Fire occurs whenever combustible fuel in the presence of oxygen at an extremely high temperature becomes gas. Flames are the visual indicator of the heated gas. Fire can also occur from lower-temperature sources. Over time, combustible materials such as smoldering embers can reach their ignition temperature.
The Fire Triangle
The fire triangle is a simple way of understanding the elements of fire. The sides of the triangle represent the interdependent ingredients needed for fire: heat, fuel and oxygen.
A heat source is responsible for the initial ignition of fire, and is also needed to maintain the fire and enable it to spread. Heat allows fire to spread by drying out and preheating nearby fuel and warming surrounding air.
Fuel is any kind of combustible material. It’s characterized by its moisture content, size, shape, quantity and the arrangement in which it is spread over the landscape. The moisture content determines how easily it will burn.
Air contains about 21 percent oxygen, and most fires require at least 16 percent oxygen content to burn. Oxygen supports the chemical processes that occur during fire. When fuel burns, it reacts with oxygen from the surrounding air, releasing heat and generating combustion products (gases, smoke, embers, etc.). This process is known as oxidation.