What is fire?
We define fire as a chemical reaction which reduces organic substances like cellulose in trees and shrubs to inorganic substances such as carbon dioxide, water and black soot, 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 is what ignites and maintains the fire as it gets used in the chemical reaction.
A fuel is anything that is capable of igniting or sustaining combustion in the presence of oxygen and heat. For instance, long dry grass, trees with a high oil content in their leaves and bark, and dead leaves are all examples of fuel because of the ease in which they can be ignited.
Temperature is the means of bringing closer the fuel to the point of igniting in the presence of oxygen.
Humidity helps to either make it harder or easier to ignite the fuel depending on how much water is in the air and/or in the fuel itself.
Wind will either dry out or add moisture to the fuel as well as increase the amount of oxygen added to the fuel when it is burning.
In essence, the optimum conditions for starting and maintaining a fire outdoors is a prolonged period of dry hot winds moving over dry land on a clear day (where air pressure is high and oxygen levels are at its maximum at ground level) and there is plenty of fuel on the ground to burn.
How do we control a fire?
Fuel, heat, oxygen, humidity and wind all help to ignite and maintain fire. Reduce the fuel, lower the temperature of the fuel, increase the humidity or amount of water in the air and/or fuel, reduce 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 to minimise wind speed or use other means to stop the flames from absorbing oxygen from the air.
You will sometimes see fire-fighters use blankets to smother a fire. This has the effect of reducing the oxygen reaching the flames and therefore increasing 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 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.