The fire triangle is a model that represents the three ingredients needed for the chemical reaction of fire, or combustion, to occur.

What You Need To Know

  • Oxygen, heat and fuel make up the fire triangle

  • Fuels in wildfires can be anything from grasses to large trees

  • Winds supply a steady flow of oxygen to sustain fire growth

  • A 10% uphill grade makes a fire spread two times faster

The three elements that make up the fire triangle are heat, fuel and oxygen. Each one of these elements make up the three sides of a triangle.

If you take away one of these elements, or sides, you would not have a triangle. The same holds true for fires.

Without heat, fuel and oxygen, fires can’t sustain themselves.

You need heat or some ignition source to start a fire. The most common natural ignition source of heat in California is lightning.

Man-made heat sources can range from a cigarette butt to a spark from an electrical power line.

Heat allows fire to spread by drying nearby fuels and the air near the flame. Think of a forest fire on a mountain slope.

Because heat rises, trees and shrubs above the flame dry out and become easier to burn.

A 10% uphill grade will make a fire burn two times faster because the heat above a flame dries out the fuel source. This allows the uphill fuels to dry out faster and burn more rapidly.

The next element in the fire triangle is the fuel source. Fuel is any kind of material that will burn.

This can be anything from grasses to large trees and timber. Fuels are characterized by how much moisture is present to sizes, shapes, and quantity.

Thin dry fuels such as grasses can ignite very easily and burn quickly. Large trees, on the other hand, take longer to ignite, but burn much more intensely. 

The last side of the fire triangle is oxygen. Air is made-up of about 21% oxygen, 78% nitrogen and less than 1% other gases including carbon dioxide and water vapor.

Fire only needs about 16% oxygen to burn. Without oxygen, fires won't burn.

Water vapor in the air, or high relative humidity values, help to keep fuel sources moist. This helps slow the spread of fires and hinders ignition of fires.  

When conditions are windy, the oxygen supply near a fire keeps getting replenished. The extra oxygen supplied by wind helps a fire burn more intensely.

To recap, in order for combustion (chemical reaction of fire) to take place you need three elements to work together. These elements are fuel, heat and oxygen. These three elements make up the fire triangle.