions, such as those prevalent at night.
Fighting a fire at night is better than during the day as the relative humidity is often higher at night and the temperature is low. This results in a lower intensity fire at night than during the day.
We can choose conditions of high humidity, such as those prevalent on cloudy days and in the tropical areas.
Fire-fighters prefer to combat a fire on a cloudy day, not only because the temperature is lower, but also because there is more moisture in the air and on the ground. Even a very hot day will not cause a fire if the humidity is high. It takes low humidity combined with high temperatures to increase the likelihood of a fire. This is why large bushfires are rare in the tropics.
We can choose a time when there is little or no wind, such as right in the middle of the night or day, but never in between (e.g. the really early mornings or late evenings), or we can somehow prevent the flames from creating their own wind at ground level when it sucks oxygen into the fire.
Firefighters agree that it is better to fight fires on a windless day. The presence of any strong, dry wind during a fire is perhaps the single most dangerous and important factor in determining how to control a fire once it has started. In general, the higher the wind speed and the more variable the wind direction, the more dangerous it is to control a fire. When fighting a fire, it is absolutely essential to know the wind speed and direction and to keep this information at all times.
But the most effective, safest and simplest way to control any fire is to control the amount of fuel available to ignite and sustain it.
You may see numerous strategically placed dirt roads running through the forest, or people deliberately burning off certain areas in an attempt to reduce fuel levels. Firefighters believe this is the most effective way to manage fires on an ecological scale.